Security-Enhanced Linux (SELinux) is an implementation of a mandatory access control mechanism in the Linux kernel, checking for allowed operations after standard discretionary access controls
are checked. It was created by the National Security Agency and can
enforce rules on files and processes in a Linux system, and on their
actions, based on defined policy.
When using SELinux, files, including directories and devices, are
referred to as objects. Processes, such as a user running a command or
the Mozilla® Firefox®
application, are referred to as subjects. Most operating systems use a
Discretionary Access Control (DAC) system that controls how subjects
interact with objects, and how subjects interact with each other. On
operating systems using DAC, users control the permissions of files
(objects) that they own. For example, on Linux®
operating systems, users could make their home directories
world-readable, giving users and processes (subjects) access to
potentially sensitive information, with no further protection over this
unwanted action.
Relying on DAC mechanisms alone is fundamentally inadequate for strong
system security. DAC access decisions are only based on user identity
and ownership, ignoring other security-relevant information such as the
role of the user, the function and trustworthiness of the program, and
the sensitivity and integrity of the data. Each user typically has
complete discretion over their files, making it difficult to enforce a
system-wide security policy. Furthermore, every program run by a user
inherits all of the permissions granted to the user and is free to
change access to the user's files, so minimal protection is provided
against malicious software. Many system services and privileged programs
run with coarse-grained privileges that far exceed their requirements,
so that a flaw in any one of these programs could be exploited to obtain
further system access.[]
The following is an example of permissions used on Linux operating
systems that do not run Security-Enhanced Linux (SELinux). The
permissions and output in these examples may differ slightly from your
system. Use the ls -l command to view file permissions:
$ ls -l file1
-rwxrw-r-- 1 user1 group1 0 2009-08-30 11:03 file1
In this example, the first three permission bits, rwx, control the access the Linux user1 user (in this case, the owner) has to file1. The next three permission bits, rw-, control the access the Linux group1 group has to file1. The last three permission bits, r--, control the access everyone else has to file1, which includes all users and processes.
Security-Enhanced Linux (SELinux) adds Mandatory Access Control (MAC)
to the Linux kernel, and is enabled by default in Red Hat Enterprise
Linux. A general purpose MAC architecture needs the ability to enforce
an administratively-set security policy over all processes and files in
the system, basing decisions on labels containing a variety of
security-relevant information. When properly implemented, it enables a
system to adequately defend itself and offers critical support for
application security by protecting against the tampering with, and
bypassing of, secured applications. MAC provides strong separation of
applications that permits the safe execution of untrustworthy
applications. Its ability to limit the privileges associated with
executing processes limits the scope of potential damage that can result
from the exploitation of vulnerabilities in applications and system
services. MAC enables information to be protected from legitimate users
with limited authorization as well as from authorized users who have
unwittingly executed malicious applications.[]
The following is an example of the labels containing security-relevant
information that are used on processes, Linux users, and files, on
Linux operating systems that run SELinux. This information is called the
SELinux context, and is viewed using the ls -Z command:
$ ls -Z file1
-rwxrw-r-- user1 group1 unconfined_u:object_r:user_home_t:s0 file1
In this example, SELinux provides a user (unconfined_u), a role (object_r), a type (user_home_t), and a level (s0).
This information is used to make access control decisions. With DAC,
access is controlled based only on Linux user and group IDs. It is
important to remember that SELinux policy rules are checked after DAC rules. SELinux policy rules are not used if DAC rules deny access first.
On Linux operating systems that run SELinux, there are Linux users as
well as SELinux users. SELinux users are part of SELinux policy. Linux
users are mapped to SELinux users. To avoid confusion, this guide uses
"Linux user" and "SELinux user" to differentiate between the two.
2.1. Benefits of running SELinux
All processes and files are labeled with a type. A type defines a
domain for processes, and a type for files. Processes are separated from
each other by running in their own domains, and SELinux policy rules
define how processes interact with files, as well as how processes
interact with each other. Access is only allowed if an SELinux policy
rule exists that specifically allows it.
Fine-grained access control. Stepping beyond traditional UNIX®
permissions that are controlled at user discretion and based on Linux
user and group IDs, SELinux access decisions are based on all available
information, such as an SELinux user, role, type, and, optionally, a
level.
SELinux policy is administratively-defined, enforced system-wide, and is not set at user discretion.
Reduced vulnerability to privilege escalation attacks. One example:
since processes run in domains, and are therefore separated from each
other, and because SELinux policy rules define how processes access
files and other processes, if a process is compromised, the attacker
only has access to the normal functions of that process, and to files
the process has been configured to have access to. For example, if the
Apache HTTP Server is compromised, an attacker can not use that process
to read files in user home directories, unless a specific SELinux policy
rule was added or configured to allow such access.
SELinux can be used to enforce data confidentiality and integrity, as well as protecting processes from untrusted inputs.
However, SELinux is not:
antivirus software.
a replacement for passwords, firewalls, or other security systems.
an all-in-one security solution.
SELinux is designed to enhance existing security solutions, not
replace them. Even when running SELinux, it is important to continue to
follow good security practices, such as keeping software up-to-date,
using hard-to-guess passwords, firewalls, and so on.
The following examples demonstrate how SELinux increases security:
The default action is deny. If an SELinux policy rule does not
exist to allow access, such as for a process opening a file, access is
denied.
SELinux can confine Linux users. A number of confined SELinux users
exist in SELinux policy. Linux users can be mapped to confined SELinux
users to take advantage of the security rules and mechanisms applied to
them. For example, mapping a Linux user to the SELinux user_u user,
results in a Linux user that is not able to run (unless configured
otherwise) set user ID (setuid) applications, such as sudo and su,
as well as preventing them from executing files and applications in
their home directory - if configured, this prevents users from executing
malicious files from their home directories.
Process separation is used. Processes run in their own domains,
preventing processes from accessing files used by other processes, as
well as preventing processes from accessing other processes. For
example, when running SELinux, unless otherwise configured, an attacker
can not compromise a Samba server, and then use that Samba server as an
attack vector to read and write to files used by other processes, such
as databases used by MySQL®.
SELinux helps limit the damage made by configuration mistakes.
Domain Name System (DNS)
servers often replicate information between each other in what is known
as a zone transfer. Attackers can use zone transfers to update DNS
servers with false information. When running the
Berkeley Internet Name Domain (BIND)
as a DNS server in Red Hat Enterprise Linux, even if an administrator
forgets to limit which servers can perform a zone transfer, the default
SELinux policy prevents zone files
[] from being updated via zone transfers, by the BIND
named daemon itself, and by other processes.
2.3. SELinux Architecture
SELinux is a Linux security module that is built into the Linux
kernel. SELinux is driven by loadable policy rules. When
security-relevant access is taking place, such as when a process
attempts to open a file, the operation is intercepted in the kernel by
SELinux. If an SELinux policy rule allows the operation, it continues,
otherwise, the operation is blocked and the process receives an error.
SELinux decisions, such as allowing or disallowing access, are
cached. This cache is known as the Access Vector Cache (AVC). Caching
decisions decrease how often SELinux policy rules need to be checked,
which increases performance. Remember that SELinux policy rules have no
effect if DAC rules deny access first.
2.4. SELinux on Other Operating Systems
Refer to the following for information about running SELinux on other Linux distributions:
Chapter 3. SELinux Contexts
Processes and files are labeled with an SELinux context that contains
additional information, such as an SELinux user, role, type, and,
optionally, a level. When running SELinux, all of this information is
used to make access control decisions. In Red Hat Enterprise Linux,
SELinux provides a combination of Role-Based Access Control (RBAC), Type Enforcement® (TE), and, optionally, Multi-Level Security (MLS).
The following is an example showing SELinux context. SELinux contexts
are used on processes, Linux users, and files, on Linux operating
systems that run SELinux. Use the ls -Z command to view the SELinux context of files and directories:
$ ls -Z file1
-rwxrw-r-- user1 group1 unconfined_u:object_r:user_home_t:s0 file1
SELinux contexts follow the SELinux user:role:type:level syntax:
- SELinux user
The SELinux user identity is an identity known to the policy that
is authorized for a specific set of roles, and for a specific MLS range.
Each Linux user is mapped to an SELinux user via SELinux policy. This
allows Linux users to inherit the restrictions placed on SELinux users.
The mapped SELinux user identity is used in the SELinux context for
processes in that session, in order to define what roles and levels they
can enter. Run the semanage login -l command as the Linux root user to view a list of mappings between SELinux and Linux user accounts:
# /usr/sbin/semanage login -l
Login Name SELinux User MLS/MCS Range
__default__ unconfined_u s0-s0:c0.c1023
root unconfined_u s0-s0:c0.c1023
system_u system_u s0-s0:c0.c1023
Output may differ slightly from system to system. The Login Name column lists Linux users, and the SELinux User
column lists which SELinux user the Linux user is mapped to. For
processes, the SELinux user limits which roles and levels are
accessible. The last column, MLS/MCS Range, is the level used by Multi-Level Security (MLS) and Multi-Category Security (MCS). Levels are briefly discussed later.
- role
Part of SELinux is the Role-Based Access Control (RBAC) security
model. The role is an attribute of RBAC. SELinux users are authorized
for roles, and roles are authorized for domains. The role serves as an
intermediary between domains and SELinux users. The roles that can be
entered determine which domains can be entered - ultimately, this
controls which object types can be accessed. This helps reduce
vulnerability to privilege escalation attacks.
- type
The type is an attribute of Type Enforcement. The type defines a
domain for processes, and a type for files. SELinux policy rules define
how types can access each other, whether it be a domain accessing a
type, or a domain accessing another domain. Access is only allowed if a
specific SELinux policy rule exists that allows it.
- level
The level is an attribute of MLS and Multi-Category Security (MCS). An MLS range is a pair of levels, written as lowlevel-highlevel if the levels differ, or lowlevel if the levels are identical (s0-s0 is the same as s0). Each level is a sensitivity-category pair, with categories being optional. If there are categories, the level is written as sensitivity:category-set. If there are no categories, it is written as sensitivity.
If the category set is a contiguous series, it can be abbreviated. For example, c0.c3 is the same as c0,c1,c2,c3. The /etc/selinux/targeted/setrans.conf file maps levels (s0:c0) to human-readable form (ie. CompanyConfidential). Do not edit setrans.conf with a text editor: use semanage to make changes. Refer to the semanage(8)
manual page for further information. In Red Hat Enterprise Linux,
targeted policy enforces MCS, and in MCS, there is just one sensitivity,
s0. MCS in Red Hat Enterprise Linux supports 1024 different categories: c0 through to c1023. s0-s0:c0.c1023 is sensitivity s0 and authorized for all categories.
MLS enforces the
Bell-La Padula Mandatory Access Model, and is used in Labeled Security Protection Profile (LSPP) environments. To use MLS restrictions, install the
selinux-policy-mls
package, and configure MLS to be the default SELinux policy. The MLS
policy shipped with Red Hat Enterprise Linux omits many program domains
that were not part of the evaluated configuration, and therefore, MLS on
a desktop workstation is unusable (no support for the X Window System);
however, an MLS policy from the
upstream SELinux Reference Policy can be built that includes all program domains.
A process in one domain transitions to another domain by executing an application that has the entrypoint type for the new domain. The entrypoint
permission is used in SELinux policy, and controls which applications
can be used to enter a domain. The following example demonstrates a
domain transition:
A user wants to change their password. To do this, they run the passwd application. The /usr/bin/passwd executable is labeled with the passwd_exec_t type:
$ ls -Z /usr/bin/passwd
-rwsr-xr-x root root system_u:object_r:passwd_exec_t:s0 /usr/bin/passwd
The passwd application accesses /etc/shadow, which is labeled with the shadow_t type:
$ ls -Z /etc/shadow
-r--------. root root system_u:object_r:shadow_t:s0 /etc/shadow
An SELinux policy rule states that processes running in the passwd_t domain are allowed to read and write to files labeled with the shadow_t type. The shadow_t type is only applied to files that are required for a password change. This includes /etc/gshadow, /etc/shadow, and their backup files.
An SELinux policy rule states that the passwd_t domain has entrypoint permission to the passwd_exec_t type.
When a user runs the /usr/bin/passwd application, the user's shell process transitions to the passwd_t
domain. With SELinux, since the default action is to deny, and a rule
exists that allows (among other things) applications running in the passwd_t domain to access files labeled with the shadow_t type, the passwd application is allowed to access /etc/shadow, and update the user's password.
This example is not exhaustive, and is used as a basic example to
explain domain transition. Although there is an actual rule that allows
subjects running in the passwd_t domain to access objects labeled with the shadow_t
file type, other SELinux policy rules must be met before the subject
can transition to a new domain. In this example, Type Enforcement
ensures:
the passwd_t domain can only be entered by executing an application labeled with the passwd_exec_t type; can only execute from authorized shared libraries, such as the lib_t type; and can not execute any other applications.
only authorized domains, such as passwd_t, can write to files labeled with the shadow_t type. Even if other processes are running with superuser privileges, those processes can not write to files labeled with the shadow_t type, as they are not running in the passwd_t domain.
only authorized domains can transition to the passwd_t domain. For example, the sendmail process running in the sendmail_t domain does not have a legitimate reason to execute passwd; therefore, it can never transition to the passwd_t domain.
processes running in the passwd_t domain can only read and write to authorized types, such as files labeled with the etc_t or shadow_t types. This prevents the passwd application from being tricked into reading or writing arbitrary files.
3.2. SELinux Contexts for Processes
Use the ps -eZ command to view the SELinux context for processes. For example:
Open a terminal, such as → → .
Run the /usr/bin/passwd command. Do not enter a new password.
Open a new tab, or another terminal, and run the ps -eZ | grep passwd command. The output is similar to the following:
unconfined_u:unconfined_r:passwd_t:s0-s0:c0.c1023 13212 pts/1 00:00:00 passwd
In the first tab/terminal, press Ctrl+C to cancel the passwd application.
In this example, when the /usr/bin/passwd application (labeled with the passwd_exec_t type) is executed, the user's shell process transitions to the passwd_t domain. Remember: the type defines a domain for processes, and a type for files.
Use the ps -eZ command to view the
SELinux contexts for running processes. The following is a truncated
example of the output, and may differ on your system:
system_u:system_r:dhcpc_t:s0 1869 ? 00:00:00 dhclient
system_u:system_r:sshd_t:s0-s0:c0.c1023 1882 ? 00:00:00 sshd
system_u:system_r:gpm_t:s0 1964 ? 00:00:00 gpm
system_u:system_r:crond_t:s0-s0:c0.c1023 1973 ? 00:00:00 crond
system_u:system_r:kerneloops_t:s0 1983 ? 00:00:05 kerneloops
system_u:system_r:crond_t:s0-s0:c0.c1023 1991 ? 00:00:00 atd
The system_r role is used for system processes, such as daemons. Type Enforcement then separates each domain.
3.3. SELinux Contexts for Users
Use the id -Z command to view the SELinux context associated with your Linux user:
unconfined_u:unconfined_r:unconfined_t:s0-s0:c0.c1023
In Red Hat Enterprise Linux, Linux users run unconfined by default.
This SELinux context shows that the Linux user is mapped to the SELinux unconfined_u user, running as the unconfined_r role, and is running in the unconfined_t domain. s0-s0 is an MLS range, which in this case, is the same as just s0. The categories the user has access to is defined by c0.c1023, which is all categories (c0 through to c1023).
Chapter 4. Targeted Policy
Targeted policy is the default SELinux policy used in Red Hat
Enterprise Linux. When using targeted policy, processes that are
targeted run in a confined domain, and processes that are not targeted
run in an unconfined domain. For example, by default, logged in users
run in the unconfined_t domain, and system processes started by init run in the initrc_t domain - both of these domains are unconfined.
Unconfined domains (as well as confined domains) are subject to
executable and writeable memory checks. By default, subjects running in
an unconfined domain can not allocate writeable memory and execute it.
This reduces vulnerability to
buffer overflow attacks.
These memory checks are disabled by setting Booleans, which allow the
SELinux policy to be modified at runtime. Boolean configuration is
discussed later.
Almost every service that listens on a network, such as sshd or httpd,
is confined in Red Hat Enterprise Linux. Also, most processes that run
as the Linux root user and perform tasks for users, such as the passwd application, are confined. When a process is confined, it runs in its own domain, such as the httpd process running in the httpd_t
domain. If a confined process is compromised by an attacker, depending
on SELinux policy configuration, an attacker's access to resources and
the possible damage they can do is limited.
The following example demonstrates how SELinux prevents the Apache HTTP Server (httpd)
from reading files that are not correctly labeled, such as files
intended for use by Samba. This is an example, and should not be used in
production. It assumes that the httpd and wget packages are installed, the SELinux targeted policy is used, and that SELinux is running in enforcing mode:
Run the sestatus command to confirm that SELinux is enabled, is running in enforcing mode, and that targeted policy is being used:
$ /usr/sbin/sestatus
SELinux status: enabled
SELinuxfs mount: /selinux
Current mode: enforcing
Mode from config file: enforcing
Policy version: 24
Policy from config file: targeted
SELinux status: enabled is returned when SELinux is enabled. Current mode: enforcing is returned when SELinux is running in enforcing mode. Policy from config file: targeted is returned when the SELinux targeted policy is used.
As the Linux root user, run the touch /var/www/html/testfile command to create a file.
Run the ls -Z /var/www/html/testfile command to view the SELinux context:
-rw-r--r-- root root unconfined_u:object_r:httpd_sys_content_t:s0 /var/www/html/testfile
By default, Linux users run unconfined in Red Hat Enterprise Linux, which is why the testfile file is labeled with the SELinux unconfined_u user. RBAC is used for processes, not files. Roles do not have a meaning for files - the object_r role is a generic role used for files (on persistent storage and network file systems). Under the /proc/ directory, files related to processes may use the system_r role.[] The httpd_sys_content_t type allows the httpd process to access this file.
As the Linux root user, run the service httpd start command to start the httpd process. The output is as follows if httpd starts successfully:
# /sbin/service httpd start
Starting httpd: [ OK ]
Change into a directory where your Linux user has write access to, and run the wget http://localhost/testfile command. Unless there are changes to the default configuration, this command succeeds:
--2009-11-06 17:43:01-- http://localhost/testfile
Resolving localhost... 127.0.0.1
Connecting to localhost|127.0.0.1|:80... connected.
HTTP request sent, awaiting response... 200 OK
Length: 0 [text/plain]
Saving to: `testfile'
[ <=> ] 0 --.-K/s in 0s
2009-11-06 17:43:01 (0.00 B/s) - `testfile' saved [0/0]
The chcon command relabels files;
however, such label changes do not survive when the file system is
relabeled. For permanent changes that survive a file system relabel, use
the semanage command, which is discussed
later. As the Linux root user, run the following command to change the
type to a type used by Samba:
chcon -t samba_share_t /var/www/html/testfile
Run the ls -Z /var/www/html/testfile command to view the changes:
-rw-r--r-- root root unconfined_u:object_r:samba_share_t:s0 /var/www/html/testfile
Note: the current DAC permissions allow the httpd process access to testfile. Change into a directory where your Linux user has write access to, and run the wget http://localhost/testfile command. Unless there are changes to the default configuration, this command fails:
--2009-11-06 14:11:23-- http://localhost/testfile
Resolving localhost... 127.0.0.1
Connecting to localhost|127.0.0.1|:80... connected.
HTTP request sent, awaiting response... 403 Forbidden
2009-11-06 14:11:23 ERROR 403: Forbidden.
As the Linux root user, run the rm -i /var/www/html/testfile command to remove testfile.
If you do not require httpd to be running, as the Linux root user, run the service httpd stop command to stop httpd:
# /sbin/service httpd stop
Stopping httpd: [ OK ]
This example demonstrates the additional security added by SELinux. Although DAC rules allowed the httpd process access to testfile in step 7, because the file was labeled with a type that the httpd process does not have access to, SELinux denied access.
An error similar to the following is logged to /var/log/audit/audit.log:
type=AVC msg=audit(1220706212.937:70): avc: denied { getattr } for pid=1904 comm="httpd" path="/var/www/html/testfile" dev=sda5 ino=247576 scontext=unconfined_u:system_r:httpd_t:s0 tcontext=unconfined_u:object_r:samba_share_t:s0 tclass=file
type=SYSCALL msg=audit(1220706212.937:70): arch=40000003 syscall=196 success=no exit=-13 a0=b9e21da0 a1=bf9581dc a2=555ff4 a3=2008171 items=0 ppid=1902 pid=1904 auid=500 uid=48 gid=48 euid=48 suid=48 fsuid=48 egid=48 sgid=48 fsgid=48 tty=(none) ses=1 comm="httpd" exe="/usr/sbin/httpd" subj=unconfined_u:system_r:httpd_t:s0 key=(null)
Also, an error similar to the following is logged to /var/log/httpd/error_log:
[Wed May 06 23:00:54 2009] [error] [client 127.0.0.1] (13)Permission denied: access to /testfile denied
4.2. Unconfined Processes
Unconfined processes run in unconfined domains, for example, init programs run in the unconfined initrc_t domain, unconfined kernel processes run in the kernel_t domain, and unconfined Linux users run in the unconfined_t
domain. For unconfined processes, SELinux policy rules are applied, but
policy rules exist that allow processes running in unconfined domains
almost all access. Processes running in unconfined domains fall back to
using DAC rules exclusively. If an unconfined process is compromised,
SELinux does not prevent an attacker from gaining access to system
resources and data, but of course, DAC rules are still used. SELinux is a
security enhancement on top of DAC rules - it does not replace them.
The following example demonstrates how the Apache HTTP Server (httpd) can access data intended for use by Samba, when running unconfined. Note: in Red Hat Enterprise Linux, the httpd process runs in the confined httpd_t domain by default. This is an example, and should not be used in production. It assumes that the httpd, wget, dbus and audit packages are installed, that the SELinux targeted policy is used, and that SELinux is running in enforcing mode:
Run the sestatus command to confirm that SELinux is enabled, is running in enforcing mode, and that targeted policy is being used:
$ /usr/sbin/sestatus
SELinux status: enabled
SELinuxfs mount: /selinux
Current mode: enforcing
Mode from config file: enforcing
Policy version: 24
Policy from config file: targeted
SELinux status: enabled is returned when SELinux is enabled. Current mode: enforcing is returned when SELinux is running in enforcing mode. Policy from config file: targeted is returned when the SELinux targeted policy is used.
As the Linux root user, run the touch /var/www/html/test2file command to create a file.
Run the ls -Z /var/www/html/test2file command to view the SELinux context:
-rw-r--r-- root root unconfined_u:object_r:httpd_sys_content_t:s0 /var/www/html/test2file
By default, Linux users run unconfined in Red Hat Enterprise Linux, which is why the test2file file is labeled with the SELinux unconfined_u user. RBAC is used for processes, not files. Roles do not have a meaning for files - the object_r role is a generic role used for files (on persistent storage and network file systems). Under the /proc/ directory, files related to processes may use the system_r role.[] The httpd_sys_content_t type allows the httpd process to access this file.
The chcon command relabels files;
however, such label changes do not survive when the file system is
relabeled. For permanent changes that survive a file system relabel, use
the semanage command, which is discussed
later. As the Linux root user, run the following command to change the
type to a type used by Samba:
chcon -t samba_share_t /var/www/html/test2file
Run the ls -Z /var/www/html/test2file command to view the changes:
-rw-r--r-- root root unconfined_u:object_r:samba_share_t:s0 /var/www/html/test2file
Run the service httpd status command to confirm that the httpd process is not running:
$ /sbin/service httpd status
httpd is stopped
If the output differs, run the service httpd stop command as the Linux root user to stop the httpd process:
# /sbin/service httpd stop
Stopping httpd: [ OK ]
To make the httpd process run unconfined, run the following command as the Linux root user to change the type of /usr/sbin/httpd, to a type that does not transition to a confined domain:
chcon -t unconfined_exec_t /usr/sbin/httpd
Run the ls -Z /usr/sbin/httpd command to confirm that /usr/sbin/httpd is labeled with the unconfined_exec_t type:
-rwxr-xr-x root root system_u:object_r:unconfined_exec_t /usr/sbin/httpd
As the Linux root user, run the service httpd start command to start the httpd process. The output is as follows if httpd starts successfully:
# /sbin/service httpd start
Starting httpd: [ OK ]
Run the ps -eZ | grep httpd command to view the httpd running in the unconfined_t domain:
$ ps -eZ | grep httpd
unconfined_u:system_r:unconfined_t 7721 ? 00:00:00 httpd
unconfined_u:system_r:unconfined_t 7723 ? 00:00:00 httpd
unconfined_u:system_r:unconfined_t 7724 ? 00:00:00 httpd
unconfined_u:system_r:unconfined_t 7725 ? 00:00:00 httpd
unconfined_u:system_r:unconfined_t 7726 ? 00:00:00 httpd
unconfined_u:system_r:unconfined_t 7727 ? 00:00:00 httpd
unconfined_u:system_r:unconfined_t 7728 ? 00:00:00 httpd
unconfined_u:system_r:unconfined_t 7729 ? 00:00:00 httpd
unconfined_u:system_r:unconfined_t 7730 ? 00:00:00 httpd
Change into a directory where your Linux user has write access to, and run the wget http://localhost/test2file command. Unless there are changes to the default configuration, this command succeeds:
--2009-05-07 01:41:10-- http://localhost/test2file
Resolving localhost... 127.0.0.1
Connecting to localhost|127.0.0.1|:80... connected.
HTTP request sent, awaiting response... 200 OK
Length: 0 [text/plain]
Saving to: `test2file.1'
[ <=> ]--.-K/s in 0s
2009-05-07 01:41:10 (0.00 B/s) - `test2file.1' saved [0/0]
Although the httpd process does not have access to files labeled with the samba_share_t type, httpd is running in the unconfined unconfined_t domain, and falls back to using DAC rules, and as such, the wget command succeeds. Had httpd been running in the confined httpd_t domain, the wget command would have failed.
The restorecon command restores the default SELinux context for files. As the Linux root user, run the restorecon -v /usr/sbin/httpd command to restore the default SELinux context for /usr/sbin/httpd:
# /sbin/restorecon -v /usr/sbin/httpd
restorecon reset /usr/sbin/httpd context system_u:object_r:unconfined_notrans_exec_t:s0->system_u:object_r:httpd_exec_t:s0
Run the ls -Z /usr/sbin/httpd command to confirm that /usr/sbin/httpd is labeled with the httpd_exec_t type:
$ ls -Z /usr/sbin/httpd
-rwxr-xr-x root root system_u:object_r:httpd_exec_t /usr/sbin/httpd
As the Linux root user, run the /sbin/service httpd restart command to restart httpd. After restarting, run the ps -eZ | grep httpd to confirm that httpd is running in the confined httpd_t domain:
# /sbin/service httpd restart
Stopping httpd: [ OK ]
Starting httpd: [ OK ]
# ps -eZ | grep httpd
unconfined_u:system_r:httpd_t 8880 ? 00:00:00 httpd
unconfined_u:system_r:httpd_t 8882 ? 00:00:00 httpd
unconfined_u:system_r:httpd_t 8883 ? 00:00:00 httpd
unconfined_u:system_r:httpd_t 8884 ? 00:00:00 httpd
unconfined_u:system_r:httpd_t 8885 ? 00:00:00 httpd
unconfined_u:system_r:httpd_t 8886 ? 00:00:00 httpd
unconfined_u:system_r:httpd_t 8887 ? 00:00:00 httpd
unconfined_u:system_r:httpd_t 8888 ? 00:00:00 httpd
unconfined_u:system_r:httpd_t 8889 ? 00:00:00 httpd
As the Linux root user, run the rm -i /var/www/html/test2file command to remove test2file.
If you do not require httpd to be running, as the Linux root user, run the service httpd stop command to stop httpd:
# /sbin/service httpd stop
Stopping httpd: [ OK ]
The examples in these sections demonstrate how data can be protected
from a compromised confined-process (protected by SELinux), as well as
how data is more accessible to an attacker from a compromised
unconfined-process (not protected by SELinux).
4.3. Confined and Unconfined Users
Each Linux user is mapped to an SELinux user via SELinux policy. This
allows Linux users to inherit the restrictions on SELinux users. This
Linux user mapping is seen by running the semanage login -l command as the Linux root user:
# /usr/sbin/semanage login -l
Login Name SELinux User MLS/MCS Range
__default__ unconfined_u s0-s0:c0.c1023
root unconfined_u s0-s0:c0.c1023
system_u system_u s0-s0:c0.c1023
In Red Hat Enterprise Linux 6, Linux users are mapped to the SELinux __default__ login by default (which is mapped to the SELinux unconfined_u user). The following defines the default-mapping:
__default__ unconfined_u s0-s0:c0.c1023
The following example demonstrates adding a new Linux user, and that Linux user being mapped to the SELinux unconfined_u user. It assumes that the Linux root user is running unconfined, as it does by default in Red Hat Enterprise Linux 6:
As the Linux root user, run the /usr/sbin/useradd newuser command to create a new Linux user named newuser.
As the Linux root user, run the passwd newuser command to assign a password to the Linux newuser user:
# passwd newuser
Changing password for user newuser.
New UNIX password: Enter a password
Retype new UNIX password: Enter the same password again
passwd: all authentication tokens updated successfully.
Log out of your current session, and log in as the Linux newuser
user. When you log in, pam_selinux maps the Linux user to an SELinux
user (in this case, unconfined_u), and sets up the resulting SELinux
context. The Linux user's shell is then launched with this context. Run
the id -Z command to view the context of a Linux user:
[newuser@localhost ~]$ id -Z
unconfined_u:unconfined_r:unconfined_t:s0-s0:c0.c1023
Log out of the Linux newuser's session, and log in with your account. If you do not want the Linux newuser user, run the /usr/sbin/userdel -r newuser command as the Linux root user to remove it, along with the Linux newuser's home directory.
Confined and unconfined Linux users are subject to executable and
writeable memory checks, and are also restricted by MCS (and MLS, if the
MLS policy is used). If unconfined Linux users execute an application
that SELinux policy defines can transition from the unconfined_t
domain to its own confined domain, unconfined Linux users are still
subject to the restrictions of that confined domain. The security
benefit of this is that, even though a Linux user is running unconfined,
the application remains confined, and therefore, the exploitation of a
flaw in the application can be limited by policy. Note: this does not
protect the system from the user. Instead, the user and the system are
being protected from possible damage caused by a flaw in the
application.
The following confined SELinux users are available in Red Hat Enterprise Linux 6:
Table 4.1. SELinux User Capabilities
|
User
|
Domain
|
X Window System
|
su and sudo
|
Execute in home directory and /tmp/
|
Networking
|
|---|
|
guest_u
|
guest_t
|
no
|
no
|
optional
|
no
|
|
xguest_u
|
xguest_t
|
yes
|
no
|
optional
|
only Firefox
|
|
user_u
|
user_t
|
yes
|
no
|
optional
|
yes
|
|
staff_u
|
staff_t
|
yes
|
only sudo
|
optional
|
yes
|
Linux users in the guest_t, xguest_t, and user_t domains can only run set user ID (setuid) applications if SELinux policy permits it (such as passwd). They can not run the su and /usr/bin/sudo setuid applications, and therefore, can not use these applications to become the Linux root user.
Linux users in the guest_t domain have no network access, and can only log in via a terminal (including ssh; they can log in via ssh, but can not use ssh to connect to another system).
The only network access Linux users in the xguest_t domain have is Firefox connecting to web pages.
Linux users in the xguest_t, user_t and staff_t domains can log in via the X Window System and a terminal.
By default, Linux users in the staff_t domain do not have permissions to execute applications with /usr/bin/sudo. These permissions must be configured by an administrator.
By default, Linux users in the guest_t and xguest_t domains can not execute applications in their home directories or /tmp/,
preventing them from executing applications (which inherit users'
permissions) in directories they have write access to. This helps
prevent flawed or malicious applications from modifying files users'
own.
By default, Linux users in the
user_t and
staff_t domains can execute applications in their home directories and
/tmp/. Refer to
Section 6.6, “Booleans for Users Executing Applications” for information about allowing and preventing users from executing applications in their home directories and
/tmp/.
Chapter 5. Working with SELinux
The following sections give a brief overview of the main SELinux
packages in Red Hat Enterprise Linux; installing and updating packages;
which log files are used; the main SELinux configuration file; enabling
and disabling SELinux; SELinux modes; configuring Booleans; temporarily
and persistently changing file and directory labels; overriding file
system labels with the mount command; mounting NFS file systems; and how to preserve SELinux contexts when copying and archiving files and directories.
In Red Hat Enterprise Linux, the SELinux packages are installed by
default, in a full installation, unless they are manually excluded
during installation. If performing a minimal installation in text mode,
the policycoreutils-python package will not
be installed by default. Also, by default, SELinux targeted policy is
used, and SELinux runs in enforcing mode. The following is a brief
description of the main SELinux packages:
policycoreutils-python: provides utilities such as semanage, audit2allow, audit2why and chcat, for operating and managing SELinux.
policycoreutils: provides utilities such as restorecon, secon, setfiles, semodule, load_policy, and setsebool, for operating and managing SELinux.
policycoreutils-gui: provides system-config-selinux, a graphical tool for managing SELinux.
selinux-policy: provides the SELinux
Reference Policy. The SELinux Reference Policy is a complete SELinux
policy, and is used as a basis for other policies, such as the SELinux
targeted policy. Refer to the Tresys Technology
SELinux Reference Policy page for further information. The
selinux-policy-devel package provides development tools, such as
/usr/share/selinux/devel/policygentool and
/usr/share/selinux/devel/policyhelp, as well as example policy files.
selinux-policy-policy: provides SELinux policies. For targeted policy, install selinux-policy-targeted. For MLS, install selinux-policy-mls.
setroubleshoot-server: translates denial messages, produced when access is denied by SELinux, into detailed descriptions that are viewed with sealert (which is provided by this package).
setools-console: this package provides the
Tresys Technology SETools distribution,
a number of tools and libraries for analyzing and querying policy,
audit log monitoring and reporting, and file context management
[]. The
setools package is a meta-package for SETools. The
setools-gui package provides the
apol,
seaudit, and
sediffx tools. The
setools-console package provides the
seaudit-report,
sechecker,
sediff,
seinfo,
sesearch,
findcon,
replcon, and
indexcon command line tools. Refer to the
Tresys Technology SETools page for information about these tools.
libselinux-utils: provides the avcstat, getenforce, getsebool, matchpathcon, selinuxconlist, selinuxdefcon, selinuxenabled, setenforce, togglesebool tools.
mcstrans: translates levels, such as s0-s0:c0.c1023, to an easier to read form, such as SystemLow-SystemHigh. This package is not installed by default.
To install packages in Red Hat Enterprise Linux, as the Linux root user, run the yum install package-name command. For example, to install the mcstrans package, run the yum install mcstrans command. To upgrade all installed packages in Red Hat Enterprise Linux, run the yum update command.
5.2. Which Log File is Used
In Red Hat Enterprise Linux 6, the dbus, setroubleshoot-server and audit packages are installed if packages are not removed from the default package selection.
SELinux denial messages, such as the following, are written to /var/log/audit/audit.log by default:
type=AVC msg=audit(1223024155.684:49): avc: denied { getattr } for pid=2000 comm="httpd" path="/var/www/html/file1" dev=dm-0 ino=399185 scontext=unconfined_u:system_r:httpd_t:s0 tcontext=system_u:object_r:samba_share_t:s0 tclass=file
May 7 18:55:56 localhost setroubleshoot: SELinux is preventing httpd (httpd_t) "getattr" to /var/www/html/file1 (samba_share_t). For complete SELinux messages. run sealert -l de7e30d6-5488-466d-a606-92c9f40d316d
In Red Hat Enterprise Linux 6, setroubleshootd
no longer constantly runs as a service, however it is still used to
analyze the AVC messages. Two new programs act as a method to start
setroubleshoot when needed: sedispatch and seapplet. sedispatch runs as part of the audit subsystem, and via dbus, sends a message when an AVC denial occurs, which will go straight to setroubleshootd if it is already running, or it will start setroubleshootd if it is not running. seapplet is a tool which runs in the system's toolbar, waiting for dbus messages in setroubleshootd, and will launch the notification bubble, allowing the user to review the denial.
To configure the auditd and rsyslogd daemons to automatically start at boot, run the following commands as the Linux root user:
/sbin/chkconfig --levels 2345 auditd on
/sbin/chkconfig --levels 2345 rsyslog on
Use the service service-name status command to check if these services are running, for example:
$ /sbin/service auditd status
auditd (pid 1318) is running...
If the above services are not running (service-name is stoppedservice service-name start command as the Linux root user to start them. For example:
# /sbin/service auditd start
Starting auditd: [ OK ]
5.3. Main Configuration File
The /etc/selinux/config file is the main SELinux configuration file. It controls the SELinux mode and the SELinux policy to use:
# This file controls the state of SELinux on the system.
# SELINUX= can take one of these three values:
# enforcing - SELinux security policy is enforced.
# permissive - SELinux prints warnings instead of enforcing.
# disabled - No SELinux policy is loaded.
SELINUX=enforcing
# SELINUXTYPE= can take one of these two values:
# targeted - Targeted processes are protected,
# mls - Multi Level Security protection.
SELINUXTYPE=targeted
SELINUX=enforcing
The SELINUX option sets the mode
SELinux runs in. SELinux has three modes: enforcing, permissive, and
disabled. When using enforcing mode, SELinux policy is enforced, and
SELinux denies access based on SELinux policy rules. Denial messages are
logged. When using permissive mode, SELinux policy is not enforced.
SELinux does not deny access, but denials are logged for actions that
would have been denied if running SELinux in enforcing mode. When using
disabled mode, SELinux is disabled (the SELinux module is not registered
with the Linux kernel), and only DAC rules are used.
SELINUXTYPE=targeted
The SELINUXTYPE option sets the
SELinux policy to use. Targeted policy is the default policy. Only
change this option if you want to use the MLS policy. To use the MLS
policy, install the selinux-policy-mls package; configure SELINUXTYPE=mls in /etc/selinux/config; and reboot your system.
When systems run with SELinux in permissive or disabled mode, users
have permission to label files incorrectly. Also, files created while
SELinux is disabled are not labeled. This causes problems when changing
to enforcing mode. To prevent incorrectly labeled and unlabeled files
from causing problems, file systems are automatically relabeled when
changing from disabled mode to permissive or enforcing mode.
5.4. Enabling and Disabling SELinux
Use the /usr/sbin/getenforce or /usr/sbin/sestatus commands to check the status of SELinux. The getenforce command returns Enforcing, Permissive, or Disabled. The getenforce command returns Enforcing when SELinux is enabled (SELinux policy rules are enforced):
$ /usr/sbin/getenforce
Enforcing
The getenforce command returns Permissive when SELinux is enabled, but SELinux policy rules are not enforced, and only DAC rules are used. The getenforce command returns Disabled if SELinux is disabled.
The sestatus command returns the SELinux status and the SELinux policy being used:
$ /usr/sbin/sestatus
SELinux status: enabled
SELinuxfs mount: /selinux
Current mode: enforcing
Mode from config file: enforcing
Policy version: 23
Policy from config file: targeted
SELinux status: enabled is returned when SELinux is enabled. Current mode: enforcing is returned when SELinux is running in enforcing mode. Policy from config file: targeted is returned when the SELinux targeted policy is used.
On systems with SELinux disabled, the SELINUX=disabled option is configured in /etc/selinux/config:
# This file controls the state of SELinux on the system.
# SELINUX= can take one of these three values:
# enforcing - SELinux security policy is enforced.
# permissive - SELinux prints warnings instead of enforcing.
# disabled - No SELinux policy is loaded.
SELINUX=disabled
# SELINUXTYPE= can take one of these two values:
# targeted - Targeted processes are protected,
# mls - Multi Level Security protection.
SELINUXTYPE=targeted
Also, the getenforce command returns Disabled:
$ /usr/sbin/getenforce
Disabled
To enable SELinux:
Use the rpm -qa | grep selinux, rpm -q policycoreutils, and rpm -qa | grep setroubleshoot commands to confirm that the SELinux packages are installed. This guide assumes the following packages are installed: selinux-policy-targeted, selinux-policy, libselinux, libselinux-python, libselinux-utils, policycoreutils, policycoreutils-python, setroubleshoot, setroubleshoot-server, setroubleshoot-plugins. If these packages are not installed, as the Linux root user, install them via the yum install package-name command. The following packages are optional: policycoreutils-gui, setroubleshoot, selinux-policy-devel, and mcstrans.
Before SELinux is enabled, each file on the file system must be
labeled with an SELinux context. Before this happens, confined domains
may be denied access, preventing your system from booting correctly. To
prevent this, configure SELINUX=permissive in /etc/selinux/config:
# This file controls the state of SELinux on the system.
# SELINUX= can take one of these three values:
# enforcing - SELinux security policy is enforced.
# permissive - SELinux prints warnings instead of enforcing.
# disabled - No SELinux policy is loaded.
SELINUX=permissive
# SELINUXTYPE= can take one of these two values:
# targeted - Targeted processes are protected,
# mls - Multi Level Security protection.
SELINUXTYPE=targeted
As the Linux root user, run the reboot
command to restart the system. During the next boot, file systems are
labeled. The label process labels all files with an SELinux context:
*** Warning -- SELinux targeted policy relabel is required.
*** Relabeling could take a very long time, depending on file
*** system size and speed of hard drives.
****
Each * character on the bottom line represents 1000 files that have been labeled. In the above example, four *
characters represent 4000 files have been labeled. The time it takes to
label all files depends upon the number of files on the system, and the
speed of the hard disk drives. On modern systems, this process can take
as little as 10 minutes.
In permissive mode, SELinux policy is not enforced, but denials
are still logged for actions that would have been denied if running in
enforcing mode. Before changing to enforcing mode, as the Linux root
user, run the
grep "SELinux is preventing" /var/log/messages
command as the Linux root user to confirm that SELinux did not deny
actions during the last boot. If SELinux did not deny actions during the
last boot, this command does not return any output. Refer to
Chapter 8, Troubleshooting for troubleshooting information if SELinux denied access during boot.
If there were no denial messages in /var/log/messages, configure SELINUX=enforcing in /etc/selinux/config:
# This file controls the state of SELinux on the system.
# SELINUX= can take one of these three values:
# enforcing - SELinux security policy is enforced.
# permissive - SELinux prints warnings instead of enforcing.
# disabled - No SELinux policy is loaded.
SELINUX=enforcing
# SELINUXTYPE= can take one of these two values:
# targeted - Targeted processes are protected,
# mls - Multi Level Security protection.
SELINUXTYPE=targeted
Reboot your system. After reboot, confirm that the getenforce command returns Enforcing:
$ /usr/sbin/getenforce
Enforcing
As the Linux root user, run the /usr/sbin/semanage login -l command to view the mapping between SELinux and Linux users. The output should be as follows:
Login Name SELinux User MLS/MCS Range
__default__ unconfined_u s0-s0:c0.c1023
root unconfined_u s0-s0:c0.c1023
system_u system_u s0-s0:c0.c1023
If this is not the case, run the following commands as the Linux
root user to fix the user mappings. It is safe to ignore the SELinux-user username is already defined warnings if they occur, where username can be unconfined_u, guest_u, or xguest_u:
/usr/sbin/semanage user -a -S targeted -P user -R "unconfined_r system_r" -r s0-s0:c0.c1023 unconfined_u
/usr/sbin/semanage login -m -S targeted -s "unconfined_u" -r s0-s0:c0.c1023 __default__
/usr/sbin/semanage login -m -S targeted -s "unconfined_u" -r s0-s0:c0.c1023 root
/usr/sbin/semanage user -a -S targeted -P user -R guest_r guest_u
/usr/sbin/semanage user -a -S targeted -P user -R xguest_r xguest_u
When systems run with SELinux in permissive or disabled mode, users
have permission to label files incorrectly. Also, files created while
SELinux is disabled are not labeled. This causes problems when changing
to enforcing mode. To prevent incorrectly labeled and unlabeled files
from causing problems, file systems are automatically relabeled when
changing from disabled mode to permissive or enforcing mode.
To disable SELinux, configure SELINUX=disabled in /etc/selinux/config:
# This file controls the state of SELinux on the system.
# SELINUX= can take one of these three values:
# enforcing - SELinux security policy is enforced.
# permissive - SELinux prints warnings instead of enforcing.
# disabled - No SELinux policy is loaded.
SELINUX=disabled
# SELINUXTYPE= can take one of these two values:
# targeted - Targeted processes are protected,
# mls - Multi Level Security protection.
SELINUXTYPE=targeted
Reboot your system. After reboot, confirm that the getenforce command returns Disabled:
$ /usr/sbin/getenforce
Disabled
SELinux has three modes:
Enforcing: SELinux policy is enforced. SELinux denies access based on SELinux policy rules.
Permissive: SELinux policy is not enforced. SELinux does not deny
access, but denials are logged for actions that would have been denied
if running in enforcing mode.
Disabled: SELinux is disabled. Only DAC rules are used.
Use the /usr/sbin/setenforce command to change between enforcing and permissive mode. Changes made with /usr/sbin/setenforce do not persist across reboots. To change to enforcing mode, as the Linux root user, run the /usr/sbin/setenforce 1 command. To change to permissive mode, run the /usr/sbin/setenforce 0 command. Use the /usr/sbin/getenforce command to view the current SELinux mode.
Booleans allow parts of SELinux policy to be changed at runtime,
without any knowledge of SELinux policy writing. This allows changes,
such as allowing services access to NFS file systems, without reloading
or recompiling SELinux policy.
For a list of Booleans, an explanation of what each one is, and whether they are on or off, run the semanage boolean -l command as the Linux root user. The following example does not list all Booleans:
# /usr/sbin/semanage boolean -l
SELinux boolean Description
ftp_home_dir -> off Allow ftp to read and write files in the user home directories
xen_use_nfs -> off Allow xen to manage nfs files
xguest_connect_network -> on Allow xguest to configure Network Manager
The SELinux boolean column lists Boolean names. The Description column lists whether the Booleans are on or off, and what they do.
In the following example, the ftp_home_dir Boolean is off, preventing the FTP daemon (vsftpd) from reading and writing to files in user home directories:
ftp_home_dir -> off Allow ftp to read and write files in the user home directories
The getsebool -a command lists
Booleans, whether they are on or off, but does not give a description of
each one. The following example does not list all Booleans:
$ /usr/sbin/getsebool -a
allow_console_login --> off
allow_cvs_read_shadow --> off
allow_daemons_dump_core --> on
Run the getsebool boolean-name command to only list the status of the boolean-name Boolean:
$ /usr/sbin/getsebool allow_console_login
allow_console_login --> off
Use a space-separated list to list multiple Booleans:
$ /usr/sbin/getsebool allow_console_login allow_cvs_read_shadow allow_daemons_dump_core
allow_console_login --> off
allow_cvs_read_shadow --> off
allow_daemons_dump_core --> on
5.6.2. Configuring Booleans
The setsebool boolean-name x command turns Booleans on or off, where boolean-name is a Boolean name, and x is either on to turn the Boolean on, or off to turn it off.
The following example demonstrates configuring the httpd_can_network_connect_db Boolean:
By default, the httpd_can_network_connect_db Boolean is off, preventing Apache HTTP Server scripts and modules from connecting to database servers:
$ /usr/sbin/getsebool httpd_can_network_connect_db
httpd_can_network_connect_db --> off
To temporarily enable Apache HTTP Server scripts and modules to connect to database servers, run the setsebool httpd_can_network_connect_db on command as the Linux root user.
Use the getsebool httpd_can_network_connect_db command to verify the Boolean is turned on:
$ /usr/sbin/getsebool httpd_can_network_connect_db
httpd_can_network_connect_db --> on
This allows Apache HTTP Server scripts and modules to connect to database servers.
This change is not persistent across reboots. To make changes persistent across reboots, run the setsebool -P boolean-name on command as the Linux root user:
# /usr/sbin/setsebool -P httpd_can_network_connect_db on
To temporarily revert to the default behavior, as the Linux root user, run the setsebool httpd_can_network_connect_db off command. For changes that persist across reboots, run the setsebool -P httpd_can_network_connect_db off command.
5.6.3. Booleans for NFS and CIFS
By default, NFS mounts on the client side are labeled with a default
context defined by policy for NFS file systems. In common policies,
this default context uses the nfs_t
type. Also, by default, Samba shares mounted on the client side are
labeled with a default context defined by policy. In common policies,
this default context uses the cifs_t type.
Depending on policy configuration, services may not be able to read files labeled with the nfs_t or cifs_t
types. This may prevent file systems labeled with these types from
being mounted and then read or exported by other services. Booleans can
be turned on or off to control which services are allowed to access the nfs_t and cifs_t types.
The setsebool and semanage commands must be run as the Linux root user. The setsebool -P command makes persistent changes. Do not use the -P option if you do not want changes to persist across reboots:
To allow access to NFS file systems (files labeled with the nfs_t type):
/usr/sbin/setsebool -P httpd_use_nfs on
To allow access to Samba file systems (files labeled with the cifs_t type):
/usr/sbin/setsebool -P httpd_use_cifs on
To export NFS file systems:
/usr/sbin/setsebool -P samba_share_nfs on
To allow access to NFS file systems:
/usr/sbin/setsebool -P allow_ftpd_use_nfs on
To allow access to Samba file systems:
/usr/sbin/setsebool -P allow_ftpd_use_cifs on
For a list of NFS related Booleans for other services:
/usr/sbin/semanage boolean -l | grep nfs
For a list of Samba related Booleans for other services:
/usr/sbin/semanage boolean -l | grep cifs
These Booleans exist in SELinux policy as shipped with Red Hat
Enterprise Linux 6. They may not exist in policy shipped with other
versions of Red Hat Enterprise Linux or other operating systems.
5.7. SELinux Contexts - Labeling Files
On systems running SELinux, all processes and files are labeled in a
way that represents security-relevant information. This information is
called the SELinux context. For files, this is viewed using the ls -Z command:
$ ls -Z file1
-rw-rw-r-- user1 group1 unconfined_u:object_r:user_home_t:s0 file1
In this example, SELinux provides a user (unconfined_u), a role (object_r), a type (user_home_t), and a level (s0).
This information is used to make access control decisions. On DAC
systems, access is controlled based on Linux user and group IDs. SELinux
policy rules are checked after DAC rules. SELinux policy rules are not
used if DAC rules deny access first.
There are multiple commands for managing the SELinux context for files, such as chcon, semanage fcontext, and restorecon.
5.7.1. Temporary Changes: chcon
The chcon command changes the SELinux context for files. However, changes made with the chcon command do not survive a file system relabel, or the execution of the /sbin/restorecon command. SELinux policy controls whether users are able to modify the SELinux context for any given file. When using chcon,
users provide all or part of the SELinux context to change. An
incorrect file type is a common cause of SELinux denying access.
The following example demonstrates changing the type, and no other attributes of the SELinux context:
Run the cd command without arguments to change into your home directory.
Run the touch file1 command to create a new file. Use the ls -Z file1 command to view the SELinux context for file1:
$ ls -Z file1
-rw-rw-r-- user1 group1 unconfined_u:object_r:user_home_t:s0 file1
In this example, the SELinux context for
file1 includes the SELinux
unconfined_u user,
object_r role,
user_home_t type, and the
s0 level. For a description of each part of the SELinux context, refer to
Chapter 3, SELinux Contexts.
Run the chcon -t samba_share_t file1 command to change the type to samba_share_t. The -t option only changes the type. View the change with ls -Z file1:
$ ls -Z file1
-rw-rw-r-- user1 group1 unconfined_u:object_r:samba_share_t:s0 file1
Use the /sbin/restorecon -v file1 command to restore the SELinux context for the file1 file. Use the -v option to view what changes:
$ /sbin/restorecon -v file1
restorecon reset file1 context unconfined_u:object_r:samba_share_t:s0->system_u:object_r:user_home_t:s0
In this example, the previous type, samba_share_t, is restored to the correct, user_home_t type. When using targeted policy (the default SELinux policy in Red Hat Enterprise Linux 6), the /sbin/restorecon command reads the files in the /etc/selinux/targeted/contexts/files/ directory, to see which SELinux context files should have.
The example in this section works the same for directories, for example, if file1 was a directory.
The following example demonstrates creating a new directory, and
changing the directory's file type (along with its contents) to a type
used by the Apache HTTP Server. The configuration in this example is
used if you want Apache HTTP Server to use a different document root
(instead of /var/www/html/):
As the Linux root user, run the mkdir /web command to create a new directory, and then the touch /web/file{1,2,3} command to create 3 empty files (file1, file2, and file3). The /web/ directory and files in it are labeled with the default_t type:
# ls -dZ /web
drwxr-xr-x root root unconfined_u:object_r:default_t:s0 /web
# ls -lZ /web
-rw-r--r-- root root unconfined_u:object_r:default_t:s0 file1
-rw-r--r-- root root unconfined_u:object_r:default_t:s0 file2
-rw-r--r-- root root unconfined_u:object_r:default_t:s0 file3
As the Linux root user, run the chcon -R -t httpd_sys_content_t /web/ command to change the type of the /web/ directory (and its contents) to httpd_sys_content_t:
# chcon -R -t httpd_sys_content_t /web/
# ls -dZ /web/
drwxr-xr-x root root unconfined_u:object_r:httpd_sys_content_t:s0 /web/
# ls -lZ /web/
-rw-r--r-- root root unconfined_u:object_r:httpd_sys_content_t:s0 file1
-rw-r--r-- root root unconfined_u:object_r:httpd_sys_content_t:s0 file2
-rw-r--r-- root root unconfined_u:object_r:httpd_sys_content_t:s0 file3
As the Linux root user, run the /sbin/restorecon -R -v /web/ command to restore the default SELinux contexts:
# /sbin/restorecon -R -v /web/
restorecon reset /web context unconfined_u:object_r:httpd_sys_content_t:s0->system_u:object_r:default_t:s0
restorecon reset /web/file2 context unconfined_u:object_r:httpd_sys_content_t:s0->system_u:object_r:default_t:s0
restorecon reset /web/file3 context unconfined_u:object_r:httpd_sys_content_t:s0->system_u:object_r:default_t:s0
restorecon reset /web/file1 context unconfined_u:object_r:httpd_sys_content_t:s0->system_u:object_r:default_t:s0
Refer to the chcon(1) manual page for further information about chcon.
Type Enforcement is the main permission control used in SELinux
targeted policy. For the most part, SELinux users and roles can be
ignored.
5.7.2. Persistent Changes: semanage fcontext
The /usr/sbin/semanage fcontext command changes the SELinux context for files. When using targeted policy, changes made with this command are added to the /etc/selinux/targeted/contexts/files/file_contexts file if the changes are to files that exists in file_contexts, or are added to file_contexts.local for new files and directories, such as creating a /web/ directory. setfiles, which is used when a file system is relabeled, and /sbin/restorecon, which restores the default SELinux contexts, read these files. This means that changes made by /usr/sbin/semanage fcontext
are persistent, even if the file system is relabeled. SELinux policy
controls whether users are able to modify the SELinux context for any
given file.
To make SELinux context changes that survive a file system relabel:
Run the /usr/sbin/semanage fcontext -a options file-name|directory-name command, remembering to use the full path to the file or directory.
Run the /sbin/restorecon -v file-name|directory-name command to apply the context changes.
The following example demonstrates changing a file's type, and no other attributes of the SELinux context:
As the Linux root user, run the touch /etc/file1 command to create a new file. By default, newly-created files in the /etc/ directory are labeled with the etc_t type:
# ls -Z /etc/file1
-rw-r--r-- root root unconfined_u:object_r:etc_t:s0 /etc/file1
As the Linux root user, run the /usr/sbin/semanage fcontext -a -t samba_share_t /etc/file1 command to change the file1 type to samba_share_t. The -a option adds a new record, and the -t option defines a type (samba_share_t). Note: running this command does not directly change the type - file1 is still labeled with the etc_t type:
# /usr/sbin/semanage fcontext -a -t samba_share_t /etc/file1
# ls -Z /etc/file1
-rw-r--r-- root root unconfined_u:object_r:etc_t:s0 /etc/file1
The /usr/sbin/semanage fcontext -a -t samba_share_t /etc/file1 command adds the following entry to /etc/selinux/targeted/contexts/files/file_contexts.local:
/etc/file1 unconfined_u:object_r:samba_share_t:s0
As the Linux root user, run the /sbin/restorecon -v /etc/file1 command to change the type. Since the semanage command added an entry to file.contexts.local for /etc/file1, the /sbin/restorecon command changes the type to samba_share_t:
# /sbin/restorecon -v /etc/file1
restorecon reset /etc/file1 context unconfined_u:object_r:etc_t:s0->system_u:object_r:samba_share_t:s0
As the Linux root user, run the rm -i /etc/file1 command to remove file1.
As the Linux root user, run the /usr/sbin/semanage fcontext -d /etc/file1 command to remove the context added for /etc/file1. When the context is removed, running restorecon changes the type to etc_t, rather than samba_share_t.
The following example demonstrates creating a new directory and
changing that directory's file type, to a type used by Apache HTTP
Server:
As the Linux root user, run the mkdir /web command to create a new directory. This directory is labeled with the default_t type:
# ls -dZ /web
drwxr-xr-x root root unconfined_u:object_r:default_t:s0 /web
The ls -d option makes ls list information about a directory, rather than its contents, and the -Z option makes ls display the SELinux context (in this example, unconfined_u:object_r:default_t:s0).
As the Linux root user, run the /usr/sbin/semanage fcontext -a -t httpd_sys_content_t /web command to change the /web/ type to httpd_sys_content_t. The -a option adds a new record, and the -t option defines a type (httpd_sys_content_t). Note: running this command does not directly change the type - /web/ is still labeled with the default_t type:
# /usr/sbin/semanage fcontext -a -t httpd_sys_content_t /web
# ls -dZ /web
drwxr-xr-x root root unconfined_u:object_r:default_t:s0 /web
The /usr/sbin/semanage fcontext -a -t httpd_sys_content_t /web command adds the following entry to /etc/selinux/targeted/contexts/files/file_contexts.local:
/web unconfined_u:object_r:httpd_sys_content_t:s0
As the Linux root user, run the /sbin/restorecon -v /web command to change the type. Since the semanage command added an entry to file.contexts.local for /web, the /sbin/restorecon command changes the type to httpd_sys_content_t:
# /sbin/restorecon -v /web
restorecon reset /web context unconfined_u:object_r:default_t:s0->system_u:object_r:httpd_sys_content_t:s0
By default, newly-created files and directories inherit the
SELinux type of their parent folders. When using this example, and
before removing the SELinux context added for /web/, files and directories created in the /web/ directory are labeled with the httpd_sys_content_t type.
As the Linux root user, run the /usr/sbin/semanage fcontext -d /web command to remove the context added for /web/.
As the Linux root user, run the /sbin/restorecon -v /web command to restore the default SELinux context.
The following example demonstrates creating a new directory, and
changing the directory's file type (along with its contents) to a type
used by Apache HTTP Server. The configuration in this example is used if
you want Apache HTTP Server to use a different document root (instead
of /var/www/html/):
As the Linux root user, run the mkdir /web command to create a new directory, and then the touch /web/file{1,2,3} command to create 3 empty files (file1, file2, and file3). The /web/ directory and files in it are labeled with the default_t type:
# ls -dZ /web
drwxr-xr-x root root unconfined_u:object_r:default_t:s0 /web
# ls -lZ /web
-rw-r--r-- root root unconfined_u:object_r:default_t:s0 file1
-rw-r--r-- root root unconfined_u:object_r:default_t:s0 file2
-rw-r--r-- root root unconfined_u:object_r:default_t:s0 file3
As the Linux root user, run the /usr/sbin/semanage fcontext -a -t httpd_sys_content_t "/web(/.*)?" command to change the type of the /web/ directory and the files in it, to httpd_sys_content_t. The -a option adds a new record, and the -t option defines a type (httpd_sys_content_t). The "/web(/.*)?" regular expression causes the semanage command to apply changes to the /web/ directory, as well as the files in it. Note: running this command does not directly change the type - /web/ and files in it are still labeled with the default_t type:
# ls -dZ /web
drwxr-xr-x root root unconfined_u:object_r:default_t:s0 /web
# ls -lZ /web
-rw-r--r-- root root unconfined_u:object_r:default_t:s0 file1
-rw-r--r-- root root unconfined_u:object_r:default_t:s0 file2
-rw-r--r-- root root unconfined_u:object_r:default_t:s0 file3
The /usr/sbin/semanage fcontext -a -t httpd_sys_content_t "/web(/.*)?" command adds the following entry to /etc/selinux/targeted/contexts/files/file_contexts.local:
/web(/.*)? system_u:object_r:httpd_sys_content_t:s0
As the Linux root user, run the /sbin/restorecon -R -v /web command to change the type of the /web/ directory, as well as all files in it. The -R is for recursive, which means all files and directories under the /web/ directory are labeled with the httpd_sys_content_t type. Since the semanage command added an entry to file.contexts.local for /web(/.*)?, the /sbin/restorecon command changes the types to httpd_sys_content_t:
# /sbin/restorecon -R -v /web
restorecon reset /web context unconfined_u:object_r:default_t:s0->system_u:object_r:httpd_sys_content_t:s0
restorecon reset /web/file2 context unconfined_u:object_r:default_t:s0->system_u:object_r:httpd_sys_content_t:s0
restorecon reset /web/file3 context unconfined_u:object_r:default_t:s0->system_u:object_r:httpd_sys_content_t:s0
restorecon reset /web/file1 context unconfined_u:object_r:default_t:s0->system_u:object_r:httpd_sys_content_t:s0
By default, newly-created files and directories inherit the
SELinux type of their parents. In this example, files and directories
created in the /web/ directory will be labeled with the httpd_sys_content_t type.
As the Linux root user, run the /usr/sbin/semanage fcontext -d "/web(/.*)?" command to remove the context added for "/web(/.*)?".
As the Linux root user, run the /sbin/restorecon -R -v /web command to restore the default SELinux contexts.
The following example demonstrates adding and removing an SELinux context:
As the Linux root user, run the /usr/sbin/semanage fcontext -a -t httpd_sys_content_t /test command. The /test/ directory does not have to exist. This command adds the following context to /etc/selinux/targeted/contexts/files/file_contexts.local:
/test system_u:object_r:httpd_sys_content_t:s0
To remove the context, as the Linux root user, run the /usr/sbin/semanage fcontext -d file-name|directory-name command, where file-name|directory-name is the first part in file_contexts.local. The following is an example of a context in file_contexts.local:
/test system_u:object_r:httpd_sys_content_t:s0
With the first part being /test. To prevent the /test/ directory from being labeled with the httpd_sys_content_t after running /sbin/restorecon, or after a file system relabel, run the following command as the Linux root user to delete the context from file_contexts.local:
/usr/sbin/semanage fcontext -d /test
If the context is part of a regular expression, for example, /web(/.*)?, use quotation marks around the regular expression:
/usr/sbin/semanage fcontext -d "/web(/.*)?"
Refer to the semanage(8) manual page for further information about /usr/sbin/semanage.
When changing the SELinux context with /usr/sbin/semanage fcontext -a, use the full path to the file or directory to avoid files being mislabeled after a file system relabel, or after the /sbin/restorecon command is run.
5.8. The file_t and default_t Types
For file systems that support extended attributes, when a file that
lacks an SELinux context on disk is accessed, it is treated as if it had
a default context as defined by SELinux policy. In common policies,
this default context uses the file_t
type. This should be the only use of this type, so that files without a
context on disk can be distinguished in policy, and generally kept
inaccessible to confined domains. The file_t
type should not exist on correctly-labeled file systems, because all
files on a system running SELinux should have an SELinux context, and
the file_t type is never used in file-context configuration[].
The
default_t type is used on
files that do not match any other pattern in file-context configuration,
so that such files can be distinguished from files that do not have a
context on disk, and generally kept inaccessible to confined domains. If
you create a new top-level directory, such as
/mydirectory/, this directory may be labeled with the
default_t type. If services need access to such a directory, update the file-contexts configuration for this location. Refer to
Section 5.7.2, “Persistent Changes: semanage fcontext” for details on adding a context to the file-context configuration.
5.9. Mounting File Systems
By default, when a file system that supports extended attributes is
mounted, the security context for each file is obtained from the security.selinux
extended attribute of the file. Files in file systems that do not
support extended attributes are assigned a single, default security
context from the policy configuration, based on file system type.
Use the mount -o context command to
override existing extended attributes, or to specify a different,
default context for file systems that do not support extended
attributes. This is useful if you do not trust a file system to supply
the correct attributes, for example, removable media used in multiple
systems. The mount -o context command can
also be used to support labeling for file systems that do not support
extended attributes, such as File Allocation Table (FAT) or NFS file
systems. The context specified with the context is not written to disk: the original contexts are preserved, and are seen when mounting without a context option (if the file system had extended attributes in the first place).
To mount a file system with the specified context, overriding
existing contexts if they exist, or to specify a different, default
context for a file system that does not support extended attributes, as
the Linux root user, use the mount -o context=SELinux_user:role:type:level
command when mounting the desired file system. Context changes are not
written to disk. By default, NFS mounts on the client side are labeled
with a default context defined by policy for NFS file systems. In common
policies, this default context uses the nfs_t
type. Without additional mount options, this may prevent sharing NFS
file systems via other services, such as the Apache HTTP Server. The
following example mounts an NFS file system so that it can be shared via
the Apache HTTP Server:
# mount server:/export /local/mount/point -o\
context="system_u:object_r:httpd_sys_content_t:s0"
Newly-created files and directories on this file system appear to have the SELinux context specified with -o context; however, since context changes are not written to disk for these situations, the context specified with the context option is only retained if the context option is used on the next mount, and if the same context is specified.
Type Enforcement is the main permission control used in SELinux
targeted policy. For the most part, SELinux users and roles can be
ignored, so, when overriding the SELinux context with -o context, use the SELinux system_u user and object_r role, and concentrate on the type. If you are not using the MLS policy or multi-category security, use the s0 level.
When a file system is mounted with a context option, context changes (by users and processes) are prohibited. For example, running chcon on a file system mounted with a context option results in a Operation not supported error.
5.9.2. Changing the Default Context
As mentioned in
Section 5.8, “The file_t and default_t Types”,
on file systems that support extended attributes, when a file that
lacks an SELinux context on disk is accessed, it is treated as if it had
a default context as defined by SELinux policy. In common policies,
this default context uses the
file_t type. If it is desirable to use a different default context, mount the file system with the
defcontext option.
The following example mounts a newly-created file system (on /dev/sda2) to the newly-created /test/ directory. It assumes that there are no rules in /etc/selinux/targeted/contexts/files/ that define a context for the /test/ directory:
# mount /dev/sda2 /test/ -o defcontext="system_u:object_r:samba_share_t:s0"
In this example:
the defcontext option defines that system_u:object_r:samba_share_t:s0 is "the default security context for unlabeled files"[].
when mounted, the root directory (/test/) of the file system is treated as if it is labeled with the context specified by defcontext (this label is not stored on disk). This affects the labeling for files created under /test/: new files inherit the samba_share_t type, and these labels are stored on disk.
files created under /test/ while the file system was mounted with a defcontext option retain their labels.
5.9.3. Mounting an NFS File System
By default, NFS mounts on the client side are labeled with a default
context defined by policy for NFS file systems. In common policies,
this default context uses the nfs_t
type. Depending on policy configuration, services, such as Apache HTTP
Server and MySQL, may not be able to read files labeled with the nfs_t type. This may prevent file systems labeled with this type from being mounted and then read or exported by other services.
If you would like to mount an NFS file system and read or export that file system with another service, use the context option when mounting to override the nfs_t type. Use the following context option to mount NFS file systems so that they can be shared via the Apache HTTP Server:
mount server:/export /local/mount/point -o\
context="system_u:object_r:httpd_sys_content_t:s0"
Since context changes are not written to disk for these situations, the context specified with the context option is only retained if the context option is used on the next mount, and if the same context is specified.
As an alternative to mounting file systems with
context options, Booleans can be turned on to allow services access to file systems labeled with the
nfs_t type. Refer to
Section 5.6.3, “Booleans for NFS and CIFS” for instructions on configuring Booleans to allow services access to the
nfs_t type.
5.9.4. Multiple NFS Mounts
When mounting multiple mounts from the same NFS export, attempting
to override the SELinux context of each mount with a different context,
results in subsequent mount commands failing. In the following example,
the NFS server has a single export, /export, which has two subdirectories, web/ and database/. The following commands attempt two mounts from a single NFS export, and try to override the context for each one:
# mount server:/export/web /local/web -o\
context="system_u:object_r:httpd_sys_content_t:s0"
# mount server:/export/database /local/database -o\
context="system_u:object_r:mysqld_db_t:s0"
The second mount command fails, and the following is logged to /var/log/messages:
kernel: SELinux: mount invalid. Same superblock, different security settings for (dev 0:15, type nfs)
To mount multiple mounts from a single NFS export, with each mount having a different context, use the -o nosharecache,context
options. The following example mounts multiple mounts from a single NFS
export, with a different context for each mount (allowing a single
service access to each one):
# mount server:/export/web /local/web -o\
nosharecache,context="system_u:object_r:httpd_sys_content_t:s0"
# mount server:/export/database /local/database -o\
nosharecache,context="system_u:object_r:mysqld_db_t:s0"
In this example, server:/export/web is mounted locally to /local/web/, with all files being labeled with the httpd_sys_content_t type, allowing Apache HTTP Server access. server:/export/database is mounted locally to /local/database, with all files being labeled with the mysqld_db_t type, allowing MySQL access. These type changes are not written to disk.
The nosharecache options allows you to mount the same subdirectory of an export multiple times with different contexts (for example, mounting /export/web
multiple times). Do not mount the same subdirectory from an export
multiple times with different contexts, as this creates an overlapping
mount, where files are accessible under two different contexts.
5.9.5. Making Context Mounts Persistent
To make context mounts persistent across remounting and reboots, add entries for the file systems in /etc/fstab or an automounter map, and use the desired context as a mount option. The following example adds an entry to /etc/fstab for an NFS context mount:
server:/export /local/mount/ nfs context="system_u:object_r:httpd_sys_content_t:s0" 0 0
5.10. Maintaining SELinux Labels
These sections describe what happens to SELinux contexts when
copying, moving, and archiving files and directories. Also, it explains
how to preserve contexts when copying and archiving.
5.10.1. Copying Files and Directories
When a file or directory is copied, a new file or directory is
created if it does not exist. That new file or directory's context is
based on default-labeling rules, not the original file or directory's
context (unless options were used to preserve the original context). For
example, files created in user home directories are labeled with the user_home_t type:
$ touch file1
$ ls -Z file1
-rw-rw-r-- user1 group1 unconfined_u:object_r:user_home_t:s0 file1
If such a file is copied to another directory, such as /etc/, the new file is created in accordance to default-labeling rules for the /etc/ directory. Copying a file (without additional options) may not preserve the original context:
$ ls -Z file1
-rw-rw-r-- user1 group1 unconfined_u:object_r:user_home_t:s0 file1
# cp file1 /etc/
$ ls -Z /etc/file1
-rw-r--r-- root root unconfined_u:object_r:etc_t:s0 /etc/file1
When file1 is copied to /etc/, if /etc/file1 does not exist, /etc/file1 is created as a new file. As shown in the example above, /etc/file1 is labeled with the etc_t type, in accordance to default-labeling rules.
When a file is copied over an existing file, the existing file's context is preserved, unless the user specified cp options to preserve the context of the original file, such as --preserve=context. SELinux policy may prevent contexts from being preserved during copies.
When copying a file with the cp command, if no options are given, the type is inherited from the targeted, parent directory:
$ touch file1
$ ls -Z file1
-rw-rw-r-- user1 group1 unconfined_u:object_r:user_home_t:s0 file1
$ ls -dZ /var/www/html/
drwxr-xr-x root root system_u:object_r:httpd_sys_content_t:s0 /var/www/html/
# cp file1 /var/www/html/
$ ls -Z /var/www/html/file1
-rw-r--r-- root root unconfined_u:object_r:httpd_sys_content_t:s0 /var/www/html/file1
In this example, file1 is created in a user's home directory, and is labeled with the user_home_t type. The /var/www/html/ directory is labeled with the httpd_sys_content_t type, as shown with the ls -dZ /var/www/html/ command. When file1 is copied to /var/www/html/, it inherits the httpd_sys_content_t type, as shown with the ls -Z /var/www/html/file1 command.
Use the cp --preserve=context command to preserve contexts when copying:
$ touch file1
$ ls -Z file1
-rw-rw-r-- user1 group1 unconfined_u:object_r:user_home_t:s0 file1
$ ls -dZ /var/www/html/
drwxr-xr-x root root system_u:object_r:httpd_sys_content_t:s0 /var/www/html/
# cp --preserve=context file1 /var/www/html/
$ ls -Z /var/www/html/file1
-rw-r--r-- root root unconfined_u:object_r:user_home_t:s0 /var/www/html/file1
In this example, file1 is created in a user's home directory, and is labeled with the user_home_t type. The /var/www/html/ directory is labeled with the httpd_sys_content_t type, as shown with the ls -dZ /var/www/html/ command. Using the --preserve=context option preserves SELinux contexts during copy operations. As shown with the ls -Z /var/www/html/file1 command, the file1 user_home_t type was preserved when the file was copied to /var/www/html/.
Use the cp -Z command to change the destination copy's context. The following example was performed in the user's home directory:
$ touch file1
$ cp -Z system_u:object_r:samba_share_t:s0 file1 file2
$ ls -Z file1 file2
-rw-rw-r-- user1 group1 unconfined_u:object_r:user_home_t:s0 file1
-rw-rw-r-- user1 group1 system_u:object_r:samba_share_t:s0 file2
$ rm file1 file2
In this example, the context is defined with the -Z option. Without the -Z option, file2 would be labeled with the unconfined_u:object_r:user_home_t context.
When a file is copied over an existing file, the existing file's
context is preserved (unless an option is used to preserve contexts).
For example:
# touch /etc/file1
# ls -Z /etc/file1
-rw-r--r-- root root unconfined_u:object_r:etc_t:s0 /etc/file1
# touch /tmp/file2
# ls -Z /tmp/file2
-rw-r--r-- root root unconfined_u:object_r:user_tmp_t:s0 /tmp/file2
# cp /tmp/file2 /etc/file1
# ls -Z /etc/file1
-rw-r--r-- root root unconfined_u:object_r:etc_t:s0 /etc/file1
In this example, two files are created: /etc/file1, labeled with the etc_t type, and /tmp/file2, labeled with the user_tmp_t type. The cp /tmp/file2 /etc/file1 command overwrites file1 with file2. After copying, the ls -Z /etc/file1 command shows file1 labeled with the etc_t type, not the user_tmp_t type from /tmp/file2 that replaced /etc/file1.
Copy files and directories, rather than moving them. This helps
ensure they are labeled with the correct SELinux contexts. Incorrect
SELinux contexts can prevent processes from accessing such files and
directories.
5.10.2. Moving Files and Directories
File and directories keep their current SELinux context when they
are moved. In many cases, this is incorrect for the location they are
being moved to. The following example demonstrates moving a file from a
user's home directory to /var/www/html/, which is used by the Apache HTTP Server. Since the file is moved, it does not inherit the correct SELinux context:
Run the cd command without any arguments to change into your home directory. Once in your home directory, run the touch file1 command to create a file. This file is labeled with the user_home_t type:
$ ls -Z file1
-rw-rw-r-- user1 group1 unconfined_u:object_r:user_home_t:s0 file1
Run the ls -dZ /var/www/html/ command to view the SELinux context of the /var/www/html/ directory:
$ ls -dZ /var/www/html/
drwxr-xr-x root root system_u:object_r:httpd_sys_content_t:s0 /var/www/html/
By default, the /var/www/html/ directory is labeled with the httpd_sys_content_t type. Files and directories created under the /var/www/html/ directory inherit this type, and as such, they are labeled with this type.
As the Linux root user, run the mv file1 /var/www/html/ command to move file1 to the /var/www/html/ directory. Since this file is moved, it keeps its current user_home_t type:
# mv file1 /var/www/html/
# ls -Z /var/www/html/file1
-rw-rw-r-- user1 group1 unconfined_u:object_r:user_home_t:s0 /var/www/html/file1
By default, the Apache HTTP Server can not read files that are labeled with the user_home_t type. If all files comprising a web page are labeled with the user_home_t
type, or another type that the Apache HTTP Server can not read,
permission is denied when attempting to access them via Firefox or
text-based Web browsers.
Moving files and directories with the mv
command may result in the wrong SELinux context, preventing processes,
such as the Apache HTTP Server and Samba, from accessing such files and
directories.
5.10.3. Checking the Default SELinux Context
Use the /usr/sbin/matchpathcon command to check if files and directories have the correct SELinux context. From the matchpathcon(8) manual page: "matchpathcon queries the system policy and outputs the default security context associated with the file path."[]. The following example demonstrates using the /usr/sbin/matchpathcon command to verify that files in /var/www/html/ directory are labeled correctly:
As the Linux root user, run the touch /var/www/html/file{1,2,3} command to create three files (file1, file2, and file3). These files inherit the httpd_sys_content_t type from the /var/www/html/ directory:
# touch /var/www/html/file{1,2,3}
# ls -Z /var/www/html/
-rw-r--r-- root root unconfined_u:object_r:httpd_sys_content_t:s0 file1
-rw-r--r-- root root unconfined_u:object_r:httpd_sys_content_t:s0 file2
-rw-r--r-- root root unconfined_u:object_r:httpd_sys_content_t:s0 file3
As the Linux root user, run the chcon -t samba_share_t /var/www/html/file1 command to change the file1 type to samba_share_t. Note: the Apache HTTP Server can not read files or directories labeled with the samba_share_t type.
The /usr/sbin/matchpathcon -V option compares the current SELinux context to the correct, default context in SELinux policy. Run the /usr/sbin/matchpathcon -V /var/www/html/* command to check all files in the /var/www/html/ directory:
$ /usr/sbin/matchpathcon -V /var/www/html/*
/var/www/html/file1 has context unconfined_u:object_r:samba_share_t:s0, should be system_u:object_r:httpd_sys_content_t:s0
/var/www/html/file2 verified.
/var/www/html/file3 verified.
The following output from the /usr/sbin/matchpathcon command explains that file1 is labeled with the samba_share_t type, but should be labeled with the httpd_sys_content_t type:
/var/www/html/file1 has context unconfined_u:object_r:samba_share_t:s0, should be system_u:object_r:httpd_sys_content_t:s0
To resolve the label problem and allow the Apache HTTP Server access to file1, as the Linux root user, run the /sbin/restorecon -v /var/www/html/file1 command:
# /sbin/restorecon -v /var/www/html/file1
restorecon reset /var/www/html/file1 context unconfined_u:object_r:samba_share_t:s0->system_u:object_r:httpd_sys_content_t:s0
5.10.4. Archiving Files with tar
tar does not retain extended attributes
by default. Since SELinux contexts are stored in extended attributes,
contexts can be lost when archiving files. Use tar --selinux
to create archives that retain contexts. If a Tar archive contains
files without extended attributes, or if you want the extended
attributes to match the system defaults, run the archive through /sbin/restorecon:
$ tar -xvf archive.tar | /sbin/restorecon -f -
Note: depending on the directory, you may need to be the Linux root user to run the /sbin/restorecon command.
The following example demonstrates creating a Tar archive that retains SELinux contexts:
As the Linux root user, run the touch /var/www/html/file{1,2,3} command to create three files (file1, file2, and file3). These files inherit the httpd_sys_content_t type from the /var/www/html/ directory:
# touch /var/www/html/file{1,2,3}
# ls -Z /var/www/html/
-rw-r--r-- root root unconfined_u:object_r:httpd_sys_content_t:s0 file1
-rw-r--r-- root root unconfined_u:object_r:httpd_sys_content_t:s0 file2
-rw-r--r-- root root unconfined_u:object_r:httpd_sys_content_t:s0 file3
Run the cd /var/www/html/ command to change into the /var/www/html/ directory. Once in this directory, as the Linux root user, run the tar --selinux -cf test.tar file{1,2,3} command to create a Tar archive named test.tar.
As the Linux root user, run the mkdir /test command to create a new directory, and then, run the chmod 777 /test/ command to allow all users full-access to the /test/ directory.
Run the cp /var/www/html/test.tar /test/ command to copy the test.tar file in to the /test/ directory.
Run the cd /test/ command to change into the /test/ directory. Once in this directory, run the tar -xvf test.tar command to extract the Tar archive.
Run the ls -lZ /test/ command to view the SELinux contexts. The httpd_sys_content_t type has been retained, rather than being changed to default_t, which would have happened had the --selinux not been used:
$ ls -lZ /test/
-rw-r--r-- user1 group1 unconfined_u:object_r:httpd_sys_content_t:s0 file1
-rw-r--r-- user1 group1 unconfined_u:object_r:httpd_sys_content_t:s0 file2
-rw-r--r-- user1 group1 unconfined_u:object_r:httpd_sys_content_t:s0 file3
-rw-r--r-- user1 group1 unconfined_u:object_r:default_t:s0 test.tar
If the /test/ directory is no longer required, as the Linux root user, run the rm -ri /test/ command to remove it, as well as all files in it.
Refer to the tar(1) manual page for further information about tar, such as the --xattrs option that retains all extended attributes.
5.10.5. Archiving Files with star
star does not retain extended
attributes by default. Since SELinux contexts are stored in extended
attributes, contexts can be lost when archiving files. Use star -xattr -H=exustar to create archives that retain contexts. The star package is not installed by default. To install star, run the yum install star command as the Linux root user.
The following example demonstrates creating a Star archive that retains SELinux contexts:
As the Linux root user, run the touch /var/www/html/file{1,2,3} command to create three files (file1, file2, and file3). These files inherit the httpd_sys_content_t type from the /var/www/html/ directory:
# touch /var/www/html/file{1,2,3}
# ls -Z /var/www/html/
-rw-r--r-- root root unconfined_u:object_r:httpd_sys_content_t:s0 file1
-rw-r--r-- root root unconfined_u:object_r:httpd_sys_content_t:s0 file2
-rw-r--r-- root root unconfined_u:object_r:httpd_sys_content_t:s0 file3
Run the cd /var/www/html/ command to change into the /var/www/html/ directory. Once in this directory, as the Linux root user, run the star -xattr -H=exustar -c -f=test.star file{1,2,3} command to create a Star archive named test.star:
# star -xattr -H=exustar -c -f=test.star file{1,2,3}
star: 1 blocks + 0 bytes (total of 10240 bytes = 10.00k).
As the Linux root user, run the mkdir /test command to create a new directory, and then, run the chmod 777 /test/ command to allow all users full-access to the /test/ directory.
Run the cp /var/www/html/test.star /test/ command to copy the test.star file in to the /test/ directory.
Run the cd /test/ command to change into the /test/ directory. Once in this directory, run the star -x -f=test.star command to extract the Star archive:
$ star -x -f=test.star
star: 1 blocks + 0 bytes (total of 10240 bytes = 10.00k).
Run the ls -lZ /test/ command to view the SELinux contexts. The httpd_sys_content_t type has been retained, rather than being changed to default_t, which would have happened had the --selinux not been used:
$ ls -lZ /test/
-rw-r--r-- user1 group1 unconfined_u:object_r:httpd_sys_content_t:s0 file1
-rw-r--r-- user1 group1 unconfined_u:object_r:httpd_sys_content_t:s0 file2
-rw-r--r-- user1 group1 unconfined_u:object_r:httpd_sys_content_t:s0 file3
-rw-r--r-- user1 group1 unconfined_u:object_r:default_t:s0 test.star
If the /test/ directory is no longer required, as the Linux root user, run the rm -ri /test/ command to remove it, as well as all files in it.
If star is no longer required, as the Linux root user, run the yum remove star command to remove the package.
Refer to the star(1) manual page for further information about star.
5.11. Information Gathering Tools
These tools are command line tools, providing formatted output. They
are harder to use as part of command line piping, but they provide
gathered and well formatted information quickly.
This command provides a short output of the access vector cache
statistics since boot. You can watch the statistics in real time by
specifying a time interval in seconds. This provides updated statistics
since the initial output. The statistics file used is /selinux/avc/cache_stats, and you can specify a different cache file with the -f /path/to/file option.
[root@localhost ~]# avcstat
lookups hits misses allocs reclaims frees
47517410 47504630 12780 12780 12176 12275
This utility is useful in describing the break-down of a policy,
such as the number of classes, types, Booleans, allow rules, etc. seinfo is a command line utility that uses either the policy.conf file or a binary policy file as input.
The output of seinfo will vary between binary and source files. For example, the policy source file uses the { }
brackets to group multiple rule elements onto a single line. A similar
effect happens with attributes, where a single attribute expands into
one or many types. Because these are expanded and no longer relevant in
the binary policy file, they have a return value of zero in the search
results. However, the number of rules greatly increases as each formerly
one line rule using brackets is now a number of individual lines.
Some items are not present in the binary policy. For example,
neverallow rules are only checked during policy compile, not during
runtime, and initial SIDs are not part of the binary policy since they
are required prior to the policy being loaded by the kernel during boot.
[root@localhost ]# seinfo
Statistics for policy file: /etc/selinux/targeted/policy/policy.24
Policy Version & Type: v.24 (binary, mls)
Classes: 77 Permissions: 229
Sensitivities: 1 Categories: 1024
Types: 3001 Attributes: 244
Users: 9 Roles: 13
Booleans: 158 Cond. Expr.: 193
Allow: 262796 Neverallow: 0
Auditallow: 44 Dontaudit: 156710
Type_trans: 10760 Type_change: 38
Type_member: 44 Role allow: 20
Role_trans: 237 Range_trans: 2546
Constraints: 62 Validatetrans: 0
Initial SIDs: 27 Fs_use: 22
Genfscon: 82 Portcon: 373
Netifcon: 0 Nodecon: 0
Permissives: 22 Polcap: 2
[root@localhost ]#
The seinfo command can also list the
number of types with the domain attribute, giving an estimate of the
number of different confined processes:
# seinfo -adomain -x | wc -l
550
Not all domain types are confined. To look at the number of unconfined domains, use the unconfined_domain attribute:
# seinfo -aunconfined_domain_type -x | wc -l
52
Permissive domains can be counted with the --permissive option.
# seinfo --permissive -x | wc -l
31
Remove the | wc -l option in the above commands to see the full lists.
You can use the sesearch command to search for a particular type in
the policy. You can search either policy source files or the binary
file. For example:
[scott@localhost ~]$ sesearch --role_allow -t httpd_sys_content_t \/etc/selinux/targeted/policy/policy.24
Found 20 role allow rules:
allow system_r sysadm_r;
allow sysadm_r system_r;
allow sysadm_r staff_r;
allow sysadm_r user_r;
allow system_r git_shell_r;
allow system_r guest_r;
allow logadm_r system_r;
allow system_r logadm_r;
allow system_r nx_server_r;
allow system_r staff_r;
allow staff_r logadm_r;
allow staff_r sysadm_r;
allow staff_r unconfined_r;
allow staff_r webadm_r;
allow unconfined_r system_r;
allow system_r unconfined_r;
allow system_r user_r;
allow webadm_r system_r;
allow system_r webadm_r;
allow system_r xguest_r;
The sesearch command can provide the number of allow rules:
# sesearch --allow | wc -l
262798
And the number of dontaudit rules:
# sesearch --dontaudit | wc -l
156712
Chapter 6. Confining Users
A number of confined SELinux users are available in Red Hat Enterprise
Linux 6. Each Linux user is mapped to an SELinux user via SELinux
policy, allowing Linux users to inherit the restrictions placed on
SELinux users, for example (depending on the user), not being able to:
run the X Window System; use networking; run setuid applications (unless
SELinux policy permits it); or run the
su and
sudo commands. This helps protect the system from the user. Refer to
Section 4.3, “Confined and Unconfined Users” for further information about confined users.
6.1. Linux and SELinux User Mappings
As the Linux root user, run the semanage login -l command to view the mapping between Linux users and SELinux users:
# /usr/sbin/semanage login -l
Login Name SELinux User MLS/MCS Range
__default__ unconfined_u s0-s0:c0.c1023
root unconfined_u s0-s0:c0.c1023
system_u system_u s0-s0:c0.c1023
In Red Hat Enterprise Linux 6, Linux users are mapped to the SELinux __default__ login by default (which is in turn mapped to the SELinux unconfined_u user). When a Linux user is created with the useradd command, if no options are specified, they are mapped to the SELinux unconfined_u user. The following defines the default-mapping:
__default__ unconfined_u s0-s0:c0.c1023
6.2. Confining New Linux Users: useradd
Linux users mapped to the SELinux unconfined_u user run in the unconfined_t domain. This is seen by running the id -Z command while logged-in as a Linux user mapped to unconfined_u:
$ id -Z
unconfined_u:unconfined_r:unconfined_t:s0-s0:c0.c1023
When Linux users run in the unconfined_t domain, SELinux policy rules are applied, but policy rules exist that allow Linux users running in the unconfined_t domain almost all access. If unconfined Linux users execute an application that SELinux policy defines can transition from the unconfined_t
domain to its own confined domain, unconfined Linux users are still
subject to the restrictions of that confined domain. The security
benefit of this is that, even though a Linux user is running unconfined,
the application remains confined, and therefore, the exploitation of a
flaw in the application can be limited by policy. Note: this does not
protect the system from the user. Instead, the user and the system are
being protected from possible damage caused by a flaw in the
application.
When creating Linux users with useradd, use the -Z
option to specify which SELinux user they are mapped to. The following
example creates a new Linux user, useruuser, and maps that user to the
SELinux user_u user. Linux users mapped to the SELinux user_u user run in the user_t domain. In this domain, Linux users are unable to run setuid applications unless SELinux policy permits it (such as passwd), and can not run su or sudo, preventing them from becoming the Linux root user with these commands.
As the Linux root user, run the /usr/sbin/useradd -Z user_u useruuser command to create a new Linux user (useruuser) that is mapped to the SELinux user_u user.
As the Linux root user, run the semanage login -l command to view the mapping between the Linux useruuser user and user_u:
# /usr/sbin/semanage login -l
Login Name SELinux User MLS/MCS Range
__default__ unconfined_u s0-s0:c0.c1023
root unconfined_u s0-s0:c0.c1023
system_u system_u s0-s0:c0.c1023
useruuser user_u s0
As the Linux root user, run the passwd useruuser command to assign a password to the Linux useruuser user:
# passwd useruuser
Changing password for user useruuser.
New UNIX password: Enter a password
Retype new UNIX password: Enter the same password again
passwd: all authentication tokens updated successfully.
Log out of your current session, and log in as the Linux useruuser
user. When you log in, pam_selinux maps the Linux user to an SELinux
user (in this case, user_u), and sets up the resulting SELinux context. The Linux user's shell is then launched with this context. Run the id -Z command to view the context of a Linux user:
[useruuser@localhost ~]$ id -Z
user_u:user_r:user_t:s0
Log out of the Linux useruuser's session, and log back in with your
account. If you do not want the Linux useruuser user, run the /usr/sbin/userdel -r useruuser command as the Linux root user to remove it, along with its home directory.
6.3. Confining Existing Linux Users: semanage login
If a Linux user is mapped to the SELinux unconfined_u user (the default behavior), and you would like to change which SELinux user they are mapped to, use the semanage login command. The following example creates a new Linux user named newuser, then maps that Linux user to the SELinux user_u user:
As the Linux root user, run the /usr/sbin/useradd newuser command to create a new Linux user (newuser). Since this user uses the default mapping, it does not appear in the /usr/sbin/semanage login -l output:
# /usr/sbin/semanage login -l
Login Name SELinux User MLS/MCS Range
__default__ unconfined_u s0-s0:c0.c1023
root unconfined_u s0-s0:c0.c1023
system_u system_u s0-s0:c0.c1023
To map the Linux newuser user to the SELinux user_u user, run the following command as the Linux root user:
/usr/sbin/semanage login -a -s user_u newuser
The -a option adds a new record, and the -s option specifies the SELinux user to map a Linux user to. The last argument, newuser, is the Linux user you want mapped to the specified SELinux user.
To view the mapping between the Linux newuser user and user_u, run the semanage login -l command as the Linux root user:
# /usr/sbin/semanage login -l
Login Name SELinux User MLS/MCS Range
__default__ unconfined_u s0-s0:c0.c1023
newuser user_u s0
root unconfined_u s0-s0:c0.c1023
system_u system_u s0-s0:c0.c1023
As the Linux root user, run the passwd newuser command to assign a password to the Linux newuser user:
# passwd newuser
Changing password for user newuser.
New UNIX password: Enter a password
Retype new UNIX password: Enter the same password again
passwd: all authentication tokens updated successfully.
Log out of your current session, and log in as the Linux newuser user. Run the id -Z command to view the newuser's SELinux context:
[newuser@rlocalhost ~]$ id -Z
user_u:user_r:user_t:s0
Log out of the Linux newuser's session, and log back in with your
account. If you do not want the Linux newuser user, run the userdel -r newuser
command as the Linux root user to remove it, along with its home
directory. Also, the mapping between the Linux newuser user and user_u is removed:
# /usr/sbin/userdel -r newuser
# /usr/sbin/semanage login -l
Login Name SELinux User MLS/MCS Range
__default__ unconfined_u s0-s0:c0.c1023
root unconfined_u s0-s0:c0.c1023
system_u system_u s0-s0:c0.c1023
6.4. Changing the Default Mapping
In Red Hat Enterprise Linux 6, Linux users are mapped to the SELinux __default__ login by default (which is in turn mapped to the SELinux unconfined_u
user). If you would like new Linux users, and Linux users not
specifically mapped to an SELinux user to be confined by default, change
the default mapping with the semanage login command.
For example, run the following command as the Linux root user to change the default mapping from unconfined_u to user_u:
/usr/sbin/semanage login -m -S targeted -s "user_u" -r s0 __default__
Run the semanage login -l command as the Linux root user to verify the __default__ login is mapped to user_u:
# /usr/sbin/semanage login -l
Login Name SELinux User MLS/MCS Range
__default__ user_u s0
root unconfined_u s0-s0:c0.c1023
system_u system_u s0-s0:c0.c1023
If a new Linux user is created and an SELinux user is not specified,
or if an existing Linux user logs in and does not match a specific entry
from the semanage login -l output, they are mapped to user_u, as per the __default__ login.
To change back to the default behavior, run the following command as the Linux root user to map the __default__ login to the SELinux unconfined_u user:
/usr/sbin/semanage login -m -S targeted -s "unconfined_u" -r\
s0-s0:c0.c1023 __default__
The xguest package provides a kiosk user
account. This account is used to secure machines that people walk up to
and use, such as those at libraries, banks, airports, information
kiosks, and coffee shops. The kiosk user account is very limited:
essentially, it only allows users to log in and use Firefox
to browse Internet websites. Any changes made while logged in with his
account, such as creating files or changing settings, are lost when you
log out.
To set up the kiosk account:
As the Linux root user, run yum install xguest command to install the xguest package. Install dependencies as required.
In order to allow the kiosk account to be used by a variety of
people, the account is not password-protected, and as such, the account
can only be protected if SELinux is running in enforcing mode. Before
logging in with this account, use the getenforce command to confirm that SELinux is running in enforcing mode:
$ /usr/sbin/getenforce
Enforcing
If this is not the case, refer to
Section 5.5, “SELinux Modes”
for information about changing to enforcing mode. It is not possible to
log in with this account if SELinux is in permissive mode or disabled.
You can only log in to this account via the GNOME Display Manager (GDM). Once the xguest package is installed, a Guest account is added to the GDM login screen.
6.6. Booleans for Users Executing Applications
Not allowing Linux users to execute applications (which inherit users' permissions) in their home directories and /tmp/,
which they have write access to, helps prevent flawed or malicious
applications from modifying files that users own. In Red Hat Enterprise
Linux 6, by default, Linux users in the guest_t and xguest_t domains can not execute applications in their home directories or /tmp/; however, by default, Linux users in the user_t and staff_t domains can.
Booleans are available to change this behavior, and are configured with the setsebool command. The setsebool command must be run as the Linux root user. The setsebool -P command makes persistent changes. Do not use the -P option if you do not want changes to persist across reboots:
To allow Linux users in the guest_t domain to execute applications in their home directories and /tmp/:
/usr/sbin/setsebool -P allow_guest_exec_content on
To allow Linux users in the xguest_t domain to execute applications in their home directories and /tmp/:
/usr/sbin/setsebool -P allow_xguest_exec_content on
To prevent Linux users in the user_t domain from executing applications in their home directories and /tmp/:
/usr/sbin/setsebool -P allow_user_exec_content off
To prevent Linux users in the staff_t domain from executing applications in their home directories and /tmp/:
/usr/sbin/setsebool -P allow_staff_exec_content off
sVirt is a technology included in Red Hat Enterprise Linux 6 that
integrates SELinux and virtualization. sVirt applies Mandatory Access
Control (MAC) to improve security when using virtual machines. The main
reasons for integrating these technologies are to improve security and
harden the system against bugs in the hypervisor that might be used as
an attack vector aimed toward the host or to another virtual machine.
This chapter describes how sVirt integrates with virtualization technologies in Red Hat Enterprise Linux 6.
In a non-virtualized environment, hosts are separated from each other
physically and each host has a self-contained environment, consisting
of services such as a Web server, or a DNS server. These services
communicate directly to their own user space, host kernel and physical
host, offering their services directly to the network. The following
image represents a non-virtualized environment:
In a virtualized environment, several operating systems can be housed
(as "guests") within a single host kernel and physical host. The
following image represents a virtualized environment:
7.1. Security and Virtualization
When services are not virtualized, machines are physically separated.
Any exploit is usually contained to the affected machine, with the
obvious exception of network attacks. When services are grouped together
in a virtualized environment, extra vulnerabilities emerge in the
system. If there is a security flaw in the hypervisor that can be
exploited by a guest instance, this guest may be able to not only attack
the host, but also other guests running on that host. This is not
theoretical; attacks already exist on hypervisors. These attacks can
extend beyond the guest instance and could expose other guests to
attack.
sVirt is an effort to isolate guests and limit their ability to
launch further attacks if exploited. This is demonstrated in the
following image, where an attack can not break out of the virtual
machine and extend to another host instance:
SELinux introduces a pluggable security framework for virtualized
instances in its implementation of Mandatory Access Control (MAC). The
sVirt framework allows guests and their resources to be uniquely
labelled. Once labelled, rules can be applied which can reject access
between different guests.
Like other services under the protection of SELinux, sVirt uses
process-based mechanisms and restrictions to provide an extra layer of
security over guest instances. Under typical use, you should not even
notice that sVirt is working in the background. This section describes
the labelling features of sVirt.
As shown in the following output, when using sVirt, each Virtual
Machine (VM) process is labelled and runs with a dynamically generated
level. Each process is isolated from other VMs with different levels:
# ps -eZ | grep qemu
system_u:system_r:svirt_t:s0:c87,c520 27950 ? 00:00:17 qemu-kvm
system_u:system_r:svirt_t:s0:c639,c757 27989 ? 00:00:06 qemu-system-x86
The actual disk images are automatically labelled to match the processes, as shown in the following output:
# ls -lZ /var/lib/libvirt/images/*
system_u:object_r:svirt_image_t:s0:c87,c520 image1
The following table outlines the different labels that can be assigned when using sVirt:
Table 7.1. sVirt Labels
|
Type
|
SELinux Context
|
Description
|
|---|
|
Virtual Machine Processes
|
system_u:system_r:svirt_t:MCS1
|
MCS1 is a randomly selected MCS field. Currently approximately 500,000 labels are supported.
|
|
Virtual Machine Image
|
system_u:object_r:svirt_image_t:MCS1
|
Only svirt_t processes with the same MCS fields are able to read/write these image files and devices.
|
|
Virtual Machine Shared Read/Write Content
|
system_u:object_r:svirt_image_t:s0
|
All svirt_t processes are allowed to write to the svirt_image_t:s0 files and devices.
|
|
Virtual Machine Shared Shared Read Only content
|
system_u:object_r:svirt_content_t:s0
|
All svirt_t processes are able to read files/devices with this label.
|
|
Virtual Machine Image
|
system_u:object_r:virt_content_t:s0
|
System default label used when an image exits. No svirt_t virtual processes are allowed to read files/devices with this label.
|
It is also possible to perform static labeling when using sVirt.
Static labels allow the administrator to select a specific label,
including the MCS/MLS field, for a virtual machine. Administrators who
run statically-labeled virtual machines are responsible for setting the
correct label on the image files. The virtual machine will always be
started with that label, and the sVirt system will never modify the
label of a statically-labelled virtual machine's content. This allows
the sVirt component to run in an MLS environment. You can also run
multiple virtual machines with different sensitivity levels on a system,
depending on your requirements.
Chapter 8. Troubleshooting
The following chapter describes what happens when SELinux denies
access; the top three causes of problems; where to find information
about correct labeling; analyzing SELinux denials; and creating custom
policy modules with audit2allow.
8.1. What Happens when Access is Denied
SELinux decisions, such as allowing or disallowing access, are
cached. This cache is known as the Access Vector Cache (AVC). Denial
messages are logged when SELinux denies access. These denials are also
known as "AVC denials", and are logged to a different location,
depending on which daemons are running:
| Daemon | Log Location |
|---|
| auditd on | /var/log/audit/audit.log |
| auditd off; rsyslogd on | /var/log/messages |
| setroubleshootd, rsyslogd, and auditd on | /var/log/audit/audit.log. Easier-to-read denial messages also sent to /var/log/messages |
If you are running the X Window System, have the setroubleshoot and setroubleshoot-server packages installed, and the setroubleshootd and auditd daemons are running, a warning is displayed when access is denied by SELinux:
Clicking on 'Show' presents a detailed analysis of why SELinux denied
access, and a possible solution for allowing access. If you are not
running the X Window System, it is less obvious when access is denied by
SELinux. For example, users browsing your website may receive an error
similar to the following:
Forbidden
You don't have permission to access file name on this server
For these situations, if DAC rules (standard Linux permissions) allow access, check /var/log/messages and /var/log/audit/audit.log for "SELinux is preventing" and "denied" errors respectively. This can be done by running the following commands as the Linux root user:
grep "SELinux is preventing" /var/log/messages
grep "denied" /var/log/audit/audit.log
8.2. Top Three Causes of Problems
The following sections describe the top three causes of problems:
labeling problems, configuring Booleans and ports for services, and
evolving SELinux rules.
On systems running SELinux, all processes and files are labeled with
a label that contains security-relevant information. This information
is called the SELinux context. If these labels are wrong, access may be
denied. If an application is labeled incorrectly, the process it
transitions to may not have the correct label, possibly causing SELinux
to deny access, and the process being able to create mislabeled files.
A common cause of labeling problems is when a non-standard directory is used for a service. For example, instead of using /var/www/html/ for a website, an administrator wants to use /srv/myweb/. On Red Hat Enterprise Linux 6, the /srv/ directory is labeled with the var_t type. Files and directories created and /srv/ inherit this type. Also, newly-created top-level directories (such as /myserver/) may be labeled with the default_t type. SELinux prevents the Apache HTTP Server (httpd) from accessing both of these types. To allow access, SELinux must know that the files in /srv/myweb/ are to be accessible to httpd:
# /usr/sbin/semanage fcontext -a -t httpd_sys_content_t \
"/srv/myweb(/.*)?"
This semanage command adds the context for the /srv/myweb/ directory (and all files and directories under it) to the SELinux file-context configuration[]. The semanage command does not change the context. As the Linux root user, run the restorecon command to apply the changes:
# /sbin/restorecon -R -v /srv/myweb
8.2.1.1. What is the Correct Context?
The matchpathcon command checks the
context of a file path and compares it to the default label for that
path. The following example demonstrates using matchpathcon on a directory that contains incorrectly labeled files:
$ /usr/sbin/matchpathcon -V /var/www/html/*
/var/www/html/index.html has context unconfined_u:object_r:user_home_t:s0, should be system_u:object_r:httpd_sys_content_t:s0
/var/www/html/page1.html has context unconfined_u:object_r:user_home_t:s0, should be system_u:object_r:httpd_sys_content_t:s0
In this example, the index.html and page1.html files are labeled with the user_home_t type. This type is used for files in user home directories. Using the mv command to move files from your home directory may result in files being labeled with the user_home_t type. This type should not exist outside of home directories. Use the restorecon command to restore such files to their correct type:
# /sbin/restorecon -v /var/www/html/index.html
restorecon reset /var/www/html/index.html context unconfined_u:object_r:user_home_t:s0->system_u:object_r:httpd_sys_content_t:s0
To restore the context for all files under a directory, use the -R option:
# /sbin/restorecon -R -v /var/www/html/
restorecon reset /var/www/html/page1.html context unconfined_u:object_r:samba_share_t:s0->system_u:object_r:httpd_sys_content_t:s0
restorecon reset /var/www/html/index.html context unconfined_u:object_r:samba_share_t:s0->system_u:object_r:httpd_sys_content_t:s0
8.2.2. How are Confined Services Running?
Services can be run in a variety of ways. To cater for this, you
must tell SELinux how you are running services. This can be achieved via
Booleans that allow parts of SELinux policy to be changed at runtime,
without any knowledge of SELinux policy writing. This allows changes,
such as allowing services access to NFS file systems, without reloading
or recompiling SELinux policy. Also, running services on non-default
port numbers requires policy configuration to be updated via the semanage command.
For example, to allow the Apache HTTP Server to communicate with MySQL, turn the httpd_can_network_connect_db Boolean on:
# /usr/sbin/setsebool -P httpd_can_network_connect_db on
If access is denied for a particular service, use the getsebool and grep commands to see if any Booleans are available to allow access. For example, use the getsebool -a | grep ftp command to search for FTP related Booleans:
$ /usr/sbin/getsebool -a | grep ftp
allow_ftpd_anon_write --> off
allow_ftpd_full_access --> off
allow_ftpd_use_cifs --> off
allow_ftpd_use_nfs --> off
ftp_home_dir --> off
httpd_enable_ftp_server --> off
tftp_anon_write --> off
For a list of Booleans and whether they are on or off, run the
/usr/sbin/getsebool -a command. For a list of Booleans, an explanation of what each one is, and whether they are on or off, run the
/usr/sbin/semanage boolean -l command as the Linux root user. Refer to
Section 5.6, “Booleans” for information about listing and configuring Booleans.
Depending on policy configuration, services may only be allowed to
run on certain port numbers. Attempting to change the port a service
runs on without changing policy may result in the service failing to
start. For example, run the semanage port -l | grep http command as the Linux root user to list http related ports:
# /usr/sbin/semanage port -l | grep http
http_cache_port_t tcp 3128, 8080, 8118
http_cache_port_t udp 3130
http_port_t tcp 80, 443, 488, 8008, 8009, 8443
pegasus_http_port_t tcp 5988
pegasus_https_port_t tcp 5989
The http_port_t port type
defines the ports Apache HTTP Server can listen on, which in this case,
are TCP ports 80, 443, 488, 8008, 8009, and 8443. If an administrator
configures httpd.conf so that httpd listens on port 9876 (Listen 9876), but policy is not updated to reflect this, the service httpd start command fails:
# /sbin/service httpd start
Starting httpd: (13)Permission denied: make_sock: could not bind to address [::]:9876
(13)Permission denied: make_sock: could not bind to address 0.0.0.0:9876
no listening sockets available, shutting down
Unable to open logs
[FAILED]
An SELinux denial similar to the following is logged to /var/log/audit/audit.log:
type=AVC msg=audit(1225948455.061:294): avc: denied { name_bind } for pid=4997 comm="httpd" src=9876 scontext=unconfined_u:system_r:httpd_t:s0 tcontext=system_u:object_r:port_t:s0 tclass=tcp_socket
To allow httpd to listen on a port that is not listed for the http_port_t port type, run the semanage port command to add a port to policy configuration[]:
# /usr/sbin/semanage port -a -t http_port_t -p tcp 9876
The -a option adds a new record; the -t option defines a type; and the -p option defines a protocol. The last argument is the port number to add.
8.2.3. Evolving Rules and Broken Applications
Applications may be broken, causing SELinux to deny access. Also,
SELinux rules are evolving - SELinux may not have seen an application
running in a certain way, possibly causing it to deny access, even
though the application is working as expected. For example, if a new
version of PostgreSQL is released, it may perform actions the current
policy has not seen before, causing access to be denied, even though
access should be allowed.
The following sections help troubleshoot issues. They go over:
checking Linux permissions, which are checked before SELinux rules;
possible causes of SELinux denying access, but no denials being logged;
manual pages for services, which contain information about labeling and
Booleans; permissive domains, for allowing one process to run
permissive, rather than the whole system; how to search for and view
denial messages; analyzing denials; and creating custom policy modules
with audit2allow.
When access is denied, check standard Linux permissions. As mentioned in
Chapter 2, Introduction,
most operating systems use a Discretionary Access Control (DAC) system
to control access, allowing users to control the permissions of files
that they own. SELinux policy rules are checked after DAC rules. SELinux
policy rules are not used if DAC rules deny access first.
If access is denied and no SELinux denials are logged, use the ls -l command to view the standard Linux permissions:
$ ls -l /var/www/html/index.html
-rw-r----- 1 root root 0 2009-05-07 11:06 index.html
In this example, index.html is owned by the root user and group. The root user has read and write permissions (-rw), and members of the root group have read permissions (-r-). Everyone else has no access (---). By default, such permissions do not allow httpd to read this file. To resolve this issue, use the chown command to change the owner and group. This command must be run as the Linux root user:
# chown apache:apache /var/www/html/index.html
This assumes the default configuration, in which httpd runs as the Linux apache user. If you run httpd with a different user, replace apache:apache with that user.
8.3.2. Possible Causes of Silent Denials
In certain situations, AVC denials may not be logged when SELinux
denies access. Applications and system library functions often probe for
more access than required to perform their tasks. To maintain least
privilege without filling audit logs with AVC denials for harmless
application probing, the policy can silence AVC denials without allowing
a permission by using dontaudit rules. These rules are common in standard policy. The downside of dontaudit is that, although SELinux denies access, denial messages are not logged, making troubleshooting hard.
To temporarily disable dontaudit rules, allowing all denials to be logged, run the following command as the Linux root user:
/usr/sbin/semodule -DB
The
-D option disables
dontaudit rules; the
-B option rebuilds policy. After running
semodule -DB,
try exercising the application that was encountering permission
problems, and see if SELinux denials — relevant to the application — are
now being logged. Take care in deciding which denials should be
allowed, as some should be ignored and handled via
dontaudit rules. If in doubt, or in search of guidance, contact other SELinux users and developers on an SELinux list, such as
fedora-selinux-list.
To rebuild policy and enable dontaudit rules, run the following command as the Linux root user:
/usr/sbin/semodule -B
This restores the policy to its original state. For a full list of dontaudit rules, run the sesearch --dontaudit command. Narrow down searches using the -s domain option and the grep command. For example:
$ sesearch --dontaudit -s smbd_t | grep squid
WARNING: This policy contained disabled aliases; they have been removed.
dontaudit smbd_t squid_port_t : tcp_socket name_bind ;
dontaudit smbd_t squid_port_t : udp_socket name_bind ;
8.3.3. Manual Pages for Services
Manual pages for services contain valuable information, such as what
file type to use for a given situation, and Booleans to change the
access a service has (such as httpd accessing NFS file systems). This information may be in the standard manual page, or a manual page with selinux prepended or appended.
For example, the httpd_selinux(8)
manual page has information about what file type to use for a given
situation, as well as Booleans to allow scripts, sharing files,
accessing directories inside user home directories, and so on. Other
manual pages with SELinux information for services include:
Samba: the samba_selinux(8) manual page describes that files and directories to be exported via Samba must be labeled with the samba_share_t type, as well as Booleans to allow files labeled with types other than samba_share_t to be exported via Samba.
NFS: the nfs_selinux(8)
manual page describes that, by default, file systems can not be
exported via NFS, and that to allow file systems to be exported,
Booleans such as nfs_export_all_ro or nfs_export_all_rw must be turned on.
Berkeley Internet Name Domain (BIND): the named(8) manual page describes what file type to use for a given situation (see the Red Hat SELinux BIND Security Profile section). The named_selinux(8) manual page describes that, by default, named can not write to master zone files, and to allow such access, the named_write_master_zones Boolean must be turned on.
The information in manual pages helps you configure the correct file
types and Booleans, helping to prevent SELinux from denying access.
8.3.4. Permissive Domains
When SELinux is running in permissive mode, SELinux does not deny
access, but denials are logged for actions that would have been denied
if running in enforcing mode. Previously, it was not possible to make a
single domain permissive (remember: processes run in domains). In
certain situations, this led to making the whole system permissive to
troubleshoot issues.
Red Hat Enterprise Linux includes permissive domains, where an
administrator can configure a single process (domain) to run permissive,
rather than making the whole system permissive. SELinux checks are
still performed for permissive domains; however, the kernel allows
access and reports an AVC denial for situations where SELinux would have
denied access.
In Red Hat Enterprise Linux 4 and 5, domain_disable_transinitrc_t. Turning such Booleans on can cause major problems. For example, if the httpd_disable_trans Boolean is turned on:
httpd runs in the unconfined initrc_t domain. Files created by processes running in the initrc_t domain may not have the same labeling rules applied as files created by a process running in the httpd_t domain, potentially allowing processes to create mislabeled files. This causes access problems later on.
confined domains that are allowed to communicate with httpd_t can not communicate with initrc_t, possibly causing additional failures.
Permissive domains can be used for:
making a single process (domain) run permissive to troubleshoot an
issue, rather than putting the entire system at risk by making the
entire system permissive.
creating policies for new applications. Previously, it was
recommended that a minimal policy be created, and then the entire
machine put into permissive mode, so that the application could run, but
SELinux denials still logged. audit2allow
could then be used to help write the policy. This put the whole system
at risk. With permissive domains, only the domain in the new policy can
be marked permissive, without putting the whole system at risk.
8.3.4.1. Making a Domain Permissive
To make a domain permissive, run the semanage permissive -a domain command, where domain is the domain you want to make permissive. For example, run the following command as the Linux root user to make the httpd_t domain (the domain the Apache HTTP Server runs in) permissive:
/usr/sbin/semanage permissive -a httpd_t
To view a list of domains you have made permissive, run the semodule -l | grep permissive command as the Linux root user. For example:
# /usr/sbin/semodule -l | grep permissive
permissive_httpd_t 1.0
If you no longer want a domain to be permissive, run the semanage permissive -d domain command as the Linux root user. For example:
/usr/sbin/semanage permissive -d httpd_t
8.3.4.2. Denials for Permissive Domains
The SYSCALL message is
different for permissive domains. The following is an example AVC denial
(and the associated system call) from the Apache HTTP Server:
type=AVC msg=audit(1226882736.442:86): avc: denied { getattr } for pid=2427 comm="httpd" path="/var/www/html/file1" dev=dm-0 ino=284133 scontext=unconfined_u:system_r:httpd_t:s0 tcontext=unconfined_u:object_r:samba_share_t:s0 tclass=file
type=SYSCALL msg=audit(1226882736.442:86): arch=40000003 syscall=196 success=no exit=-13 a0=b9a1e198 a1=bfc2921c a2=54dff4 a3=2008171 items=0 ppid=2425 pid=2427 auid=502 uid=48 gid=48 euid=48 suid=48 fsuid=48 egid=48 sgid=48 fsgid=48 tty=(none) ses=4 comm="httpd" exe="/usr/sbin/httpd" subj=unconfined_u:system_r:httpd_t:s0 key=(null)
By default, the httpd_t domain is not permissive, and as such, the action is denied, and the SYSCALL message contains success=no. The following is an example AVC denial for the same situation, except the semanage permissive -a httpd_t command has been run to make the httpd_t domain permissive:
type=AVC msg=audit(1226882925.714:136): avc: denied { read } for pid=2512 comm="httpd" name="file1" dev=dm-0 ino=284133 scontext=unconfined_u:system_r:httpd_t:s0 tcontext=unconfined_u:object_r:samba_share_t:s0 tclass=file
type=SYSCALL msg=audit(1226882925.714:136): arch=40000003 syscall=5 success=yes exit=11 a0=b962a1e8 a1=8000 a2=0 a3=8000 items=0 ppid=2511 pid=2512 auid=502 uid=48 gid=48 euid=48 suid=48 fsuid=48 egid=48 sgid=48 fsgid=48 tty=(none) ses=4 comm="httpd" exe="/usr/sbin/httpd" subj=unconfined_u:system_r:httpd_t:s0 key=(null)
In this case, although an AVC denial was logged, access was not denied, as shown by success=yes in the SYSCALL message.
Refer to Dan Walsh's
"Permissive Domains" blog entry for further information about permissive domains.
8.3.5. Searching For and Viewing Denials
This section assumes the
setroubleshoot,
setroubleshoot-server,
dbus and
audit packages are installed, and that the
auditd,
rsyslogd, and
setroubleshootd daemons are running. Refer to
Section 5.2, “Which Log File is Used”
for information about starting these daemons. A number of tools are
available for searching for and viewing SELinux denials, such as
ausearch,
aureport, and
sealert.
The audit package provides ausearch. From the ausearch(8) manual page: "ausearch is a tool that can query the audit daemon logs based for events based on different search criteria"[]. The ausearch tool accesses /var/log/audit/audit.log, and as such, must be run as the Linux root user:
| Searching For | Command |
|---|
| all denials | /sbin/ausearch -m avc |
| denials for that today | /sbin/ausearch -m avc -ts today |
| denials from the last 10 minutes | /sbin/ausearch -m avc -ts recent |
To search for SELinux denials for a particular service, use the -c comm-name option, where comm-name "is the executable’s name"[], for example, httpd for the Apache HTTP Server, and smbd for Samba:
/sbin/ausearch -m avc -c httpd
/sbin/ausearch -m avc -c smbd
Refer to the ausearch(8) manual page for further ausearch options.
The audit package provides aureport. From the aureport(8) manual page: "aureport is a tool that produces summary reports of the audit system logs"[]. The aureport tool accesses /var/log/audit/audit.log, and as such, must be run as the Linux root user. To view a list of SELinux denials and how often each one occurred, run the aureport -a command. The following is example output that includes two denials:
# /sbin/aureport -a
AVC Report
========================================================
# date time comm subj syscall class permission obj event
========================================================
1. 05/01/2009 21:41:39 httpd unconfined_u:system_r:httpd_t:s0 195 file getattr system_u:object_r:samba_share_t:s0 denied 2
2. 05/03/2009 22:00:25 vsftpd unconfined_u:system_r:ftpd_t:s0 5 file read unconfined_u:object_r:cifs_t:s0 denied 4
Refer to the aureport(8) manual page for further aureport options.
The setroubleshoot-server package provides sealert, which reads denial messages translated by setroubleshoot-server. Denials are assigned IDs, as seen in /var/log/messages. The following is an example denial from messages:
setroubleshoot: SELinux is preventing httpd (httpd_t) "getattr" to /var/www/html/file1 (samba_share_t). For complete SELinux messages. run sealert -l 84e0b04d-d0ad-4347-8317-22e74f6cd020
In this example, the denial ID is 84e0b04d-d0ad-4347-8317-22e74f6cd020. The -l option takes an ID as an argument. Running the sealert -l 84e0b04d-d0ad-4347-8317-22e74f6cd020 command presents a detailed analysis of why SELinux denied access, and a possible solution for allowing access.
If you are running the X Window System, have the setroubleshoot and setroubleshoot-server packages installed, and the setroubleshootd, dbus and auditd daemons are running, a warning is displayed when access is denied by SELinux. Clicking on 'Show' launches the sealert GUI, and displays denials in HTML output:
Run the sealert -b command to launch the sealert GUI.
Run the sealert -l \* command to view a detailed analysis of all denials.
As the Linux root user, run the sealert -a /var/log/audit/audit.log -H > audit.html command to create a HTML version of the sealert analysis, as seen with the sealert GUI.
Refer to the sealert(8) manual page for further sealert options.
8.3.6. Raw Audit Messages
Raw audit messages are logged to /var/log/audit/audit.log.
The following is an example AVC denial (and the associated system call)
that occurred when the Apache HTTP Server (running in the httpd_t domain) attempted to access the /var/www/html/file1 file (labeled with the samba_share_t type):
type=AVC msg=audit(1226874073.147:96): avc: denied { getattr } for pid=2465 comm="httpd" path="/var/www/html/file1" dev=dm-0 ino=284133 scontext=unconfined_u:system_r:httpd_t:s0 tcontext=unconfined_u:object_r:samba_share_t:s0 tclass=file
type=SYSCALL msg=audit(1226874073.147:96): arch=40000003 syscall=196 success=no exit=-13 a0=b98df198 a1=bfec85dc a2=54dff4 a3=2008171 items=0 ppid=2463 pid=2465 auid=502 uid=48 gid=48 euid=48 suid=48 fsuid=48 egid=48 sgid=48 fsgid=48 tty=(none) ses=6 comm="httpd" exe="/usr/sbin/httpd" subj=unconfined_u:system_r:httpd_t:s0 key=(null)
{ getattr }
The item in braces indicates the permission that was denied. getattr
indicates the source process was trying to read the target file's
status information. This occurs before reading files. This action is
denied due to the file being accessed having the wrong label. Commonly
seen permissions include getattr, read, and write.
- comm="
httpd"
The executable that launched the process. The full path of the executable is found in the exe= section of the system call (SYSCALL) message, which in this case, is exe="/usr/sbin/httpd".
- path="
/var/www/html/file1"
The path to the object (target) the process attempted to access.
- scontext="
unconfined_u:system_r:httpd_t:s0"
The SELinux context of the process that attempted the denied
action. In this case, it is the SELinux context of the Apache HTTP
Server, which is running in the httpd_t domain.
- tcontext="
unconfined_u:object_r:samba_share_t:s0"
The SELinux context of the object (target) the process attempted to access. In this case, it is the SELinux context of file1. Note: the samba_share_t type is not accessible to processes running in the httpd_t domain.
In certain situations, the tcontext may match the scontext,
for example, when a process attempts to execute a system service that
will change characteristics of that running process, such as the user
ID. Also, the tcontext may match the scontext
when a process tries to use more resources (such as memory) than normal
limits allow, resulting in a security check to see if that process is
allowed to break those limits.
From the system call (SYSCALL) message, two items are of interest:
success=no: indicates whether the denial (AVC) was enforced or not. success=no indicates the system call was not successful (SELinux denied access). success=yes indicates the system call was successful - this can be seen for permissive domains or unconfined domains, such as initrc_t and kernel_t.
exe="/usr/sbin/httpd": the full path to the executable that launched the process, which in this case, is exe="/usr/sbin/httpd".
An incorrect file type is a common cause for SELinux denying access. To start troubleshooting, compare the source context (scontext) with the target context (tcontext). Should the process (scontext) be accessing such an object (tcontext)? For example, the Apache HTTP Server (httpd_t) should only be accessing types specified in the httpd_selinux(8) manual page, such as httpd_sys_content_t, public_content_t, and so on, unless configured otherwise.
Denials are assigned IDs, as seen in /var/log/messages. The following is an example AVC denial (logged to messages) that occurred when the Apache HTTP Server (running in the httpd_t domain) attempted to access the /var/www/html/file1 file (labeled with the samba_share_t type):
hostname setroubleshoot: SELinux is preventing httpd (httpd_t) "getattr" to /var/www/html/file1 (samba_share_t). For complete SELinux messages. run sealert -l 84e0b04d-d0ad-4347-8317-22e74f6cd020
As suggested, run the sealert -l 84e0b04d-d0ad-4347-8317-22e74f6cd020 command to view the complete message. This command only works on the local machine, and presents the same information as the sealert GUI:
$ sealert -l 84e0b04d-d0ad-4347-8317-22e74f6cd020
Summary:
SELinux is preventing httpd (httpd_t) "getattr" to /var/www/html/file1
(samba_share_t).
Detailed Description:
SELinux denied access to /var/www/html/file1 requested by httpd.
/var/www/html/file1 has a context used for sharing by different program. If you
would like to share /var/www/html/file1 from httpd also, you need to change its
file context to public_content_t. If you did not intend to this access, this
could signal a intrusion attempt.
Allowing Access:
You can alter the file context by executing chcon -t public_content_t
'/var/www/html/file1'
Fix Command:
chcon -t public_content_t '/var/www/html/file1'
Additional Information:
Source Context unconfined_u:system_r:httpd_t:s0
Target Context unconfined_u:object_r:samba_share_t:s0
Target Objects /var/www/html/file1 [ file ]
Source httpd
Source Path /usr/sbin/httpd
Port <Unknown>
Host hostname
Source RPM Packages httpd-2.2.10-2
Target RPM Packages
Policy RPM selinux-policy-3.5.13-11.fc12
Selinux Enabled True
Policy Type targeted
MLS Enabled True
Enforcing Mode Enforcing
Plugin Name public_content
Host Name hostname
Platform Linux hostname 2.6.27.4-68.fc12.i686 #1 SMP Thu Oct
30 00:49:42 EDT 2008 i686 i686
Alert Count 4
First Seen Wed Nov 5 18:53:05 2008
Last Seen Wed Nov 5 01:22:58 2008
Local ID 84e0b04d-d0ad-4347-8317-22e74f6cd020
Line Numbers
Raw Audit Messages
node=hostname type=AVC msg=audit(1225812178.788:101): avc: denied { getattr } for pid=2441 comm="httpd" path="/var/www/html/file1" dev=dm-0 ino=284916 scontext=unconfined_u:system_r:httpd_t:s0 tcontext=unconfined_u:object_r:samba_share_t:s0 tclass=file
node=hostname type=SYSCALL msg=audit(1225812178.788:101): arch=40000003 syscall=196 success=no exit=-13 a0=b8e97188 a1=bf87aaac a2=54dff4 a3=2008171 items=0 ppid=2439 pid=2441 auid=502 uid=48 gid=48 euid=48 suid=48 fsuid=48 egid=48 sgid=48 fsgid=48 tty=(none) ses=3 comm="httpd" exe="/usr/sbin/httpd" subj=unconfined_u:system_r:httpd_t:s0 key=(null)
- Summary
A brief summary of the denied action. This is the same as the denial in /var/log/messages. In this example, the httpd process was denied access to a file (file1), which is labeled with the samba_share_t type.
- Detailed Description
A more verbose description. In this example, file1 is labeled with the samba_share_t
type. This type is used for files and directories that you want to
export via Samba. The description suggests changing the type to a type
that can be accessed by the Apache HTTP Server and Samba, if such access
is desired.
- Allowing Access
A suggestion for how to allow access. This may be relabeling
files, turning a Boolean on, or making a local policy module. In this
case, the suggestion is to label the file with a type accessible to both
the Apache HTTP Server and Samba.
- Fix Command
A suggested command to allow access and resolve the denial. In this example, it gives the command to change the file1 type to public_content_t, which is accessible to the Apache HTTP Server and Samba.
- Additional Information
Information that is useful in bug reports, such as the policy package name and version (selinux-policy-3.5.13-11.fc12), but may not help towards solving why the denial occurred.
- Raw Audit Messages
The raw audit messages from
/var/log/audit/audit.log that are associated with the denial. Refer to
Section 8.3.6, “Raw Audit Messages” for information about each item in the AVC denial.
8.3.8. Allowing Access: audit2allow
Do not use the example in this section in production. It is used only to demonstrate the use of audit2allow.
From the
audit2allow(1) manual page: "
audit2allow - generate SELinux policy allow rules from logs of denied operations"
[]. After analyzing denials as per
Section 8.3.7, “sealert Messages”, and if no label changes or Booleans allowed access, use
audit2allow to create a local policy module. After access is denied by SELinux, running the
audit2allow command presents Type Enforcement rules that allow the previously denied access.
The following example demonstrates using audit2allow to create a policy module:
A denial and the associated system call are logged to /var/log/audit/audit.log:
type=AVC msg=audit(1226270358.848:238): avc: denied { write } for pid=13349 comm="certwatch" name="cache" dev=dm-0 ino=218171 scontext=system_u:system_r:certwatch_t:s0 tcontext=system_u:object_r:var_t:s0 tclass=dir
type=SYSCALL msg=audit(1226270358.848:238): arch=40000003 syscall=39 success=no exit=-13 a0=39a2bf a1=3ff a2=3a0354 a3=94703c8 items=0 ppid=13344 pid=13349 auid=4294967295 uid=0 gid=0 euid=0 suid=0 fsuid=0 egid=0 sgid=0 fsgid=0 tty=(none) ses=4294967295 comm="certwatch" exe="/usr/bin/certwatch" subj=system_u:system_r:certwatch_t:s0 key=(null)
In this example,
certwatch (
comm="certwatch") was denied write access (
{ write }) to a directory labeled with the
var_t type (
tcontext=system_u:object_r:var_t:s0). Analyze the denial as per
Section 8.3.7, “sealert Messages”. If no label changes or Booleans allowed access, use
audit2allow to create a local policy module.
With a denial logged, such as the certwatch denial in step 1, run the audit2allow -w -a command to produce a human-readable description of why access was denied. The -a option causes all audit logs to be read. The -w option produces the human-readable description. The audit2allow tool accesses /var/log/audit/audit.log, and as such, must be run as the Linux root user:
# audit2allow -w -a
type=AVC msg=audit(1226270358.848:238): avc: denied { write } for pid=13349 comm="certwatch" name="cache" dev=dm-0 ino=218171 scontext=system_u:system_r:certwatch_t:s0 tcontext=system_u:object_r:var_t:s0 tclass=dir
Was caused by:
Missing type enforcement (TE) allow rule.
You can use audit2allow to generate a loadable module to allow this access.
As shown, access was denied due to a missing Type Enforcement rule.
Run the audit2allow -a command to view the Type Enforcement rule that allows the denied access:
# audit2allow -a
#============= certwatch_t ==============
allow certwatch_t var_t:dir write;
Missing Type Enforcement rules are usually caused by bugs in SELinux policy, and should be reported in
Red Hat Bugzilla. For Red Hat Enterprise Linux, create bugs against the
Red Hat Enterprise Linux product, and select the
selinux-policy component. Include the output of the
audit2allow -w -a and
audit2allow -a commands in such bug reports.
To use the rule displayed by audit2allow -a, run the audit2allow -a -M mycertwatch command as the Linux root user to create custom module. The -M option creates a Type Enforcement file (.te) with the name specified with -M, in your current working directory:
# audit2allow -a -M mycertwatch
******************** IMPORTANT ***********************
To make this policy package active, execute:
semodule -i mycertwatch.pp
# ls
mycertwatch.pp mycertwatch.te
Also, audit2allow compiles the Type Enforcement rule into a policy package (.pp). To install the module, run the /usr/sbin/semodule -i mycertwatch.pp command as the Linux root user.
Modules created with
audit2allow may allow more access than required. It is recommended that policy created with
audit2allow be posted to an SELinux list, such as
fedora-selinux-list, for review. If you believe their is a bug in policy, create a bug in
Red Hat Bugzilla.
If you have multiple denials from multiple processes, but only want to create a custom policy for a single process, use the grep command to narrow down the input for audit2allow. The following example demonstrates using grep to only send denials related to certwatch through audit2allow:
# grep certwatch /var/log/audit/audit.log | audit2allow -M mycertwatch2
******************** IMPORTANT ***********************
To make this policy package active, execute:
# /usr/sbin/semodule -i mycertwatch2.pp
