SELinux Cheat Sheet

CHAPTER 41. TROUBLESHOOTING PROBLEMS RELATED TO SELINUX

If you plan to enable SELinux on systems where it has been previously disabled or if you run a service in a non-standard configuration, you might need to troubleshoot situations potentially blocked by SELinux. Note that in most cases, SELinux denials are signs of misconfiguration.

TROUBLESHOOTING PROBLEMS RELATED TO SELINUX

41.1. Identifying SELinux denials

Follow only the necessary steps from this procedure; in most cases, you need to perform just step 1.

Procedure

  1. When your scenario is blocked by SELinux, the /var/log/audit/audit.log file is the first place to check for more information about a denial. To query Audit logs, use the ausearch tool. Because the SELinux decisions, such as allowing or disallowing access, are cached and this cache is known as the Access Vector Cache (AVC), use the AVC and USER_AVC values for the message type parameter, for example:

    # ausearch -m AVC,USER_AVC,SELINUX_ERR,USER_SELINUX_ERR -ts recent

    If there are no matches, check if the Audit daemon is running. If it does not, repeat the denied scenario after you start auditd and check the Audit log again.

  2. In case auditd is running, but there are no matches in the output of ausearch, check messages provided by the systemd Journal:

    # journalctl -t setroubleshoot
  3. If SELinux is active and the Audit daemon is not running on your system, then search for certain SELinux messages in the output of the dmesg command:

    # dmesg | grep -i -e type=1300 -e type=1400
  4. Even after the previous three checks, it is still possible that you have not found anything. In this case, AVC denials can be silenced because of dontaudit rules.

    To temporarily disable dontaudit rules, allowing all denials to be logged:

    # semodule -DB

    After re-running your denied scenario and finding denial messages using the previous steps, the following command enables dontaudit rules in the policy again:

    # semodule -B
  5. If you apply all four previous steps, and the problem still remains unidentified, consider if SELinux really blocks your scenario:

    1. Switch to permissive mode:

      # setenforce 0
      $ getenforce
      Permissive
    2. Repeat your scenario.

    If the problem still occurs, something different than SELinux is blocking your scenario.

41.2. Analyzing SELinux denial messages

After identifying that SELinux is blocking your scenario, you might need to analyze the root cause before you choose a fix.

Prerequisites

  • The policycoreutils-python-utils and setroubleshoot-server packages are installed on your system.

Procedure

  1. List more details about a logged denial using the sealert command, for example:

    $ sealert -l "*"
    SELinux is preventing /usr/bin/passwd from write access on the file
    /root/test.
    
    *****  Plugin leaks (86.2 confidence) suggests *****************************
    
    If you want to ignore passwd trying to write access the test file,
    because you believe it should not need this access.
    Then you should report this as a bug.
    You can generate a local policy module to dontaudit this access.
    Do
    # ausearch -x /usr/bin/passwd --raw | audit2allow -D -M my-passwd
    # semodule -X 300 -i my-passwd.pp
    
    *****  Plugin catchall (14.7 confidence) suggests **************************
    
    ...
    [trimmed for clarity]
    ...
    
    Raw Audit Messages
    type=AVC msg=audit(1553609555.619:127): avc:  denied  { write } for
    pid=4097 comm="passwd" path="/root/test" dev="dm-0" ino=17142697
    scontext=unconfined_u:unconfined_r:passwd_t:s0-s0:c0.c1023
    tcontext=unconfined_u:object_r:admin_home_t:s0 tclass=file permissive=0
    
    ...
    [trimmed for clarity]
    ...
    
    Hash: passwd,passwd_t,admin_home_t,file,write
  2. If the output obtained in the previous step does not contain clear suggestions:

    • Enable full-path auditing to see full paths to accessed objects and to make additional Linux Audit event fields visible:

      # auditctl -w /etc/shadow -p w -k shadow-write
    • Clear the setroubleshoot cache:

      # rm -f /var/lib/setroubleshoot/setroubleshoot.xml
    • Reproduce the problem.

    • Repeat step 1.

  3. If sealert returns only catchall suggestions or suggests adding a new rule using the audit2allow tool, match your problem with examples listed and explained in SELinux denials in the Audit log.

Additional resources

  • The sealert(8) man page.

41.3. Fixing analyzed SELinux denials

In most cases, suggestions provided by the sealert tool give you the right guidance about how to fix problems related to the SELinux policy. See Analyzing SELinux denial messages for information how to use sealert to analyze SELinux denials.

Be careful when the tool suggests using the audit2allow tool for configuration changes. You should not use audit2allow to generate a local policy module as your first option when you see an SELinux denial. Troubleshooting should start with a check if there is a labeling problem. The second most often case is that you have changed a process configuration, and you forgot to tell SELinux about it.

Labeling problems

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 might want to use /srv/myweb/. On Red Hat Enterprise Linux, the /srv directory is labeled with the var_t type. Files and directories created in /srv inherit this type. Also, newly-created objects in top-level directories, such as /myserver, can 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 by httpd:

# 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 utility does not change the context. As root, use the restorecon utility to apply the changes:

~]# restorecon -R -v /srv/myweb

Incorrect context

The matchpathcon utility checks the context of a file path and compares it to the default label for that path. The following example demonstrates the use of matchpathcon on a directory that contains incorrectly labeled files:

$ 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 utility to restore such files to their correct type:

~]# 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:

# 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

Confined applications configured in non-standard ways

Services can be run in a variety of ways. To account for that, you need to specify how you run your services. You can achieve this through SELinux booleans that allow parts of SELinux policy to be changed at runtime. This enables changes, such as allowing services access to NFS volumes, without reloading or recompiling SELinux policy. Also, running services on non-default port numbers requires policy configuration to be updated using the semanage command.

For example, to allow the Apache HTTP Server to communicate with MariaDB, enable the httpd_can_network_connect_db boolean:

# setsebool -P httpd_can_network_connect_db on

Note that the 

  • P

 option makes the setting persistent across reboots of the system.

If access is denied for a particular service, use the getsebool and grep utilities 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:

$ getsebool -a | grep ftp
ftpd_anon_write --> off
ftpd_full_access --> off
ftpd_use_cifs --> off
ftpd_use_nfs --> off

ftpd_connect_db --> off
httpd_enable_ftp_server --> off
tftp_anon_write --> off

To get a list of booleans and to find out if they are enabled or disabled, use the getsebool -a command. To get a list of booleans including their meaning, and to find out if they are enabled or disabled, install the selinux-policy-devel package and use the semanage boolean -l command as root.

Port numbers

Depending on policy configuration, services can 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 root to list http related ports:

# 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 following command fails:

# systemctl start httpd.service
Job for httpd.service failed. See 'systemctl status httpd.service' and 'journalctl -xn' for details.

# systemctl status httpd.service
httpd.service - The Apache HTTP Server
   Loaded: loaded (/usr/lib/systemd/system/httpd.service; disabled)
   Active: failed (Result: exit-code) since Thu 2013-08-15 09:57:05 CEST; 59s ago
  Process: 16874 ExecStop=/usr/sbin/httpd $OPTIONS -k graceful-stop (code=exited, status=0/SUCCESS)
  Process: 16870 ExecStart=/usr/sbin/httpd $OPTIONS -DFOREGROUND (code=exited, status=1/FAILURE)

An SELinux denial message 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, use the semanage port command to assign a different label to the port:

# 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.

Corner cases, evolving or broken applications, and compromised systems

Applications may contain bugs, 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 does not account for, causing access to be denied, even though access should be allowed.

For these situations, after access is denied, use the audit2allow utility to create a custom policy module to allow access. You can report missing rules in the SELinux policy in Red Hat Bugzilla. For Red Hat Enterprise Linux 8, create bugs against the Red Hat Enterprise Linux 8 product, and select the selinux-policy component. Include the output of the audit2allow -w -a and audit2allow -a commands in such bug reports.

If an application asks for major security privileges, it could be a signal that the application is compromised. Use intrusion detection tools to inspect such suspicious behavior.

The Solution Engine on the Red Hat Customer Portal can also provide guidance in the form of an article containing a possible solution for the same or very similar problem you have. Select the relevant product and version and use SELinux-related keywords, such as selinux or avc, together with the name of your blocked service or application, for example: selinux samba.

41.4. SELinux denials in the Audit log

The Linux Audit system stores log entries in the /var/log/audit/audit.log file by default. To list only SELinux-related records, use the ausearch command with the message type parameter set to AVC and AVC_USER at a minimum, for example:

# ausearch -m AVC,USER_AVC,SELINUX_ERR,USER_SELINUX_ERR

An SELinux denial entry in the Audit log file can look as follows:

type=AVC msg=audit(1395177286.929:1638): avc:  denied  { read } for  pid=6591 comm="httpd" name="webpages" dev="0:37" ino=2112 scontext=system_u:system_r:httpd_t:s0 tcontext=system_u:object_r:nfs_t:s0 tclass=dir

The most important parts of this entry are:

  • avc: denied

    • the action performed by SELinux and recorded in Access Vector Cache (AVC)

  • { read }

    • the denied action

  • pid=6591

    • the process identifier of the subject that tried to perform the denied action

  • comm="httpd"

    • the name of the command that was used to invoke the analyzed process

  • httpd_t

    • the SELinux type of the process

  • nfs_t

    • the SELinux type of the object affected by the process action

  • tclass=dir

    • the target object class

The previous log entry can be translated to:

SELinux denied the httpd process with PID 6591 and the httpd_t type to read from a directory with the nfs_t type.

The following SELinux denial message occurs when the Apache HTTP Server attempts to access a directory labeled with a type for the Samba suite:

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
  • { getattr }

    • the 

    getattr entry indicates the source process was trying to read the target file’s status information. This occurs before reading files. SELinux denies this action because the process accesses the file and it does not have an appropriate label. Commonly seen permissions include getattrread, and write.

  • 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 (source) that attempted the denied action. In this case, it is the SELinux context of the Apache HTTP Server, which is running with the 

    httpd_t type.

  • 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.

This SELinux denial can be translated to:

SELinux denied the httpd process with PID 2465 to access the /var/www/html/file1 file with the samba_share_t type, which is not accessible to processes running in the httpd_t domain unless configured otherwise.

Additional resources

  • For more information, see the auditd(8) and ausearch(8) man pages.

41.5. Related information

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