25.2.7 Troubleshooting Access-Denial Messages

The decisions that SELinux has made about allowing denying access are stored in the Access Vector Cache (AVC). If the auditing service (auditd) is not running, SELinux logs AVC denial messages to /var/log/messages. Otherwise, the messages are logged to /var/log/audit/audit.log. If the setroubleshootd daemon is running, easier-to-read versions of the denial messages are also written to /var/log/messages.

If you have installed the setroubleshoot and setroubleshoot-server packages, the auditd and setroubleshoot services are running, and you are using the X Window System, you can use the sealert -b command to run the SELinux Alert Browser, which displays information about SELinux AVC denials. To view the details of the alert, click Show. To view a recommended solution, click Troubleshoot.

If you do not use the SELinux Alert Browser, you can search in /var/log/audit/audit.log for messages containing the string denied, and in /var/log/messages for messages containing the string SELinux is preventing. For example:

# grep denied /var/log/audit/audit.log 
type=AVC msg=audit(1364486257.632:26178): avc:  denied  { read } for  
pid=5177 comm="httpd" name="index.html" dev=dm-0 ino=396075 
scontext=unconfined_u:system_r:httpd_t:s0 
tcontext=unconfined_u:object_r:acct_data_t:s0 tclass=file

The main causes of access-denial problems are:

  • The context labels for an application or file are incorrect.

    A solution might be to change the default file type of the directory hierarchy. For example, change the default file type from /var/webcontent to httpd_sys_content_t:

    # /usr/sbin/semanage fcontext -a -t httpd_sys_content_t "/var/webcontent(/.*)?"
    # /sbin/restorecon -R -v /var/webcontent
  • A Boolean that configures a security policy for a service is set incorrectly.

    A solution might be to change the value of a Boolean. For example, allow users' home directories to be browsable by turning on httpd_enable_homedirs:

    # setsebool -P httpd_enable_homedirs on
  • A service attempts to access a port to which a security policy does not allow access.

    If the service's use of the port is valid, a solution is to use semanage to add the port to the policy configuration. For example, allow the Apache HTTP server to listen on port 8000:

    # semanage port -a -t http_port_t -p tcp 8000
  • An update to a package causes an application to behave in a way that breaks an existing security policy.

    You can use the audit2allow -w -a command to view the reason why an access denial occurred.

    If you then run the audit2allow -a -M module command, it creates a type enforcement (.te) file and a policy package (.pp) file. You can use the policy package file with the semodule -i module.pp command to stop the error from reoccurring. This procedure is usually intended to allow package updates to function until an amended policy is available. If used incorrectly, it can create potential security holes on your system.