SELINUX: Diagnose and address routine SELinux policy violations.

Note: This is an RHCSA 7 exam objective.

Introduction

In RHEL 7, each package doesn’t store its own SELinux policy. The SELinux policy is stored in one and only one package called selinux-policy-targeted.

When a policy has been written for a given process and SELinux is in Permissive or Enforcing mode, all action not allowed by the SELinux policy will trigger a violation.

The following procedure will give you some details about any SELinux policy violation.

Main Procedure

Install the setroubleshoot-server package:

# yum install -y setroubleshoot-server

Note: In fact, it’s the policycoreutils-python package that really contains the semanage command. However, I have always found the setroubleshoot-server package name, that contains the policycoreutils-python package itself, easier to remember!

Display the SELinux policy violations:

# sealert -a /var/log/audit/audit.log

In addition, when an AVC (Access Vector Cache) event occurs, you can grab the associated line displayed in the /var/log/audit/audit.log file and send it to the audit2why command to get a diagnostic.

For example, let’s assume you’ve got this line in your /var/log/audit/audit.log file:

type=AVC msg=audit(1415714880.156:29): avc:  denied  { name_connect } for  pid=1349 \
  comm="nginx" dest=8080 scontext=unconfined_u:system_r:httpd_t:s0 \
  tcontext=system_u:object_r:http_cache_port_t:s0 tclass=tcp_socket

Execute this command to get a diagnostic:

# grep 1415714880.156:29 /var/log/audit/audit.log | audit2why

        Was caused by:
        One of the following booleans was set incorrectly.
        Description:
        Allow httpd to act as a relay

        Allow access by executing:
        # setsebool -P httpd_can_network_relay 1
        Description:
        Allow HTTPD scripts and modules to connect to the network using TCP.

        Allow access by executing:
        # setsebool -P httpd_can_network_connect 1

This will make your investigation much easier!

Additional Resources

Jens Depuydt’s blog provides a good article called SELinux in a practical way about this topic.
Sander van Vugt offers an interesting video about Fixing SELinux Issues (48min/2015).
In addition, Red Hat provides a video about Monitoring SELinux Violations (10min/2016).
During the 2016 DevConf.cz a presentation was given about the Big SElinux Troubleshooting Chart (95min/2016) (pdf here).

Beyond the exam objectives, you could be interested in this post from Dan Walsh about SELinux Users and Roles.
The mgrepl website also provides very interesting articles about SELinux security policy.