Creating local module quickly in CIL!


Today, I’ll show you how to create local policy module for testing purposes or workaround while issue will be fixed in our distro selinux-policy. Local modules will be written in CIL (common intermediate language) so this post concerns Fedora 23 and higher.

As an example we can look on following BZ:

In description of BZ we can find this AVC:

type=AVC msg=audit(1466793837.663:1360): avc:  denied  { setrlimit }
for  pid=15770 comm="zabbix_agentd" scontext=system_u:system_r:zabbix_agent_t:s0
tcontext=system_u:system_r:zabbix_agent_t:s0 tclass=process permissive=0

Important parts of AVC for us:

avc:  denied  { setrlimit }

From these parts we can create allowing rule in CIL. Template for creating allow rules:

(allow source_context target_context (tclass (permissions)))

If we apply template on our AVC it looks like this:

(allow zabbix_agent_t zabbix_agent_t(process (setrlimit)))

Now, we add this line to file:
(Note: Name of your local module must be different than any other used in distro policy!)

$ cat zabbix_setrlimit.cil 
(allow zabbix_agent_t zabbix_agent_t(process (setrlimit)))

Finally, we could load this local module into kernel:

# semodule -i zabbix_setrlimit.cil

And that’s it! Rule from AVC report is loaded into kernel! You can verify it using:

$ sesearch -A -s zabbix_agent_t -t zabbix_agent_t -c process -p setrlimit
Found 1 semantic av rules:
   allow zabbix_agent_t zabbix_agent_t : process { fork sigchld sigkill sigstop signull signal getsched setsched setpgid getcap setrlimit } ;

Pretty easy, don’t you think? 😉

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