How does foo_t get this privilege?


Sven Vermeulen Sun 05 January 2014

Today a question was raised how the unprivileged user domain user_t was allowed to write to cgroup_t files. There is nothing obvious about that in the roles/unprivuser.te file, so what gives?

I used a simple script (which I've been using for a while already) called seshowtree which presents the SELinux rules for a particular domain in a tree-like structure, expanding the interfaces as it finds them. The script is far from perfect, but does enough to help me answer such questions. If you're interested, the script is also available on my github small.coding project.

~# seshowtree user roles/unprivuser.te > output.txt

In the resulting output, I search for the cgroup_t and work my way up, which gives:

. userdom_common_user_template($1)
. . fs_rw_cgroup_files($1_t)
. . . rw_files_pattern($1, cgroup_t, cgroup_t)
. . . . allow $1 $3:file rw_file_perms;

So in this case, the user forgot to look into userdom_common_user_template, which is called by userdom_unpriv_user_template to find the path to this privilege. Of course, that still doesn't explain why the privileges are assigned in the first place. As the policy file itself does not contain the necessary comments to deduce this, I had to ask the git repository for more information:

~$ git annotate userdomain.if

In the end, it was a commit from 2010, informing me that "Common users can read and write cgroup files (access governed by dac)". So the privilege is by design, referring to the regular DAC permissions to properly govern access to the files.