Re: About commit 901ef845fa2469c ("selinux: allow per-file labeling for cgroupfs")

[Stephen Smalley <sds@tycho.nsa.gov>]

On Fri, 2017-10-06 at 13:53 -0400, Waiman Long wrote:
> Antonio,
> 
> I have a question about your 4.14 upstream commit 901ef845fa2469c
> ("selinux: allow per-file labeling for cgroupfs"). With that, I am no
> longer able to mount the cgroup2 filesystem with a 4.14 kernel. The
> problem is that your commit sets the SE_SBGENFS flag, which causes
> selinux to lookup the genfs database for a filesystem type match.
> However, the filesystem type "cgroup2" isn't in the genfs database in
> my
> RHEL7 based test system. The "cgroup" filesystem type is in the genfs
> database,
> so I have no problem with v1 cgroup mount.
> 
> Do you know where the genfs database is defined? I need some way to
> add cgroup2
> as a valid genfs fstype, or I have to manually back out the commit in
> order to
> do my cgroup2 testing.

It is part of the policy; you could add it via a policy module ala:
$ cat cgroup2.cil
(genfscon cgroup2 / (system_u object_r cgroup_t ((s0) (s0))))

$ sudo semodule -i cgroup2.cil

That said, the fact that you can't even mount it without that is
arguably a bug/regression.  I guess this is due to the ENOENT from
security_genfs_sid being propagated all the way up instead of just
leaving it unlabeled and permitting the mount to proceed.