Re: virtiofs and its optional xattr support vs. fs_use_xattr

From: Paul Moore <paul@paul-moore.com>
To: James Carter <jwcart2@gmail.com>
Cc: Ondrej Mosnacek <omosnace@redhat.com>,
	SElinux list <selinux@vger.kernel.org>,
	Stephen Smalley <stephen.smalley.work@gmail.com>,
	Vivek Goyal <vgoyal@redhat.com>, Daniel Walsh <dwalsh@redhat.com>,
	Zdenek Pytela <zpytela@redhat.com>
Subject: Re: virtiofs and its optional xattr support vs. fs_use_xattr
Date: Tue, 8 Dec 2020 18:45:00 -0500	[thread overview]
Message-ID: <CAHC9VhSYt86rQJGg9JFD1ubL4N3PNmLg02sACOvRNHr94=41dg@mail.gmail.com> (raw)
In-Reply-To: <CAP+JOzSWW6HFtN1VSyvKacKeZdtRkBo7TWR7JS4a0ewiopuKww@mail.gmail.com>

On Mon, Dec 7, 2020 at 12:17 PM James Carter <jwcart2@gmail.com> wrote:
> On Mon, Dec 7, 2020 at 9:45 AM Ondrej Mosnacek <omosnace@redhat.com> wrote:
> >
> > Hi everyone,
> >
> > In [1] we ran into a problem with the current handling of filesystem
> > labeling rules. Basically, it is only possible to specify either
> > genfscon or fs_use_xattr for a given filesystem, but in the case of
> > virtiofs, certain mounts may support security xattrs, while other ones
> > may not.
> >
> > So we can't use the xattr support by adding fs_use_xattr virtiofs
> > (...); to the policy, because then a non-xattr mount will fail
> > (SELinux does a mount-time check on the root inode to make sure that
> > the xattr handler works), but we also don't want to stay on genfscon,
> > because then we can't relabel files.
> >
> > So my question is how to best address this? One option is to use a
> > similar "hack" as for cgroupfs; i.e. do a kind of mixed genfs-xattr
> > labeling, but that's ugly and requires hard-coding another FS name in
> > the selinux code. The only other alternative I could come up with is
> > to add a new FS labeling statement that would specify some kind of
> > mixed genfscon / fs_use_xattr behavior. That would be a better
> > long-term solution, but leads to more questions on how such statement
> > should actually work... Should it work the cgroupfs way, giving a
> > default label to everything and allowing to set/change labels via
> > xattrs? Or should it rather just detect xattrs support and switch
> > between SECURITY_FS_USE_XATTR and SECURITY_FS_USE_GENFS behavior based
> > on that? In the latter case, should the statement specify two contexts
> > (one for fs_use_xattr and another one for genfscon) or just one for
> > both behaviors?
>
> I don't think adding a new statement is necessary. It seems like
> allowing both fs_use_xattr and genfscon rules for the filesystem in
> policy and then using the fs_use_xattr rule if xattrs are supported
> while falling back to the genfscon rule if they are not would do what
> you need.

That seems reasonable to me so long as this ambiguity is okay with the
folks who do policy analysis.  Thinking quickly I'm not sure why it
would be a problem, but the thought did occur while I was typing up
this reply ...

-- 
paul moore
www.paul-moore.com