linux-kernel - Re: [PATCH v19 0/4] overlayfs override

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Date:   Wed, 22 Mar 2023 08:28:50 +0100
From:   Johannes Segitz <jsegitz@...e.com>
To:     Paul Moore <paul@...l-moore.com>
Cc:     Amir Goldstein <amir73il@...il.com>,
        Vivek Goyal <vgoyal@...hat.com>,
        Miklos Szeredi <miklos@...redi.hu>,
        David Anderson <dvander@...gle.com>,
        Mark Salyzyn <salyzyn@...roid.com>,
        Jonathan Corbet <corbet@....net>,
        "Eric W . Biederman" <ebiederm@...ssion.com>,
        Randy Dunlap <rdunlap@...radead.org>,
        Stephen Smalley <sds@...ho.nsa.gov>,
        John Stultz <john.stultz@...aro.org>,
        linux-doc@...r.kernel.org,
        linux-kernel <linux-kernel@...r.kernel.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        overlayfs <linux-unionfs@...r.kernel.org>,
        LSM List <linux-security-module@...r.kernel.org>,
        kernel-team <kernel-team@...roid.com>, selinux@...r.kernel.org,
        paulmoore@...rosoft.com, luca.boccassi@...rosoft.com
Subject: Re: [PATCH v19 0/4] overlayfs override_creds=off & nested get xattr
 fix

On Fri, Mar 11, 2022 at 03:52:54PM -0500, Paul Moore wrote:
> On Fri, Mar 11, 2022 at 9:01 AM Vivek Goyal <vgoyal@...hat.com> wrote:
> > Agreed. After going through the patch set, I was wondering what's the
> > overall security model and how to visualize that.
> >
> > So probably there needs to be a documentation patch which explains
> > what's the new security model and how does it work.
> 
> Yes, of course.  I'll be sure to add a section to the existing docs.
> 
> > Also think both in terms of DAC and MAC. (Instead of just focussing too
> > hard on SELinux).
> 
> Definitely.  Most of what I've been thinking about the past day or so
> has been how to properly handle some of the DAC/capability issues; I
> have yet to start playing with the code, but for the most part I think
> the MAC/SELinux bits are already working properly.
> 
> > My understanding is that in current model, some of the overlayfs
> > operations require priviliges. So mounter is supposed to be priviliged
> > and does the operation on underlying layers.
> >
> > Now in this new model, there will be two levels of check. Both overlay
> > level and underlying layer checks will happen in the context of task
> > which is doing the operation. So first of all, all tasks will need
> > to have enough priviliges to be able to perform various operations
> > on lower layer.
> >
> > If we do checks at both the levels in with the creds of calling task,
> > I guess that probably is fine. (But will require a closer code inspection
> > to make sure there is no privilege escalation both for mounter as well
> > calling task).
> 
> I have thoughts on this, but I don't think I'm yet in a position to
> debate this in depth just yet; I still need to finish poking around
> the code and playing with a few things :)
> 
> It may take some time before I'm back with patches, but I appreciate
> all of the tips and insight - thank you!

Let me resurrect this discussion. With 
https://github.com/fedora-selinux/selinux-policy/commit/1e8688ea694393c9d918939322b72dfb44a01792
the Fedora policy changed kernel_t to a confined domain. This means that
many overlayfs setups that are created in initrd will now run into issues,
as it will have kernel_t as part of the saved credentials. So while the
original use case that inspired the patch set was probably not very common
that now changed.

It's tricky to work around this. Loading a policy in initrd causes a lot of
issues now that kernel_t isn't unconfined anymore. Once the policy is
loaded by systemd changing the mounts is tough since we use it for /etc and
at this time systemd already has open file handles for policy files in
/etc.

Johannes
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