Re: [PATCH v19 0/4] overlayfs override_creds=off & nested get xattr fix

From: David Anderson
Date: Thu Nov 18 2021 - 04:54:27 EST

On Tue, Nov 16, 2021 at 11:36 PM Amir Goldstein <amir73il@xxxxxxxxx> wrote:
> Hi David,
> I see that the patch set has changed hands (presumably to Android upstreaming
> team), but you just rebased v18 without addressing the maintainers concerns [1].

Indeed I'm carrying this forward as Mark is no longer working on it.
My apologies for
missing those comments!

> Specifically, the patch 2/4 is very wrong for unprivileged mount and
> I think that the very noisy patch 1/4 could be completely avoided:
> Can't you use -o userxattr mount option for Android use case and limit
> the manipulation of user.ovrelay.* xattr based on sepolicy for actors
> that are allowed
> to make changes in overlayfs mount? or not limit at all?
> The access to those xattr is forbidden via "incoming" xattr ops on
> overlay inodes.

Can you clarify a bit more? The patch is definitely super noisy and I'd love
to have a better solution. The problem it's trying to solve is:
1. Kernel-privileged init mounts /mnt/blah-lower and /mnt/blah-upper.
2. Kernel-privileged init mounts /blah with overlayfs using the above dirs.
2. Kernel-privileged init loads sepolicy off /blah/policy. Enforcing begins.
3. Kernel-privileged init tries to execute /blah/init to initiate a
domain transition.
4. exec() fails because the overlayfs mounter creds (kernel domain) does
not have getxattr permission to /blah/init.

Eg, we're hitting this problem without even making changes to the mount, and
without anything being written to /mnt/blah-upper.

> Can an unprivileged user create an overlay over a directory that they have
> access to and redirect an innocent looking file name to an underlying file that
> said the mounting user has no access to and by doing that, tricking a privileged
> user to modify the innocent looking file on the mounter's behalf?
> Of course this could be avoided by forbidding unprivileged mount with
> override_creds=off, but there could be other scenarios, so a clear model
> would help to understand the risks.
> For example:
> If user 1 was able to read in lower dir A, now the content of overlay dir A
> is cached and user 2, that has permissions to read upper dir A and does
> not have read permissions on lower dir A will see the content of lower dir A.

I'll need to think about this more and test to verify. It's not a scenario that
would come up in our use case (both dirs effectively have the same permissions).

If the answer is "yes, that can happen" - do you see this as a problem of
clarifying the model, or a problem of fixing that loophole?

>> I think that the core problem with the approach is using Non-uniform
> credentials to access underlying layers. I don't see a simple way around
> a big audit that checks all those cases, but maybe I'm missing some quick
> shortcut or maybe your use case can add some restrictions about the
> users that could access this overlay that would simplify the generic problem.

In a security model like ours, I think there's no way around it, that
we really need
accesses to be from the caller's credentials and not the mounter's. It's even
worse than earlier iterations of this patch perhaps let on: we mount
before sepolicy
is loaded (so we can overlay the policy itself), and thus the
mounter's creds are
effectively "the kernel". This domain is highly restricted in our
sepolicy for obvious
reasons. There's no way our security team will let us unrestrict it.



> Thanks,
> Amir.
> [1]