Re: [PATCH v8 2/2] overlayfs: override_creds=off option bypass creator_cred

[Vivek Goyal <vgoyal@redhat.com>]

On Thu, Nov 08, 2018 at 01:28:32PM -0800, Mark Salyzyn wrote:
> On 11/08/2018 12:01 PM, Vivek Goyal wrote:
> > On Tue, Nov 06, 2018 at 03:01:15PM -0800, Mark Salyzyn wrote:
> > > By default, all access to the upper, lower and work directories is the
> > > recorded mounter's MAC and DAC credentials.  The incoming accesses are
> > > checked against the caller's credentials.
> > Ok, I am trying to think of scenarios where override_creds=off can
> > provide any privilege escalation. How about following.
> > 
> > $ mkdir lower lower/foo upper upper/foo work merged
> > $ touch lower/foo/bar.txt
> > $ chmod 700 lower/foo/
> > 
> > # Mount overlay with override_creds=off
> > 
> > $ mount -t overlay -o
> > lowerdir=lower,upperdir=upper,workdir=work,override_creds=off none merged
> > 
> > # Try to read lower/foo as unpriviliged user. Say "test"
> > # su test
> > # ls merged/foo/
> > ls: cannot access 'merged/foo/': Operation not permitted
> > 
> > # Now first try to do same operation as root and retry as test user.
> > $ exit
> > $ ls merged/foo
> > bar.txt
> > $ su test
> > $ ls merged/foo
> > bar.txt
> > 
> > lower/foo/ is not readable by user "test". So it fails in first try. Later
> > "root" accesses it and it populates cache in overlayfs. When test retries,
> > it gets these entries from cache.
> > 
> > With override_creds=on this is not a problem because overlay provides
> > this as functionality as long as mounter as access to lower/foo/.
> > 
> > But with override_creds=off, mounter is not providing any such
> > functionality and we are exposing an issue where cache will make
> > something available which is not normally available.
> > 
> > I think it probably is a good idea to do something about it?
> > 
> > Thanks
> > Vivek
> > 
> Good stuff.
> 
> That sounds like a bug in cache (!) to not recheck caller's credentials.
> Currently unsure how/where to force bypass of the cache (performance hit) as
> it is wired in throughout the code without a clear off switch, or rechecking
> of the credentials at access. This does need to be addressed to make this
> 'feature' more useful/trusted for non-MAC controlled, use cases.

DAC is just an example. There is no reason same issue will not happen
with MAC? Proacess A with correct MAC priviliges will fill overlay
cache and process B without correct MAC priviliges will still be able
to get information about dentry.

As Amir suggested, for now documenting this probably is fine. I can't
think of any other good options either. May be Miklos has some ideas here.

Thanks
Vivek