linux-kernel - Re: [PATCH] cgroup1: fix leaked context root causing sporadic NULL deref in LTP

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Date:   Wed, 30 Jun 2021 17:10:36 +0100
From:   Mark Brown <broonie@...nel.org>
To:     Tejun Heo <tj@...nel.org>
Cc:     Paul Gortmaker <paul.gortmaker@...driver.com>,
        linux-kernel@...r.kernel.org, cgroups@...r.kernel.org,
        Al Viro <viro@...iv.linux.org.uk>,
        Zefan Li <lizefan.x@...edance.com>,
        Johannes Weiner <hannes@...xchg.org>, stable@...r.kernel.org,
        Richard Purdie <richard.purdie@...uxfoundation.org>
Subject: Re: [PATCH] cgroup1: fix leaked context root causing sporadic NULL
 deref in LTP

On Wed, Jun 16, 2021 at 11:23:34AM -0400, Tejun Heo wrote:
> On Wed, Jun 16, 2021 at 08:51:57AM -0400, Paul Gortmaker wrote:

> > A fix would be to not leave the stale reference in fc->root as follows:

> >    --------------
> >                   dput(fc->root);
> >   +               fc->root = NULL;
> >                   deactivate_locked_super(sb);
> >    --------------

> > ...but then we are just open-coding a duplicate of fc_drop_locked() so we
> > simply use that instead.

> As this is unlikely to be a real-world problem both in probability and
> circumstances, I'm applying this to cgroup/for-5.14 instead of
> cgroup/for-5.13-fixes.

FWIW at Arm we've started seeing what appears to be this issue blow up
very frequently in some of our internal LTP CI runs against -next, seems
to be mostly on lower end platforms.  We seem to have started finding it
at roughly the same time that the Yocto people did, I guess some other
change made it more likely to trigger.  Not exactly real world usage
obviously but it's creating quite a bit of noise in testing which is
disruptive so it'd be good to get it into -next as a fix.

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