linux-kernel - Re: [PATCH RESEND] locking/ww_mutex: Adjust to lockdep nest

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Message-ID: <c8f3278c442de947a167876a565c2c96c6264cc3.camel@linux.intel.com>
Date: Fri, 04 Oct 2024 12:50:27 +0200
From: Thomas Hellström <thomas.hellstrom@...ux.intel.com>
To: Peter Zijlstra <peterz@...radead.org>
Cc: intel-xe@...ts.freedesktop.org, Ingo Molnar <mingo@...hat.com>, Will
 Deacon <will@...nel.org>, Waiman Long <longman@...hat.com>, Boqun Feng
 <boqun.feng@...il.com>, Maarten Lankhorst <maarten@...khorst.se>, Christian
 König <christian.koenig@....com>,
 dri-devel@...ts.freedesktop.org,  linux-kernel@...r.kernel.org
Subject: Re: [PATCH RESEND] locking/ww_mutex: Adjust to lockdep nest_lock
 requirements

On Fri, 2024-10-04 at 12:16 +0200, Peter Zijlstra wrote:
> On Wed, Oct 02, 2024 at 02:56:11PM +0200, Thomas Hellström wrote:
> > When using mutex_acquire_nest() with a nest_lock, lockdep refcounts
> > the
> > number of acquired lockdep_maps of mutexes of the same class, and
> > also
> > keeps a pointer to the first acquired lockdep_map of a class. That
> > pointer
> > is then used for various comparison-, printing- and checking
> > purposes,
> > but there is no mechanism to actively ensure that lockdep_map stays
> > in
> > memory. Instead, a warning is printed if the lockdep_map is freed
> > and
> > there are still held locks of the same lock class, even if the
> > lockdep_map
> > itself has been released.
> > 
> > In the context of WW/WD transactions that means that if a user
> > unlocks
> > and frees a ww_mutex from within an ongoing ww transaction, and
> > that
> > mutex happens to be the first ww_mutex grabbed in the transaction,
> > such a warning is printed and there might be a risk of a UAF.
> 
> I'm assuming you actually hit this?

Yes, but there was a change merged to drm_exec, a main ww_mutex user
that makes it less likely to happen. (Unlocking in reverse unless the
user explicitly requests an unlock which will be a more common use-case
moving forward).

> 
> Anyway, work around seems sane enough, thanks!

Thanks,
Thomas