linux-kernel - Re: [PATCH v2] locking/ww_mutex: Adjust to lockdep nest

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Message-ID: <20241009131015.GP17263@noisy.programming.kicks-ass.net>
Date: Wed, 9 Oct 2024 15:10:15 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: Thomas Hellström <thomas.hellstrom@...ux.intel.com>
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 v2] locking/ww_mutex: Adjust to lockdep nest_lock
 requirements

On Wed, Oct 09, 2024 at 11:20:31AM +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.
> 
> Note that this is only problem when lockdep is enabled and affects only
> dereferences of struct lockdep_map.
> 
> Adjust to this by adding a fake lockdep_map to the acquired context and
> make sure it is the first acquired lockdep map of the associated
> ww_mutex class. Then hold it for the duration of the WW/WD transaction.
> 
> This has the side effect that trying to lock a ww mutex *without* a
> ww_acquire_context but where a such context has been acquire, we'd see
> a lockdep splat. The test-ww_mutex.c selftest attempts to do that, so
> modify that particular test to not acquire a ww_acquire_context if it
> is not going to be used.
> 
> v2:
> - Lower the number of locks in the test-ww_mutex
>   stress(STRESS_ALL) test to accommodate the dummy lock
>   introduced in this patch without overflowing lockdep held lock
>   references.

Thanks, I rebased tip/locking/core, which should now have this patch.