Re: [PATCH RESEND] locking/ww_mutex: Adjust to lockdep nest_lock requirements

[Thomas Hellström]

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