Re: [PATCH v2] locking/ww_mutex: Adjust to lockdep nest_lock requirements

["Thomas Hellström" <thomas.hellstrom@linux.intel.com>]

On Wed, 2024-10-09 at 15:10 +0200, Peter Zijlstra wrote:
> 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.

Thanks.
It takes some time for that failing CI test to run, though so, and
since I can't repro the failure locally I'll keep a watch out.

/Thomas