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
