Re: [PATCH v2] mm/mmu_notifier.c: Fix race in mmu_interval_notifier_remove()

[Jason Gunthorpe <jgg@nvidia.com>]

On Wed, Apr 20, 2022 at 03:11:42PM -0700, Andrew Morton wrote:
> On Wed, 20 Apr 2022 14:37:34 +1000 Alistair Popple <apopple@nvidia.com> wrote:
> 
> > In some cases it is possible for mmu_interval_notifier_remove() to race
> > with mn_tree_inv_end() allowing it to return while the notifier data
> > structure is still in use. Consider the following sequence:
> > 
> > CPU0 - mn_tree_inv_end()            CPU1 - mmu_interval_notifier_remove()
> >                                     spin_lock(subscriptions->lock);
> >                                     seq = subscriptions->invalidate_seq;
> > spin_lock(subscriptions->lock);     spin_unlock(subscriptions->lock);
> > subscriptions->invalidate_seq++;
> >                                     wait_event(invalidate_seq != seq);
> >                                     return;
> > interval_tree_remove(interval_sub); kfree(interval_sub);
> > spin_unlock(subscriptions->lock);
> > wake_up_all();
> > 
> > As the wait_event() condition is true it will return immediately. This
> > can lead to use-after-free type errors if the caller frees the data
> > structure containing the interval notifier subscription while it is
> > still on a deferred list. Fix this by taking the appropriate lock when
> > reading invalidate_seq to ensure proper synchronisation.
> > 
> > ...
> >
> > Fixes: 99cb252f5e68 ("mm/mmu_notifier: add an interval tree notifier")
> 
> Do you think fix this should be backported into older kernels?

I think it should be tagged stable, yes

Jason