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

[Andrew Morton <akpm@linux-foundation.org>]

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?