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

[Jason Gunthorpe]

On Thu, Apr 14, 2022 at 01:18:10PM +1000, Alistair Popple 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 changing invalidate_seq to an
> atomic type as it is read outside of the lock and moving the increment
> until after deferred lists have been updated.

Oh, yes, that is a mistake.

I would not solve it with more unlocked atomics though, this is just a
simple case of a missing lock - can you look at this and if you like
it post it as a patch please?

diff --git a/mm/mmu_notifier.c b/mm/mmu_notifier.c
index 459d195d2ff64b..f45ff1b7626a62 100644
--- a/mm/mmu_notifier.c
+++ b/mm/mmu_notifier.c
@@ -1036,6 +1036,18 @@ int mmu_interval_notifier_insert_locked(
 }
 EXPORT_SYMBOL_GPL(mmu_interval_notifier_insert_locked);
 
+static bool
+mmu_interval_seq_released(struct mmu_notifier_subscriptions *subscriptions,
+			  unsigned long seq)
+{
+	bool ret;
+
+	spin_lock(&subscriptions->lock);
+	ret = subscriptions->invalidate_seq != seq;
+	spin_unlock(&subscriptions->lock);
+	return ret;
+}
+
 /**
  * mmu_interval_notifier_remove - Remove a interval notifier
  * @interval_sub: Interval subscription to unregister
@@ -1083,7 +1095,7 @@ void mmu_interval_notifier_remove(struct mmu_interval_notifier *interval_sub)
 	lock_map_release(&__mmu_notifier_invalidate_range_start_map);
 	if (seq)
 		wait_event(subscriptions->wq,
-			   READ_ONCE(subscriptions->invalidate_seq) != seq);
+			   mmu_interval_seq_released(subscriptions, seq));
 
 	/* pairs with mmgrab in mmu_interval_notifier_insert() */
 	mmdrop(mm);

Signed-off-by: Jason Gunthorpe <jgg@xxxxxxxxxx>