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>
