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

[Alistair Popple <apopple@nvidia.com>]

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.

Signed-off-by: Alistair Popple <apopple@nvidia.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>
Fixes: 99cb252f5e68 ("mm/mmu_notifier: add an interval tree notifier")
---
 mm/mmu_notifier.c | 14 +++++++++++++-
 1 file changed, 13 insertions(+), 1 deletion(-)

diff --git a/mm/mmu_notifier.c b/mm/mmu_notifier.c
index 3f3bbcd298c6..e0275b9f6b81 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
@@ -1086,7 +1098,7 @@ 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);
-- 
2.34.1