[PATCH] drm/sched: Fix UB in spsc_queue

[Philipp Stanner <phasta@kernel.org>]

The spsc_queue is an unlocked, highly asynchronous piece of
infrastructure. Its inline function spsc_queue_peek() obtains the head
entry of the queue.

This access is performed without READ_ONCE() and is, therefore,
undefined behavior. In order to prevent the compiler from ever
reordering that access, or even optimizing it away, a READ_ONCE() is
strictly necessary. This is easily proven by the fact that
spsc_queue_pop() uses this very pattern to access the head.

Add READ_ONCE() to spsc_queue_peek().

Cc: stable@vger.kernel.org # v4.16+
Fixes: 27105db6c63a ("drm/amdgpu: Add SPSC queue to scheduler.")
Signed-off-by: Philipp Stanner <phasta@kernel.org>
---
I think this makes it less broken, but I'm not even sure if it's enough
or more memory barriers or an rcu_dereference() would be correct. The
spsc_queue is, of course, not documented and the existing barrier
comments are either false or not telling.

If someone has an idea, shoot us the info. Otherwise I think this is the
right thing to do for now.

P.
---
 include/drm/spsc_queue.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/include/drm/spsc_queue.h b/include/drm/spsc_queue.h
index ee9df8cc67b7..39bada748ffc 100644
--- a/include/drm/spsc_queue.h
+++ b/include/drm/spsc_queue.h
@@ -54,7 +54,7 @@ static inline void spsc_queue_init(struct spsc_queue *queue)
 
 static inline struct spsc_node *spsc_queue_peek(struct spsc_queue *queue)
 {
-	return queue->head;
+	return READ_ONCE(queue->head);
 }
 
 static inline int spsc_queue_count(struct spsc_queue *queue)
-- 
2.49.0