io_uring: fix race condition reading SQ entries

A read memory barrier is required between reading SQ tail and reading
the actual data belonging to the SQ entry.

Userspace needs a matching write barrier between writing SQ entries and
updating SQ tail (using smp_store_release to update tail will do).
Signed-off-by: Stefan Bühler <source@stbuehler.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

fs/io_uring.c

@@ -1739,7 +1739,8 @@ static bool io_get_sqring(struct io_ring_ctx *ctx, struct sqe_submit *s)
 	head = ctx->cached_sq_head;
 	/* See comment at the top of this file */
 	smp_rmb();
-	if (head == READ_ONCE(ring->r.tail))
+	/* make sure SQ entry isn't read before tail */
+	if (head == smp_load_acquire(&ring->r.tail))
 		return false;
 
 	head = READ_ONCE(ring->array[head & ctx->sq_mask]);