io_uring: fix io_prep_async_link locking

io_prep_async_link() may be called after arming a linked timeout,
automatically making it unsafe to traverse the linked list. Guard
with completion_lock if there was a linked timeout.

Cc: stable@vger.kernel.org # 5.9+
Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/93f7c617e2b4f012a2a175b3dab6bc2f27cebc48.1627304436.git.asml.silence@gmail.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>

fs/io_uring.c

@@ -1279,8 +1279,17 @@ static void io_prep_async_link(struct io_kiocb *req)
 {
 	struct io_kiocb *cur;
 
-	io_for_each_link(cur, req)
-		io_prep_async_work(cur);
+	if (req->flags & REQ_F_LINK_TIMEOUT) {
+		struct io_ring_ctx *ctx = req->ctx;
+
+		spin_lock_irq(&ctx->completion_lock);
+		io_for_each_link(cur, req)
+			io_prep_async_work(cur);
+		spin_unlock_irq(&ctx->completion_lock);
+	} else {
+		io_for_each_link(cur, req)
+			io_prep_async_work(cur);
+	}
 }
 
 static void io_queue_async_work(struct io_kiocb *req)