io_uring: fix complete_post races for linked req

Calling io_queue_next() after spin_unlock in io_req_complete_post()
races with the other side extracting and reusing this request. Hand
coded parts of io_req_find_next() considering that io_disarm_next()
and io_req_task_queue() have (and safe) to be called with
completion_lock held.

It already does io_commit_cqring() and io_cqring_ev_posted(), so just
reuse it for post io_disarm_next().

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/5672a62f3150ee7c55849f40c0037655c4f2840f.1615250156.git.asml.silence@gmail.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>

fs/io_uring.c

@@ -985,6 +985,7 @@ static const struct io_op_def io_op_defs[] = {
 	[IORING_OP_UNLINKAT] = {},
 };
 
+static bool io_disarm_next(struct io_kiocb *req);
 static void io_uring_del_task_file(unsigned long index);
 static void io_uring_try_cancel_requests(struct io_ring_ctx *ctx,
 					 struct task_struct *task,
@@ -1525,15 +1526,14 @@ static void io_cqring_fill_event(struct io_kiocb *req, long res)
 	__io_cqring_fill_event(req, res, 0);
 }
 
-static inline void io_req_complete_post(struct io_kiocb *req, long res,
-					unsigned int cflags)
+static void io_req_complete_post(struct io_kiocb *req, long res,
+				 unsigned int cflags)
 {
 	struct io_ring_ctx *ctx = req->ctx;
 	unsigned long flags;
 
 	spin_lock_irqsave(&ctx->completion_lock, flags);
 	__io_cqring_fill_event(req, res, cflags);
-	io_commit_cqring(ctx);
 	/*
 	 * If we're the last reference to this request, add to our locked
 	 * free_list cache.
@@ -1541,19 +1541,26 @@ static inline void io_req_complete_post(struct io_kiocb *req, long res,
 	if (refcount_dec_and_test(&req->refs)) {
 		struct io_comp_state *cs = &ctx->submit_state.comp;
 
+		if (req->flags & (REQ_F_LINK | REQ_F_HARDLINK)) {
+			if (req->flags & (REQ_F_LINK_TIMEOUT | REQ_F_FAIL_LINK))
+				io_disarm_next(req);
+			if (req->link) {
+				io_req_task_queue(req->link);
+				req->link = NULL;
+			}
+		}
 		io_dismantle_req(req);
 		io_put_task(req->task, 1);
 		list_add(&req->compl.list, &cs->locked_free_list);
 		cs->locked_free_nr++;
 	} else
 		req = NULL;
+	io_commit_cqring(ctx);
 	spin_unlock_irqrestore(&ctx->completion_lock, flags);
-
 	io_cqring_ev_posted(ctx);
-	if (req) {
-		io_queue_next(req);
+
+	if (req)
 		percpu_ref_put(&ctx->refs);
-	}
 }
 
 static void io_req_complete_state(struct io_kiocb *req, long res,