io_uring/msg_ring: check for dead submitter task

The change for improving the handling of the target CQE posting
inadvertently dropped the NULL check for the submitter task on the target
ring, reinstate that.

Fixes: 0617bb5 ("io_uring/msg_ring: improve handling of target CQE posting")
Reported-by: Pavel Begunkov <asml.silence@gmail.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

io_uring/msg_ring.c

@@ -86,16 +86,21 @@ static void io_msg_tw_complete(struct io_kiocb *req, struct io_tw_state *ts)
 	percpu_ref_put(&ctx->refs);
 }
 
-static void io_msg_remote_post(struct io_ring_ctx *ctx, struct io_kiocb *req,
-			       int res, u32 cflags, u64 user_data)
+static int io_msg_remote_post(struct io_ring_ctx *ctx, struct io_kiocb *req,
+			      int res, u32 cflags, u64 user_data)
 {
+	req->task = READ_ONCE(ctx->submitter_task);
+	if (!req->task) {
+		kmem_cache_free(req_cachep, req);
+		return -EOWNERDEAD;
+	}
 	req->cqe.user_data = user_data;
 	io_req_set_res(req, res, cflags);
 	percpu_ref_get(&ctx->refs);
 	req->ctx = ctx;
-	req->task = READ_ONCE(ctx->submitter_task);
 	req->io_task_work.func = io_msg_tw_complete;
 	io_req_task_work_add_remote(req, ctx, IOU_F_TWQ_LAZY_WAKE);
+	return 0;
 }
 
 static struct io_kiocb *io_msg_get_kiocb(struct io_ring_ctx *ctx)
@@ -125,8 +130,8 @@ static int io_msg_data_remote(struct io_kiocb *req)
 	if (msg->flags & IORING_MSG_RING_FLAGS_PASS)
 		flags = msg->cqe_flags;
 
-	io_msg_remote_post(target_ctx, target, msg->len, flags, msg->user_data);
-	return 0;
+	return io_msg_remote_post(target_ctx, target, msg->len, flags,
+					msg->user_data);
 }
 
 static int io_msg_ring_data(struct io_kiocb *req, unsigned int issue_flags)