io_uring: explicitly catch any illegal async queue attempt

Catch an illegal case to queue async from an unrelated task that got
the ring fd passed to it. This should not be possible to hit, but
better be proactive and catch it explicitly. io-wq is extended to
check for early IO_WQ_WORK_CANCEL being set on a work item as well,
so it can run the request through the normal cancelation path.
Signed-off-by: Jens Axboe <axboe@kernel.dk>

fs/io-wq.c

@@ -731,7 +731,12 @@ static void io_wqe_enqueue(struct io_wqe *wqe, struct io_wq_work *work)
 	int work_flags;
 	unsigned long flags;
 
-	if (test_bit(IO_WQ_BIT_EXIT, &wqe->wq->state)) {
+	/*
+	 * If io-wq is exiting for this task, or if the request has explicitly
+	 * been marked as one that should not get executed, cancel it here.
+	 */
+	if (test_bit(IO_WQ_BIT_EXIT, &wqe->wq->state) ||
+	    (work->flags & IO_WQ_WORK_CANCEL)) {
 		io_run_cancel(work, wqe);
 		return;
 	}

fs/io_uring.c

@@ -1294,6 +1294,17 @@ static void io_queue_async_work(struct io_kiocb *req)
 
 	/* init ->work of the whole link before punting */
 	io_prep_async_link(req);
+
+	/*
+	 * Not expected to happen, but if we do have a bug where this _can_
+	 * happen, catch it here and ensure the request is marked as
+	 * canceled. That will make io-wq go through the usual work cancel
+	 * procedure rather than attempt to run this request (or create a new
+	 * worker for it).
+	 */
+	if (WARN_ON_ONCE(!same_thread_group(req->task, current)))
+		req->work.flags |= IO_WQ_WORK_CANCEL;
+
 	trace_io_uring_queue_async_work(ctx, io_wq_is_hashed(&req->work), req,
 					&req->work, req->flags);
 	io_wq_enqueue(tctx->io_wq, &req->work);