ublk: make sure ubq->canceling is set when queue is frozen

Now ublk driver depends on `ubq->canceling` for deciding if the request
can be dispatched via uring_cmd & io_uring_cmd_complete_in_task().

Once ubq->canceling is set, the uring_cmd can be done via ublk_cancel_cmd()
and io_uring_cmd_done().

So set ubq->canceling when queue is frozen, this way makes sure that the
flag can be observed from ublk_queue_rq() reliably, and avoids
use-after-free on uring_cmd.

Fixes: 216c8f5 ("ublk: replace monitor with cancelable uring_cmd")
Signed-off-by: Ming Lei <ming.lei@redhat.com>
Link: https://lore.kernel.org/r/20250327095123.179113-2-ming.lei@redhat.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>

drivers/block/ublk_drv.c

@@ -1446,17 +1446,27 @@ static void ublk_abort_queue(struct ublk_device *ub, struct ublk_queue *ubq)
 	}
 }
 
+/* Must be called when queue is frozen */
+static bool ublk_mark_queue_canceling(struct ublk_queue *ubq)
+{
+	bool canceled;
+
+	spin_lock(&ubq->cancel_lock);
+	canceled = ubq->canceling;
+	if (!canceled)
+		ubq->canceling = true;
+	spin_unlock(&ubq->cancel_lock);
+
+	return canceled;
+}
+
 static bool ublk_abort_requests(struct ublk_device *ub, struct ublk_queue *ubq)
 {
+	bool was_canceled = ubq->canceling;
 	struct gendisk *disk;
 
-	spin_lock(&ubq->cancel_lock);
-	if (ubq->canceling) {
-		spin_unlock(&ubq->cancel_lock);
+	if (was_canceled)
 		return false;
-	}
-	ubq->canceling = true;
-	spin_unlock(&ubq->cancel_lock);
 
 	spin_lock(&ub->lock);
 	disk = ub->ub_disk;
@@ -1468,14 +1478,23 @@ static bool ublk_abort_requests(struct ublk_device *ub, struct ublk_queue *ubq)
 	if (!disk)
 		return false;
 
-	/* Now we are serialized with ublk_queue_rq() */
+	/*
+	 * Now we are serialized with ublk_queue_rq()
+	 *
+	 * Make sure that ubq->canceling is set when queue is frozen,
+	 * because ublk_queue_rq() has to rely on this flag for avoiding to
+	 * touch completed uring_cmd
+	 */
 	blk_mq_quiesce_queue(disk->queue);
-	/* abort queue is for making forward progress */
-	ublk_abort_queue(ub, ubq);
+	was_canceled = ublk_mark_queue_canceling(ubq);
+	if (!was_canceled) {
+		/* abort queue is for making forward progress */
+		ublk_abort_queue(ub, ubq);
+	}
 	blk_mq_unquiesce_queue(disk->queue);
 	put_device(disk_to_dev(disk));
 
-	return true;
+	return !was_canceled;
 }
 
 static void ublk_cancel_cmd(struct ublk_queue *ubq, struct ublk_io *io,