[PATCH] ublk: set canceling flag even when disk is not allocated

[Ming Lei <tom.leiming@gmail.com>]

ublk_start_cancel() previously bailed out early when ublk_get_disk()
returned NULL, treating it as "our disk has been dead".  That is correct
for the post-teardown case, but it also wrongly covers the pre-start
case: ublk_ctrl_start_dev() has not assigned ub->ub_disk yet, while
io_uring is already tearing down the daemon's uring_cmds via
ublk_uring_cmd_cancel_fn().

In that window, the cancel path skips ublk_set_canceling(), so
ubq->canceling stays false, even though ublk_cancel_cmd() goes on to
NULL out every io->cmd.  ublk_ctrl_start_dev() then proceeds to set
ub->ub_disk, call add_disk(), and schedule partition_scan_work.  When
ublk_partition_scan_work() runs bdev_disk_changed() and the resulting
read reaches ublk_queue_rq() -> ublk_queue_cmd(), the ubq->canceling
check passes and the code dereferences the NULL io->cmd:

  BUG: kernel NULL pointer dereference, address: 0000000000000018
  RIP: ublk_queue_cmd drivers/block/ublk_drv.c [inline]
  RIP: ublk_queue_rq+0x73/0x100
  Call Trace:
   blk_mq_dispatch_rq_list+0x1c5/0xca0
   ...
   bdev_disk_changed+0x3d4/0x5e0
   ublk_partition_scan_work+0x89/0xe0
   process_one_work+0x344/0x8a0

Fix it by always setting ub->canceling / ubq->canceling under
cancel_mutex.  When the disk is allocated, keep the existing
quiesce/unquiesce dance so the flag is observed across the
ublk_queue_rq() barrier.  When the disk is not yet allocated, there is
no request_queue and ublk_queue_rq() cannot be running concurrently, so
simply flipping the flag is sufficient: any subsequent I/O - including
the partition scan started by ublk_ctrl_start_dev() - will see
canceling set and be aborted via __ublk_queue_rq_common().

Fixes: 7fc4da6a304b ("ublk: scan partition in async way")
Signed-off-by: Ming Lei <tom.leiming@gmail.com>
---
 drivers/block/ublk_drv.c | 32 ++++++++++++++++++--------------
 1 file changed, 18 insertions(+), 14 deletions(-)

diff --git a/drivers/block/ublk_drv.c b/drivers/block/ublk_drv.c
index a08593b9f862..4f6d9e652187 100644
--- a/drivers/block/ublk_drv.c
+++ b/drivers/block/ublk_drv.c
@@ -2727,23 +2727,27 @@ static void ublk_start_cancel(struct ublk_device *ub)
 {
 	struct gendisk *disk = ublk_get_disk(ub);
 
-	/* Our disk has been dead */
-	if (!disk)
-		return;
-
 	mutex_lock(&ub->cancel_mutex);
 	if (ub->canceling)
 		goto out;
-	/*
-	 * 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);
-	ublk_set_canceling(ub, true);
-	blk_mq_unquiesce_queue(disk->queue);
+
+	if (disk) {
+		/*
+		 * Quiesce to serialize with ublk_queue_rq(), ensuring
+		 * ubq->canceling is visible when the queue resumes.
+		 */
+		blk_mq_quiesce_queue(disk->queue);
+		ublk_set_canceling(ub, true);
+		blk_mq_unquiesce_queue(disk->queue);
+	} else {
+		/*
+		 * Disk not yet allocated by ublk_ctrl_start_dev(), so
+		 * there is no request queue and ublk_queue_rq() cannot
+		 * be running.  Just set the flag; if start_dev proceeds
+		 * later, new I/O will see canceling and be aborted.
+		 */
+		ublk_set_canceling(ub, true);
+	}
 out:
 	mutex_unlock(&ub->cancel_mutex);
 	ublk_put_disk(disk);
-- 
2.54.0