[PATCH AUTOSEL 6.4 10/15] blk-mq: fix NULL dereference on q->elevator in blk_mq_elv_switch_none

[Sasha Levin]

From: Ming Lei <ming.lei@xxxxxxxxxx>

[ Upstream commit 245165658e1c9f95c0fecfe02b9b1ebd30a1198a ]

After grabbing q->sysfs_lock, q->elevator may become NULL because of
elevator switch.

Fix the NULL dereference on q->elevator by checking it with lock.

Reported-by: Guangwu Zhang <guazhang@xxxxxxxxxx>
Signed-off-by: Ming Lei <ming.lei@xxxxxxxxxx>
Link: https://lore.kernel.org/r/20230616132354.415109-1-ming.lei@xxxxxxxxxx
Signed-off-by: Jens Axboe <axboe@xxxxxxxxx>
Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>
---
 block/blk-mq.c | 10 +++++++---
 1 file changed, 7 insertions(+), 3 deletions(-)

diff --git a/block/blk-mq.c b/block/blk-mq.c
index 850bfb844ed2f..9516f65a50ea4 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -4608,9 +4608,6 @@ static bool blk_mq_elv_switch_none(struct list_head *head,
 {
 	struct blk_mq_qe_pair *qe;
 
-	if (!q->elevator)
-		return true;
-
 	qe = kmalloc(sizeof(*qe), GFP_NOIO | __GFP_NOWARN | __GFP_NORETRY);
 	if (!qe)
 		return false;
@@ -4618,6 +4615,12 @@ static bool blk_mq_elv_switch_none(struct list_head *head,
 	/* q->elevator needs protection from ->sysfs_lock */
 	mutex_lock(&q->sysfs_lock);
 
+	/* the check has to be done with holding sysfs_lock */
+	if (!q->elevator) {
+		kfree(qe);
+		goto unlock;
+	}
+
 	INIT_LIST_HEAD(&qe->node);
 	qe->q = q;
 	qe->type = q->elevator->type;
@@ -4625,6 +4628,7 @@ static bool blk_mq_elv_switch_none(struct list_head *head,
 	__elevator_get(qe->type);
 	list_add(&qe->node, head);
 	elevator_disable(q);
+unlock:
 	mutex_unlock(&q->sysfs_lock);
 
 	return true;
-- 
2.39.2