[PATCH 6.1.y] blk-mq: fix NULL dereference on q->elevator in blk_mq_elv_switch_none

[Leon Chen <leonchen.oss@139.com>]

From: Ming Lei <ming.lei@redhat.com>

[ 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@redhat.com>
Signed-off-by: Ming Lei <ming.lei@redhat.com>
Link: https://lore.kernel.org/r/20230616132354.415109-1-ming.lei@redhat.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Leon Chen <leonchen.oss@139.com>
---
 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 f480b6ddba5e..8a9d9e3db166 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -4732,9 +4732,6 @@ 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;
@@ -4742,6 +4739,12 @@ 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;
@@ -4756,6 +4759,7 @@ static bool blk_mq_elv_switch_none(struct list_head *head,
 	 */
 	__module_get(qe->type->elevator_owner);
 	elevator_switch(q, NULL);
+unlock:
 	mutex_unlock(&q->sysfs_lock);
 
 	return true;
-- 
2.35.3