[PATCH AUTOSEL 4.19 6/7] blk-mq: protect q->elevator by ->sysfs_lock in blk_mq_elv_switch_none

[Sasha Levin]

From: Ming Lei <ming.lei@xxxxxxxxxx>

[ Upstream commit 5fd7a84a09e640016fe106dd3e992f5210e23dc7 ]

elevator can be tore down by sysfs switch interface or disk release, so
hold ->sysfs_lock before referring to q->elevator, then potential
use-after-free can be avoided.

Reviewed-by: Christoph Hellwig <hch@xxxxxx>
Signed-off-by: Ming Lei <ming.lei@xxxxxxxxxx>
Link: https://lore.kernel.org/r/20220616014401.817001-2-ming.lei@xxxxxxxxxx
Signed-off-by: Jens Axboe <axboe@xxxxxxxxx>
Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>
---
 block/blk-mq.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/block/blk-mq.c b/block/blk-mq.c
index ae70b4809bec..3a791597c534 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -2940,12 +2940,14 @@ static bool blk_mq_elv_switch_none(struct list_head *head,
 	if (!qe)
 		return false;
 
+	/* q->elevator needs protection from ->sysfs_lock */
+	mutex_lock(&q->sysfs_lock);
+
 	INIT_LIST_HEAD(&qe->node);
 	qe->q = q;
 	qe->type = q->elevator->type;
 	list_add(&qe->node, head);
 
-	mutex_lock(&q->sysfs_lock);
 	/*
 	 * After elevator_switch_mq, the previous elevator_queue will be
 	 * released by elevator_release. The reference of the io scheduler
-- 
2.35.1