[PATCH] blk-mq: fix use-after-free in blk_mq_exit_sched

[Ming Lei <ming.lei@redhat.com>]

tagset can't be used after blk_cleanup_queue() is returned because
freeing tagset usually follows blk_clenup_queue(). Commit d97e594c5166
("blk-mq: Use request queue-wide tags for tagset-wide sbitmap") adds
check on q->tag_set->flags in blk_mq_exit_sched(), and causes
use-after-free.

Fixes it by using hctx->flags.

Reported-by: syzbot+77ba3d171a25c56756ea@syzkaller.appspotmail.com
Fixes: d97e594c5166 ("blk-mq: Use request queue-wide tags for tagset-wide sbitmap")
Cc: John Garry <john.garry@huawei.com>
Signed-off-by: Ming Lei <ming.lei@redhat.com>
---
 block/blk-mq-sched.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/block/blk-mq-sched.c b/block/blk-mq-sched.c
index a9182d2f8ad3..80273245d11a 100644
--- a/block/blk-mq-sched.c
+++ b/block/blk-mq-sched.c
@@ -680,6 +680,7 @@ void blk_mq_exit_sched(struct request_queue *q, struct elevator_queue *e)
 {
 	struct blk_mq_hw_ctx *hctx;
 	unsigned int i;
+	unsigned int flags = 0;
 
 	queue_for_each_hw_ctx(q, hctx, i) {
 		blk_mq_debugfs_unregister_sched_hctx(hctx);
@@ -687,12 +688,13 @@ void blk_mq_exit_sched(struct request_queue *q, struct elevator_queue *e)
 			e->type->ops.exit_hctx(hctx, i);
 			hctx->sched_data = NULL;
 		}
+		flags = hctx->flags;
 	}
 	blk_mq_debugfs_unregister_sched(q);
 	if (e->type->ops.exit_sched)
 		e->type->ops.exit_sched(e);
 	blk_mq_sched_tags_teardown(q);
-	if (blk_mq_is_sbitmap_shared(q->tag_set->flags))
+	if (blk_mq_is_sbitmap_shared(flags))
 		blk_mq_exit_sched_shared_sbitmap(q);
 	q->elevator = NULL;
 }
-- 
2.31.1