blk-mq: fix use-after-free in blk_mq_exit_sched

tagset can't be used after blk_cleanup_queue() is returned because
freeing tagset usually follows blk_clenup_queue(). Commit d97e594
("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: d97e594 ("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>
Tested-by: John Garry <john.garry@huawei.com>
Reviewed-by: John Garry <john.garry@huawei.com>
Link: https://lore.kernel.org/r/20210609063046.122843-1-ming.lei@redhat.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>

block/blk-mq-sched.c

@@ -680,19 +680,21 @@ 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);
 		if (e->type->ops.exit_hctx && hctx->sched_data) {
 			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;
 }