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

mainline inclusion
from mainline-v5.14-rc1
commit f0c1c4d2
category: bugfix
bugzilla: https://gitee.com/openeuler/kernel/issues/I597XM
CVE: NA

Reference: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=f0c1c4d2864ed614f90d2da1bab1a1c42907b940

--------------------------------

tagset can't be used after blk_cleanup_queue() is returned because
freeing tagset usually follows blk_clenup_queue(). Commit d97e594c
("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: d97e594c ("blk-mq: Use request queue-wide tags for tagset-wide sbitmap")
Cc: John Garry <john.garry@huawei.com>
Signed-off-by: NMing Lei <ming.lei@redhat.com>
Tested-by: NJohn Garry <john.garry@huawei.com>
Reviewed-by: NJohn Garry <john.garry@huawei.com>
Link: https://lore.kernel.org/r/20210609063046.122843-1-ming.lei@redhat.comSigned-off-by: NJens Axboe <axboe@kernel.dk>
Signed-off-by: NYufen Yu <yuyufen@huawei.com>
Reviewed-by: NJason Yan <yanaijie@huawei.com>
Signed-off-by: NZheng Zengkai <zhengzengkai@huawei.com>

block/blk-mq-sched.c

@@ -663,6 +663,7 @@ 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);
@@ -670,12 +671,13 @@ 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;
 }