[PATCH] block: fix use-after-free of q->q_usage_counter

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

For blk-mq, queue release handler is usually called into after
blk_mq_freeze_queue_wait() returns. However, q_usage_counter->release()
handler may not be started yet at that time, so cause user-after-free.

Fix the issue by moving percpu_ref_exit() into blk_free_queue_rcu()
since ->release() is called with rcu read lock held, since it is
concluded that the race should be covered in caller per discussion
from the two links.

Reported-by: Zhang Wensheng <zhangwensheng@huaweicloud.com>
Reported-by: Zhong Jinghua <zhongjinghua@huawei.com>
Link: https://lore.kernel.org/linux-block/Y5prfOjyyjQKUrtH@T590/T/#u
Link: https://lore.kernel.org/lkml/Y4%2FmzMd4evRg9yDi@fedora/
Cc: Hillf Danton <hdanton@sina.com>
Cc: Yu Kuai <yukuai3@huawei.com>
Cc: Dennis Zhou <dennis@kernel.org>
Fixes: 2b0d3d3e4fcf ("percpu_ref: reduce memory footprint of percpu_ref in fast path")
Signed-off-by: Ming Lei <ming.lei@redhat.com>
---
 block/blk-core.c | 9 +++++----
 1 file changed, 5 insertions(+), 4 deletions(-)

diff --git a/block/blk-core.c b/block/blk-core.c
index 3866b6c4cd88..9321767470dc 100644
--- a/block/blk-core.c
+++ b/block/blk-core.c
@@ -254,14 +254,15 @@ EXPORT_SYMBOL_GPL(blk_clear_pm_only);
 
 static void blk_free_queue_rcu(struct rcu_head *rcu_head)
 {
-	kmem_cache_free(blk_requestq_cachep,
-			container_of(rcu_head, struct request_queue, rcu_head));
+	struct request_queue *q = container_of(rcu_head,
+			struct request_queue, rcu_head);
+
+	percpu_ref_exit(&q->q_usage_counter);
+	kmem_cache_free(blk_requestq_cachep, q);
 }
 
 static void blk_free_queue(struct request_queue *q)
 {
-	percpu_ref_exit(&q->q_usage_counter);
-
 	if (q->poll_stat)
 		blk_stat_remove_callback(q, q->poll_cb);
 	blk_stat_free_callback(q->poll_cb);
-- 
2.31.1