[PATCH -next] [RFC] block: fix null-deref in percpu_ref_put

[Zhang Wensheng]

From: Zhang Wensheng <zhangwensheng5@xxxxxxxxxx>

A problem was find in stable 5.10 and the root cause of it like below.

In the use of q_usage_counter of request_queue, blk_cleanup_queue using
"wait_event(q->mq_freeze_wq, percpu_ref_is_zero(&q->q_usage_counter))"
to wait q_usage_counter becoming zero. however, if the q_usage_counter
becoming zero quickly, and percpu_ref_exit will execute and ref->data
will be freed, maybe another process will cause a null-defef problem
like below:

	CPU0                             CPU1
blk_cleanup_queue
 blk_freeze_queue
  blk_mq_freeze_queue_wait
				scsi_end_request
				 percpu_ref_get
				 ...
				 percpu_ref_put
				  atomic_long_sub_and_test
  percpu_ref_exit
   ref->data -> NULL
   				   ref->data->release(ref) -> null-deref

Fix it by setting flag(QUEUE_FLAG_USAGE_COUNT_SYNC) to add synchronization
mechanism, when ref->data->release is called, the flag will be setted,
and the "wait_event" in blk_mq_freeze_queue_wait must wait flag becoming
true as well, which will limit percpu_ref_exit to execute ahead of time.

Although the problem was not reproduced in mainline, it may also has
problem when the passthrough IO which will go directly to
blk_cleanup_queue and cause the problem as well.

Signed-off-by: Zhang Wensheng <zhangwensheng5@xxxxxxxxxx>
---
 block/blk-core.c       | 4 +++-
 block/blk-mq.c         | 7 +++++++
 include/linux/blk-mq.h | 1 +
 include/linux/blkdev.h | 2 ++
 4 files changed, 13 insertions(+), 1 deletion(-)

diff --git a/block/blk-core.c b/block/blk-core.c
index 27fb1357ad4b..4b73f46e62ec 100644
--- a/block/blk-core.c
+++ b/block/blk-core.c
@@ -312,7 +312,8 @@ void blk_cleanup_queue(struct request_queue *q)
 	 * prevent that blk_mq_run_hw_queues() accesses the hardware queues
 	 * after draining finished.
 	 */
-	blk_freeze_queue(q);
+	blk_freeze_queue_start(q);
+	blk_mq_freeze_queue_wait_sync(q);
 
 	blk_queue_flag_set(QUEUE_FLAG_DEAD, q);
 
@@ -403,6 +404,7 @@ static void blk_queue_usage_counter_release(struct percpu_ref *ref)
 	struct request_queue *q =
 		container_of(ref, struct request_queue, q_usage_counter);
 
+	blk_queue_flag_set(QUEUE_FLAG_USAGE_COUNT_SYNC, q);
 	wake_up_all(&q->mq_freeze_wq);
 }
 
diff --git a/block/blk-mq.c b/block/blk-mq.c
index 93d9d60980fb..44e764257511 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -165,6 +165,7 @@ void blk_freeze_queue_start(struct request_queue *q)
 {
 	mutex_lock(&q->mq_freeze_lock);
 	if (++q->mq_freeze_depth == 1) {
+		blk_queue_flag_clear(QUEUE_FLAG_USAGE_COUNT_SYNC, q);
 		percpu_ref_kill(&q->q_usage_counter);
 		mutex_unlock(&q->mq_freeze_lock);
 		if (queue_is_mq(q))
@@ -175,6 +176,12 @@ void blk_freeze_queue_start(struct request_queue *q)
 }
 EXPORT_SYMBOL_GPL(blk_freeze_queue_start);
 
+void blk_mq_freeze_queue_wait_sync(struct request_queue *q)
+{
+	wait_event(q->mq_freeze_wq, percpu_ref_is_zero(&q->q_usage_counter) &&
+			test_bit(QUEUE_FLAG_USAGE_COUNT_SYNC, &q->queue_flags));
+}
+
 void blk_mq_freeze_queue_wait(struct request_queue *q)
 {
 	wait_event(q->mq_freeze_wq, percpu_ref_is_zero(&q->q_usage_counter));
diff --git a/include/linux/blk-mq.h b/include/linux/blk-mq.h
index e2d9daf7e8dd..50fd56f85b31 100644
--- a/include/linux/blk-mq.h
+++ b/include/linux/blk-mq.h
@@ -868,6 +868,7 @@ void blk_mq_freeze_queue(struct request_queue *q);
 void blk_mq_unfreeze_queue(struct request_queue *q);
 void blk_freeze_queue_start(struct request_queue *q);
 void blk_mq_freeze_queue_wait(struct request_queue *q);
+void blk_mq_freeze_queue_wait_sync(struct request_queue *q);
 int blk_mq_freeze_queue_wait_timeout(struct request_queue *q,
 				     unsigned long timeout);
 
diff --git a/include/linux/blkdev.h b/include/linux/blkdev.h
index 2f7b43444c5f..93ed8b166d66 100644
--- a/include/linux/blkdev.h
+++ b/include/linux/blkdev.h
@@ -575,6 +575,8 @@ struct request_queue {
 #define QUEUE_FLAG_HCTX_ACTIVE	28	/* at least one blk-mq hctx is active */
 #define QUEUE_FLAG_NOWAIT       29	/* device supports NOWAIT */
 #define QUEUE_FLAG_SQ_SCHED     30	/* single queue style io dispatch */
+/* sync for q_usage_counter */
+#define QUEUE_FLAG_USAGE_COUNT_SYNC    31
 
 #define QUEUE_FLAG_MQ_DEFAULT	((1 << QUEUE_FLAG_IO_STAT) |		\
 				 (1 << QUEUE_FLAG_SAME_COMP) |		\
-- 
2.31.1