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

[Ming Lei]

On Fri, Jul 29, 2022 at 06:50:36PM +0800, Zhang Wensheng wrote:
> 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
> 

Looks it is one generic issue in percpu_ref, I think the following patch
should address it.


diff --git a/include/linux/percpu-refcount.h b/include/linux/percpu-refcount.h
index d73a1c08c3e3..07308bd36d83 100644
--- a/include/linux/percpu-refcount.h
+++ b/include/linux/percpu-refcount.h
@@ -331,8 +331,12 @@ static inline void percpu_ref_put_many(struct percpu_ref *ref, unsigned long nr)
 
 	if (__ref_is_percpu(ref, &percpu_count))
 		this_cpu_sub(*percpu_count, nr);
-	else if (unlikely(atomic_long_sub_and_test(nr, &ref->data->count)))
-		ref->data->release(ref);
+	else {
+		percpu_ref_func_t	*release = ref->data->release;
+
+		if (unlikely(atomic_long_sub_and_test(nr, &ref->data->count)))
+			release(ref);
+	}
 
 	rcu_read_unlock();
 }


Thanks,
Ming