Re: [PATCH 5.10] block: fix null-deref in percpu_ref_put

[Greg KH]

On Fri, Jul 29, 2022 at 02:52:43PM +0800, Zhang Wensheng wrote:
> From: Zhang Wensheng <zhangwensheng5@xxxxxxxxxx>
> 
> 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.
> 
> 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(-)


<formletter>

This is not the correct way to submit patches for inclusion in the
stable kernel tree.  Please read:
    https://www.kernel.org/doc/html/latest/process/stable-kernel-rules.html
for how to do this properly.

</formletter>