Re: [PATCH] blk-throttle: fix NULL pointer dereference in throtl_schedule_pending_timer

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On Wed, Jun 07, 2017 at 11:36:14AM +0800, Joseph Qi wrote:
> From: Joseph Qi <qijiang.qj@xxxxxxxxxxxxxxx>
> 
> I have encountered a NULL pointer dereference in
> throtl_schedule_pending_timer:
>   [  413.735396] BUG: unable to handle kernel NULL pointer dereference at 0000000000000038
>   [  413.735535] IP: [<ffffffff812ebbbf>] throtl_schedule_pending_timer+0x3f/0x210
>   [  413.735643] PGD 22c8cf067 PUD 22cb34067 PMD 0
>   [  413.735713] Oops: 0000 [#1] SMP
>   ......
> 
> This is caused by the following case:
>   blk_throtl_bio
>     throtl_schedule_next_dispatch  <= sq is top level one without parent
>       throtl_schedule_pending_timer
>         sq_to_tg(sq)->td->throtl_slice  <= sq_to_tg(sq) returns NULL
> 
> Fix it by using sq_to_td instead of sq_to_tg(sq)->td, which will always
> return a valid td.
> 
> Fixes: 297e3d854784 ("blk-throttle: make throtl_slice tunable")
> Signed-off-by: Joseph Qi <qijiang.qj@xxxxxxxxxxxxxxx>

Thanks!

Reviewed-by: Shaohua Li <shli@xxxxxx>
> ---
>  block/blk-throttle.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/block/blk-throttle.c b/block/blk-throttle.c
> index fc13dd0..3b751f7 100644
> --- a/block/blk-throttle.c
> +++ b/block/blk-throttle.c
> @@ -698,7 +698,7 @@ static void throtl_dequeue_tg(struct throtl_grp *tg)
>  static void throtl_schedule_pending_timer(struct throtl_service_queue *sq,
>  					  unsigned long expires)
>  {
> -	unsigned long max_expire = jiffies + 8 * sq_to_tg(sq)->td->throtl_slice;
> +	unsigned long max_expire = jiffies + 8 * sq_to_td(sq)->throtl_slice;
>  
>  	/*
>  	 * Since we are adjusting the throttle limit dynamically, the sleep
> -- 
> 1.9.4



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