Re: [PATCH] block, bfq: fix uaf for 'stable_merge_bfqq'

[Shinichiro Kawasaki]

On Mar 08, 2023 / 10:32, Yu Kuai wrote:
> From: Yu Kuai <yukuai3@xxxxxxxxxx>
> 
> Before commit fd571df0ac5b ("block, bfq: turn bfqq_data into an array
> in bfq_io_cq"), process reference is read before bfq_put_stable_ref(),
> and it's safe if bfq_put_stable_ref() put the last reference, because
> process reference will be 0 and 'stable_merge_bfqq' won't be accessed
> in this case. However, the commit changed the order and  will cause
> uaf for 'stable_merge_bfqq'.
> 
> In order to emphasize that bfq_put_stable_ref() can drop the last
> reference, fix the problem by moving bfq_put_stable_ref() to the end of
> bfq_setup_stable_merge().
> 
> Fixes: fd571df0ac5b ("block, bfq: turn bfqq_data into an array in bfq_io_cq")
> Reported-and-tested-by: Shinichiro Kawasaki <shinichiro.kawasaki@xxxxxxx>
> Link: https://lore.kernel.org/linux-block/20230307071448.rzihxbm4jhbf5krj@shindev/
> Signed-off-by: Yu Kuai <yukuai3@xxxxxxxxxx>
> ---
>  block/bfq-iosched.c | 18 +++++++++---------
>  1 file changed, 9 insertions(+), 9 deletions(-)
> 
> diff --git a/block/bfq-iosched.c b/block/bfq-iosched.c
> index 8a8d4441519c..d9ed3108c17a 100644
> --- a/block/bfq-iosched.c
> +++ b/block/bfq-iosched.c
> @@ -2854,11 +2854,11 @@ bfq_setup_stable_merge(struct bfq_data *bfqd, struct bfq_queue *bfqq,
>  {
>  	int proc_ref = min(bfqq_process_refs(bfqq),
>  			   bfqq_process_refs(stable_merge_bfqq));
> -	struct bfq_queue *new_bfqq;
> +	struct bfq_queue *new_bfqq = NULL;
>  
> -	if (idling_boosts_thr_without_issues(bfqd, bfqq) ||
> -	    proc_ref == 0)
> -		return NULL;
> +	bfqq_data->stable_merge_bfqq = NULL;
> +	if (idling_boosts_thr_without_issues(bfqd, bfqq) || proc_ref == 0)
> +		goto out;
>  
>  	/* next function will take at least one ref */
>  	new_bfqq = bfq_setup_merge(bfqq, stable_merge_bfqq);
> @@ -2873,6 +2873,11 @@ bfq_setup_stable_merge(struct bfq_data *bfqd, struct bfq_queue *bfqq,
>  			new_bfqq_data->stably_merged = true;
>  		}
>  	}
> +
> +out:
> +	/* deschedule stable merge, because done or aborted here */
> +	bfq_put_stable_ref(stable_merge_bfqq);
> +
>  	return new_bfqq;
>  }
>  
> @@ -2933,11 +2938,6 @@ bfq_setup_cooperator(struct bfq_data *bfqd, struct bfq_queue *bfqq,
>  			struct bfq_queue *stable_merge_bfqq =
>  				bfqq_data->stable_merge_bfqq;

Nit: I suggest to remove the local variable above. The two references to it
below are removed, and the argument of bfq_setup_stable_merge() can be replaced
with bfqq_data->stable_merge_bfqq.

>  
> -			/* deschedule stable merge, because done or aborted here */
> -			bfq_put_stable_ref(stable_merge_bfqq);
> -
> -			bfqq_data->stable_merge_bfqq = NULL;
> -
>  			return bfq_setup_stable_merge(bfqd, bfqq,
>  						      stable_merge_bfqq,
>  						      bfqq_data);
> -- 
> 2.31.1
> 

-- 
Shin'ichiro Kawasaki