linux-kernel - Re: [PATCH] block, bfq: free 'sync_bfqq' after bic_set_bfqq() in bfq_sync_bfqq

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Message-ID: <78830e38-e7a4-24e5-277e-f8e5022c59ef@opensource.wdc.com>
Date:   Tue, 21 Feb 2023 18:14:14 +0900
From:   Damien Le Moal <damien.lemoal@...nsource.wdc.com>
To:     Yu Kuai <yukuai1@...weicloud.com>, jack@...e.cz, axboe@...nel.dk,
        paolo.valente@...aro.org
Cc:     linux-block@...r.kernel.org, linux-kernel@...r.kernel.org,
        yukuai3@...wei.com, yi.zhang@...wei.com, yangerkun@...wei.com
Subject: Re: [PATCH] block, bfq: free 'sync_bfqq' after bic_set_bfqq() in
 bfq_sync_bfqq_move()

On 2/21/23 17:29, Yu Kuai wrote:
> From: Yu Kuai <yukuai3@...wei.com>
> 
> As explained in commit b600de2d7d3a ("block, bfq: fix uaf for bfqq in
> bic_set_bfqq()"), bfqq should not be freed before bic_set_bfqq().
> However, this is broken while merging commit 9778369a2d6c ("block, bfq:
> split sync bfq_queues on a per-actuator basis") from branch
> for-6.3/block.

The patch looks OK to me, but the commit message is not super clear. What is
broken exactly ?

> 
> Fixes: 9778369a2d6c ("block, bfq: split sync bfq_queues on a per-actuator basis")
> Signed-off-by: Yu Kuai <yukuai3@...wei.com>
> ---
>  block/bfq-cgroup.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/block/bfq-cgroup.c b/block/bfq-cgroup.c
> index ea3638e06e04..89ffb3aa992c 100644
> --- a/block/bfq-cgroup.c
> +++ b/block/bfq-cgroup.c
> @@ -746,8 +746,8 @@ static void bfq_sync_bfqq_move(struct bfq_data *bfqd,
>  		 * old cgroup.
>  		 */
>  		bfq_put_cooperator(sync_bfqq);
> -		bfq_release_process_ref(bfqd, sync_bfqq);
>  		bic_set_bfqq(bic, NULL, true, act_idx);
> +		bfq_release_process_ref(bfqd, sync_bfqq);
>  	}
>  }
>  

-- 
Damien Le Moal
Western Digital Research