Subject: Re: [PATCH] block, bfq: free 'sync_bfqq' after bic_set_bfqq() in
 bfq_sync_bfqq_move()
To:     Damien Le Moal <damien.lemoal@opensource.wdc.com>,
        Yu Kuai <yukuai1@huaweicloud.com>, jack@suse.cz,
        axboe@kernel.dk, paolo.valente@linaro.org
Cc:     linux-block@vger.kernel.org, linux-kernel@vger.kernel.org,
        yi.zhang@huawei.com, yangerkun@huawei.com,
        "yukuai (C)" <yukuai3@huawei.com>
References: <e2071a24-cd25-e5bd-9166-a3b575b7bf4a@huaweicloud.com>
 <20230221082905.3389012-1-yukuai1@huaweicloud.com>
 <78830e38-e7a4-24e5-277e-f8e5022c59ef@opensource.wdc.com>
From:   Yu Kuai <yukuai1@huaweicloud.com>
Message-ID: <826a699f-8c35-700a-de54-e4bbfbc6e081@huaweicloud.com>
Date:   Tue, 21 Feb 2023 17:35:34 +0800
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Hi,

在 2023/02/21 17:14, Damien Le Moal 写道:
> On 2/21/23 17:29, Yu Kuai wrote:
>> From: Yu Kuai <yukuai3@huawei.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 ?

1) bfq_sync_bfqq_move() is introduced in commit 9778369a2d6c ("block,
bfq: split sync bfq_queues on a per-actuator basis"), which is merged to
block/for-6.3 branch.

2) commit 64dc8c732f5c ("block, bfq: fix possible uaf for 'bfqq->bic'")
is merged to mainline.

3) later, the fix for 2) b600de2d7d3a ("block, bfq: fix uaf for bfqq in
bic_set_bfqq()") is merged to mainline as well, however,
bfq_sync_bfqq_move() in block/for-6.3 branch is not changed.

4) At last, 1) is merged to mainline and bfq_sync_bfqq_move() is
still problematic.

Thanks,
Kuai
> 
>>
>> Fixes: 9778369a2d6c ("block, bfq: split sync bfq_queues on a per-actuator basis")
>> Signed-off-by: Yu Kuai <yukuai3@huawei.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);
>>   	}
>>   }
>>   
>