linux-kernel - Re: [PATCH] quota: fix race condition between dqput() and dquot_mark_dquot

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Message-ID: <2486ec73-55e0-00cb-fc76-97b9b285a9ce@huawei.com>
Date:   Mon, 26 Jun 2023 21:55:49 +0800
From:   Baokun Li <libaokun1@...wei.com>
To:     Jan Kara <jack@...e.cz>
CC:     <linux-fsdevel@...r.kernel.org>, <linux-ext4@...r.kernel.org>,
        <linux-kernel@...r.kernel.org>, <yi.zhang@...wei.com>,
        <yangerkun@...wei.com>, <chengzhihao1@...wei.com>,
        <yukuai3@...wei.com>, Baokun Li <libaokun1@...wei.com>
Subject: Re: [PATCH] quota: fix race condition between dqput() and
 dquot_mark_dquot_dirty()

Hello!

On 2023/6/26 21:09, Jan Kara wrote:
> Hello!
>
> On Sun 25-06-23 15:56:10, Baokun Li wrote:
>>>> I think we can simply focus on the race between the DQ_ACTIVE_B flag and
>>>> the DQ_MOD_B flag, which is the core problem, because the same quota
>>>> should not have both flags. These two flags are protected by dq_list_lock
>>>> and dquot->dq_lock respectively, so it makes sense to add a
>>>> wait_on_dquot() to ensure the accuracy of DQ_ACTIVE_B.
>>> But the fundamental problem is not only the race with DQ_MOD_B setting. The
>>> dquot structure can be completely freed by the time
>>> dquot_claim_space_nodirty() calls dquot_mark_dquot_dirty() on it. That's
>>> why I think making __dquot_transfer() obey dquot_srcu rules is the right
>>> solution.
>> Yes, now I also think that making __dquot_transfer() obey dquot_srcu
>> rules is a better solution. But with inode->i_lock protection, why would
>> the dquot structure be completely freed?
> Well, when dquot_claim_space_nodirty() calls mark_all_dquot_dirty() it does
> not hold any locks (only dquot_srcu). So nothing prevents dquot_transfer()
> to go, swap dquot structure pointers and drop dquot references and after
> that mark_all_dquot_dirty() can use a stale pointer to call
> mark_dquot_dirty() on already freed memory.
>
> 								Honza
No, this doesn't look like it's going to happen.
The mark_all_dquot_dirty() uses a pointer array pointer, the dquot in 
the array is
dynamically changing, so after swap dquot structure pointers, 
mark_all_dquot_dirty()
uses the new pointer, and the stale pointer is always destroyed after 
swap, so there
is no case of using the stale pointer here.

-- 
With Best Regards,
Baokun Li
.