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Subject: Re: [PATCH 4/6] mm, hwpoison: fix possible use-after-free in
 mf_dax_kill_procs()
To:     =?UTF-8?B?SE9SSUdVQ0hJIE5BT1lBKOWggOWPoyDnm7TkuZ8p?= 
        <naoya.horiguchi@nec.com>
CC:     "akpm@linux-foundation.org" <akpm@linux-foundation.org>,
        "linux-mm@kvack.org" <linux-mm@kvack.org>,
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References: <20220818130016.45313-1-linmiaohe@huawei.com>
 <20220818130016.45313-5-linmiaohe@huawei.com>
 <20220819052324.GD613144@hori.linux.bs1.fc.nec.co.jp>
From:   Miaohe Lin <linmiaohe@huawei.com>
Message-ID: <6dff6aec-833c-a3db-1180-09e395f6265a@huawei.com>
Date:   Fri, 19 Aug 2022 15:32:27 +0800
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On 2022/8/19 13:23, HORIGUCHI NAOYA(堀口 直也) wrote:
> On Thu, Aug 18, 2022 at 09:00:14PM +0800, Miaohe Lin wrote:
>> After kill_procs(), tk will be freed without being removed from the to_kill
>> list. In the next iteration, the freed list entry in the to_kill list will
>> be accessed, thus leading to use-after-free issue.
> 
> kill_procs() runs over the to_kill list and frees all listed items in each
> iteration.  So just after returning from unmap_and_kill(), to_kill->next and
> to_kill->prev still point to the addresses of struct to_kill which was the
> first or last item (already freed!).  This is bad-manered, but
> collect_procs_fsdax() in the next iteration calls list_add_tail() and
> overwrites the dangling pointers with newly allocated item.  So this problem

list_add_tail will do WRITE_ONCE(prev->next, new) where prev is already freed!
Or am I miss something?

> should not be so critical?  Anyway, I agree with fixing this fragile code.
> 
>> Fix it by reinitializing
>> the to_kill list after unmap_and_kill().
>>
>> Fixes: c36e20249571 ("mm: introduce mf_dax_kill_procs() for fsdax case")
>> Signed-off-by: Miaohe Lin <linmiaohe@huawei.com>
> 
>> ---
>>  mm/memory-failure.c | 2 ++
>>  1 file changed, 2 insertions(+)
>>
>> diff --git a/mm/memory-failure.c b/mm/memory-failure.c
>> index 7023c3d81273..a2f4e8b00a26 100644
>> --- a/mm/memory-failure.c
>> +++ b/mm/memory-failure.c
>> @@ -1658,6 +1658,8 @@ int mf_dax_kill_procs(struct address_space *mapping, pgoff_t index,
>>  		collect_procs_fsdax(page, mapping, index, &to_kill);
>>  		unmap_and_kill(&to_kill, page_to_pfn(page), mapping,
>>  				index, mf_flags);
>> +		/* Reinitialize to_kill list for later resuing. */
> 
> s/resuing/reusing/ ?

OK.

> 
>> +		INIT_LIST_HEAD(&to_kill);
> 
> How about adding list_del() in kill_procs()?  Other callers now use
> to_kill only once, but fixing generally looks tidier to me.

That's a good idea. Will do it in v2. Many thanks for your review, Naoya!

Thanks,
Miaohe Lin

> 
> Thanks,
> Naoya Horiguchi
>