linux-kernel - Re: [PATCH 2/4] mm/hwpoison: fix potential pte_unmap

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Message-ID: <4b0a5fcd-0b1b-6198-b6a7-d9cab5b9fae2@huawei.com>
Date:   Tue, 17 Aug 2021 16:24:43 +0800
From:   Miaohe Lin <linmiaohe@...wei.com>
To:     HORIGUCHI NAOYA(堀口 直也) 
        <naoya.horiguchi@....com>
CC:     "akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
        "sfr@...b.auug.org.au" <sfr@...b.auug.org.au>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 2/4] mm/hwpoison: fix potential pte_unmap_unlock pte error

On 2021/8/17 15:29, HORIGUCHI NAOYA(堀口 直也) wrote:
> On Sat, Aug 14, 2021 at 06:51:29PM +0800, Miaohe Lin wrote:
>> If the first pte is equal to poisoned_pfn, i.e. check_hwpoisoned_entry()
>> return 1, the wrong ptep - 1 would be passed to pte_unmap_unlock().
>>
>> Fixes: ad9c59c24095 ("mm,hwpoison: send SIGBUS with error virutal address")
>> Signed-off-by: Miaohe Lin <linmiaohe@...wei.com>
> 
> I agree with the change itself, so
> 
> Acked-by: Naoya Horiguchi <naoya.horiguchi@....com>
> 

Many thanks for your review and Acked-by tag!

> One question is that according to "grep -r pte_unmap_unlock ." command over
> whole kernel source code, pte_unmap_unlock() is called with "ptep - 1" in some places.
> I think that none of them seems to have "break in for loop" in locked period,
> so the same problem does not occur there.  But I'm still not sure why some place
> call with "ptep - 1" and the others call with pte returned by pte_offset_map_lock().

IMO pte_unmap_unlock() works as long as the passed in pte belongs to the same page returned
from pte_offset_map_lock(). I have fixed some similar place where pte_unmap_unlock() is called
with wrong "ptep - 1" when I was learning the related mm code.

>