linux-kernel - Re: [PATCH] mm: fix potential pte_unmap

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Date:   Thu, 15 Oct 2020 21:19:06 +0800
From:   Shijie Luo <luoshijie1@...wei.com>
To:     <osalvador@...e.de>
CC:     <akpm@...ux-foundation.org>, <linux-mm@...ck.org>,
        <linux-kernel@...r.kernel.org>, <linmiaohe@...wei.com>,
        <linfeilong@...wei.com>
Subject: Re: [PATCH] mm: fix potential pte_unmap_unlock pte error


On 2020/10/15 20:58, osalvador@...e.de wrote:
> On 2020-10-15 14:15, Shijie Luo wrote:
>> When flags don't have MPOL_MF_MOVE or MPOL_MF_MOVE_ALL bits, code breaks
>>  and passing origin pte - 1 to pte_unmap_unlock seems like not a good 
>> idea.
>>
>> Signed-off-by: Shijie Luo <luoshijie1@...wei.com>
>> Signed-off-by: linmiaohe <linmiaohe@...wei.com>
>> ---
>>  mm/mempolicy.c | 6 +++++-
>>  1 file changed, 5 insertions(+), 1 deletion(-)
>>
>> diff --git a/mm/mempolicy.c b/mm/mempolicy.c
>> index 3fde772ef5ef..01f088630d1d 100644
>> --- a/mm/mempolicy.c
>> +++ b/mm/mempolicy.c
>> @@ -571,7 +571,11 @@ static int queue_pages_pte_range(pmd_t *pmd,
>> unsigned long addr,
>>          } else
>>              break;
>>      }
>> -    pte_unmap_unlock(pte - 1, ptl);
>> +
>> +    if (addr >= end)
>> +        pte = pte - 1;
>> +
>> +    pte_unmap_unlock(pte, ptl);
>
> But this is still wrong, isn't it?
> Unless I am missing something, this is "only" important under 
> CONFIG_HIGHPTE.
>
> We have:
>
> pte = pte_offset_map_lock(walk->mm, pmd, addr, &ptl);
>
> which under CONFIG_HIGHPTE does a kmap_atomoc.
>
> Now, we either break the loop in the first pass because of 
> !(MPOL_MF_MOVE | MPOL_MF_MOVE_ALL),
> or we keep incrementing pte by every pass.
> Either way is wrong, because the pointer kunmap_atomic gets will not 
> be the same (since we incremented pte).
>
> Or is the loop meant to be running only once, so pte - 1 will bring us 
> back to the original pte?
>
> .

Thanks for your reply, if we break the loop in the first pass, the pte 
pointer will not be incremented,

pte - 1 equals original pte - 1,  because we only increase pte pointer 
when not break the loop.