linux-kernel - Re: [PATCH mm-new 1/1] mm/khugepaged: guard is_zero_pfn() calls with pte

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Message-ID: <debdccdd-1a4a-4039-bdc3-64b68525d03c@arm.com>
Date: Thu, 16 Oct 2025 11:45:00 +0530
From: Dev Jain <dev.jain@....com>
To: Lance Yang <lance.yang@...ux.dev>
Cc: ziy@...dia.com, baolin.wang@...ux.alibaba.com, Liam.Howlett@...cle.com,
 npache@...hat.com, ryan.roberts@....com, baohua@...nel.org,
 ioworker0@...il.com, linux-kernel@...r.kernel.org, linux-mm@...ck.org,
 lorenzo.stoakes@...cle.com, akpm@...ux-foundation.org, david@...hat.com
Subject: Re: [PATCH mm-new 1/1] mm/khugepaged: guard is_zero_pfn() calls with
 pte_present()


On 16/10/25 11:29 am, Lance Yang wrote:
>
>
> On 2025/10/16 13:34, Dev Jain wrote:
>>
>> On 16/10/25 9:06 am, Lance Yang wrote:
>>> From: Lance Yang <lance.yang@...ux.dev>
>>>
>>> A non-present entry, like a swap PTE, contains completely different 
>>> data
>>> (swap type and offset). pte_pfn() doesn't know this, so if we feed it a
>>> non-present entry, it will spit out a junk PFN.
>>>
>>> What if that junk PFN happens to match the zeropage's PFN by sheer
>>> chance? While really unlikely, this would be really bad if it did.
>>>
>>> So, let's fix this potential bug by ensuring all calls to is_zero_pfn()
>>> in khugepaged.c are properly guarded by a pte_present() check.
>>>
>>> Suggested-by: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
>>> Signed-off-by: Lance Yang <lance.yang@...ux.dev>
>>> ---
>>
>> Thanks, I missed this.
>
> Me too ...
>
>>
>>>   mm/khugepaged.c | 13 ++++++++-----
>>>   1 file changed, 8 insertions(+), 5 deletions(-)
>>>
>>> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
>>> index d635d821f611..0341c3d13e9e 100644
>>> --- a/mm/khugepaged.c
>>> +++ b/mm/khugepaged.c
>>> @@ -516,7 +516,7 @@ static void release_pte_pages(pte_t *pte, pte_t 
>>> *_pte,
>>>           pte_t pteval = ptep_get(_pte);
>>>           unsigned long pfn;
>>> -        if (pte_none(pteval))
>>> +        if (!pte_present(pteval))
>>
>> There should be no chance that we end up with a pteval which is not 
>> none *and*
>> not present, if you look at the callers of release_pte_pages. So 
>> perhaps we
>> should keep this either the same, or, after "if(pte_none(pteval))", do a
>> WARN_ON_ONCE(!pte_present(pteval))?
>
> Good catch! Yeah, but I'd rather not rely on the callers ...
>
> Wouldn't it just be simpler and safer to always have is_zero_pfn() 
> guarded
> by pte_present()?
>
> I don't have a strong opinon here, though ;p

Yeah same, I think we can leave it to what you have done.


>
> Dev, Thanks!
>