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

[Lance Yang <lance.yang@linux.dev>]

On 2025/10/16 13:34, Dev Jain wrote:
> 
> On 16/10/25 9:06 am, Lance Yang wrote:
>> From: Lance Yang <lance.yang@linux.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@oracle.com>
>> Signed-off-by: Lance Yang <lance.yang@linux.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

Dev, Thanks!