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 14:17, 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>
>> ---
>>   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))
>>               continue;
>>           pfn = pte_pfn(pteval);
>>           if (is_zero_pfn(pfn))
>> @@ -690,9 +690,10 @@ static void 
>> __collapse_huge_page_copy_succeeded(pte_t *pte,
>>            address += nr_ptes * PAGE_SIZE) {
>>           nr_ptes = 1;
>>           pteval = ptep_get(_pte);
>> -        if (pte_none(pteval) || is_zero_pfn(pte_pfn(pteval))) {
>> +        if (pte_none(pteval) ||
>> +            (pte_present(pteval) && is_zero_pfn(pte_pfn(pteval)))) {
>>               add_mm_counter(vma->vm_mm, MM_ANONPAGES, 1);
>> -            if (is_zero_pfn(pte_pfn(pteval))) {
>> +            if (!pte_none(pteval)) {
> 
> Could save a level of indentation by saying
> if (pte_none(pteval))
>      continue;
> 
> That would make it crystal clear that we do nothing when pte is none,
> and we do something when pte is pointing to zero pfn.

Yes! That does look clearer ;)

I'll pick that up in the next spin, if one is needed.

> 
> Reviewed-by: Dev Jain <dev.jain@arm.com>

Cheers!

> 
>>                   /*
>>                    * ptl mostly unnecessary.
>>                    */
>> @@ -794,7 +795,8 @@ static int __collapse_huge_page_copy(pte_t *pte, 
>> struct folio *folio,
>>           unsigned long src_addr = address + i * PAGE_SIZE;
>>           struct page *src_page;
>> -        if (pte_none(pteval) || is_zero_pfn(pte_pfn(pteval))) {
>> +        if (pte_none(pteval) ||
>> +            (pte_present(pteval) && is_zero_pfn(pte_pfn(pteval)))) {
>>               clear_user_highpage(page, src_addr);
>>               continue;
>>           }
>> @@ -1294,7 +1296,8 @@ static int hpage_collapse_scan_pmd(struct 
>> mm_struct *mm,
>>                   goto out_unmap;
>>               }
>>           }
>> -        if (pte_none(pteval) || is_zero_pfn(pte_pfn(pteval))) {
>> +        if (pte_none(pteval) ||
>> +            (pte_present(pteval) && is_zero_pfn(pte_pfn(pteval)))) {
>>               ++none_or_zero;
>>               if (!userfaultfd_armed(vma) &&
>>                   (!cc->is_khugepaged ||