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

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Message-ID: <17c4c5f9-6ac8-4914-838f-f511dfbf948f@arm.com>
Date: Thu, 16 Oct 2025 11:47:06 +0530
From: Dev Jain <dev.jain@....com>
To: Lance Yang <lance.yang@...ux.dev>, akpm@...ux-foundation.org,
 david@...hat.com, lorenzo.stoakes@...cle.com
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
Subject: Re: [PATCH mm-new 1/1] mm/khugepaged: guard is_zero_pfn() calls with
 pte_present()


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>
> ---
>   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.

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

>   				/*
>   				 * 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 ||