linux-kernel - Re: [PATCH mm-new v2 1/1] mm/khugepaged: guard is_zero_pfn() calls with pte

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Message-ID: <CAA1CXcBSCezWLFNJFf+7yw8aVSAvbAMWe4OUa9EJ14yX=MjfPw@mail.gmail.com>
Date: Fri, 17 Oct 2025 08:42:41 -0600
From: Nico Pache <npache@...hat.com>
To: Lance Yang <lance.yang@...ux.dev>
Cc: akpm@...ux-foundation.org, david@...hat.com, lorenzo.stoakes@...cle.com, 
	ziy@...dia.com, baolin.wang@...ux.alibaba.com, Liam.Howlett@...cle.com, 
	ryan.roberts@....com, dev.jain@....com, baohua@...nel.org, 
	ioworker0@...il.com, linux-kernel@...r.kernel.org, linux-mm@...ck.org, 
	Wei Yang <richard.weiyang@...il.com>
Subject: Re: [PATCH mm-new v2 1/1] mm/khugepaged: guard is_zero_pfn() calls
 with pte_present()

On Fri, Oct 17, 2025 at 3:39 AM Lance Yang <lance.yang@...ux.dev> 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>
> Reviewed-by: Dev Jain <dev.jain@....com>
> Reviewed-by: Baolin Wang <baolin.wang@...ux.alibaba.com>
> Reviewed-by: Wei Yang <richard.weiyang@...il.com>
> Signed-off-by: Lance Yang <lance.yang@...ux.dev>
LGTM!

Reviewed-by: Nico Pache <npache@...hat.com>
> ---
> Applies against commit 0f22abd9096e in mm-new.
>
> v1 -> v2:
>  - Collect Reviewed-by from Dev, Wei and Baolin - thanks!
>  - Reduce a level of indentation (per Dev)
>  - https://lore.kernel.org/linux-mm/20251016033643.10848-1-lance.yang@linux.dev/
>
>  mm/khugepaged.c | 29 ++++++++++++++++-------------
>  1 file changed, 16 insertions(+), 13 deletions(-)
>
> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> index d635d821f611..648d9335de00 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))
> @@ -690,17 +690,18 @@ 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))) {
> -                               /*
> -                                * ptl mostly unnecessary.
> -                                */
> -                               spin_lock(ptl);
> -                               ptep_clear(vma->vm_mm, address, _pte);
> -                               spin_unlock(ptl);
> -                               ksm_might_unmap_zero_page(vma->vm_mm, pteval);
> -                       }
> +                       if (pte_none(pteval))
> +                               continue;
> +                       /*
> +                        * ptl mostly unnecessary.
> +                        */
> +                       spin_lock(ptl);
> +                       ptep_clear(vma->vm_mm, address, _pte);
> +                       spin_unlock(ptl);
> +                       ksm_might_unmap_zero_page(vma->vm_mm, pteval);
>                 } else {
>                         struct page *src_page = pte_page(pteval);
>
> @@ -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 ||
> --
> 2.49.0
>