linux-kernel - Re: [PATCH mm-new v3 1/1] mm/khugepaged: guard is_zero_pfn() calls with pte

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Message-ID: <749beb24-f38f-42a1-9d7f-64a41de20b8b@lucifer.local>
Date: Mon, 20 Oct 2025 18:46:21 +0100
From: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
To: Lance Yang <lance.yang@...ux.dev>
Cc: akpm@...ux-foundation.org, david@...hat.com, ziy@...dia.com,
        baolin.wang@...ux.alibaba.com, Liam.Howlett@...cle.com,
        npache@...hat.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 v3 1/1] mm/khugepaged: guard is_zero_pfn() calls
 with pte_present()

On Mon, Oct 20, 2025 at 11:11:11PM +0800, 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>
> Reviewed-by: Nico Pache <npache@...hat.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>

This LGTM thanks for this, so:

Reviewed-by: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>

> ---
> Applies against commit a61ca1246ad3 in mm-new.
>
> v2 -> v3:
>  - Collect Reviewed-by from Nico - thanks!
>  - Add a VM_WARN_ON_ONCE() for unexpected PTEs (per David)
>  - Introduce a pte_is_none_or_zero() helper to reduce duplication
>    (per David and Lorenzo)

Wow I hadn't realised David had suggested that too, that was actually both of us
doing that independently by chance lol.

I guess we agree then :)

>  - https://lore.kernel.org/linux-mm/20251017093847.36436-1-lance.yang@linux.dev/
>
> 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 | 35 +++++++++++++++++++++--------------
>  1 file changed, 21 insertions(+), 14 deletions(-)
>
> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> index d635d821f611..6f2ae2238b5b 100644
> --- a/mm/khugepaged.c
> +++ b/mm/khugepaged.c
> @@ -337,6 +337,13 @@ struct attribute_group khugepaged_attr_group = {
>  };
>  #endif /* CONFIG_SYSFS */
>
> +static bool pte_none_or_zero(pte_t pte)
> +{
> +	if (pte_none(pte))
> +		return true;
> +	return pte_present(pte) && is_zero_pfn(pte_pfn(pte));
> +}
> +
>  int hugepage_madvise(struct vm_area_struct *vma,
>  		     vm_flags_t *vm_flags, int advice)
>  {
> @@ -518,6 +525,7 @@ static void release_pte_pages(pte_t *pte, pte_t *_pte,
>
>  		if (pte_none(pteval))
>  			continue;
> +		VM_WARN_ON_ONCE(!pte_present(pteval));
>  		pfn = pte_pfn(pteval);
>  		if (is_zero_pfn(pfn))
>  			continue;
> @@ -548,8 +556,7 @@ static int __collapse_huge_page_isolate(struct vm_area_struct *vma,
>  	for (_pte = pte; _pte < pte + HPAGE_PMD_NR;
>  	     _pte++, addr += PAGE_SIZE) {
>  		pte_t pteval = ptep_get(_pte);
> -		if (pte_none(pteval) || (pte_present(pteval) &&
> -				is_zero_pfn(pte_pfn(pteval)))) {
> +		if (pte_none_or_zero(pteval)) {
>  			++none_or_zero;
>  			if (!userfaultfd_armed(vma) &&
>  			    (!cc->is_khugepaged ||
> @@ -690,17 +697,17 @@ 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_or_zero(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 +801,7 @@ 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_or_zero(pteval)) {
>  			clear_user_highpage(page, src_addr);
>  			continue;
>  		}
> @@ -1294,7 +1301,7 @@ 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_or_zero(pteval)) {
>  			++none_or_zero;
>  			if (!userfaultfd_armed(vma) &&
>  			    (!cc->is_khugepaged ||
> --
> 2.49.0
>