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

Lorenzo Stoakes
2025-10-20T17:46:49Z

On Mon, Oct 20, 2025 at 11:11:11PM +0800, 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>
> Reviewed-by: Nico Pache <npache@redhat.com>
> Reviewed-by: Dev Jain <dev.jain@arm.com>
> Reviewed-by: Baolin Wang <baolin.wang@linux.alibaba.com>
> Reviewed-by: Wei Yang <richard.weiyang@gmail.com>
> Signed-off-by: Lance Yang <lance.yang@linux.dev>

This LGTM thanks for this, so:

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