From 074f027d15c10cb376b3ad88405b8e512fa5b3a8 Mon Sep 17 00:00:00 2001 From: Lance Yang Date: Mon, 20 Oct 2025 23:11:11 +0800 Subject: [PATCH] mm/khugepaged: guard is_zero_pfn() calls with pte_present() 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. Link: https://lkml.kernel.org/r/20251020151111.53561-1-lance.yang@linux.dev Signed-off-by: Lance Yang Suggested-by: Lorenzo Stoakes Reviewed-by: Nico Pache Reviewed-by: Dev Jain Reviewed-by: Baolin Wang Reviewed-by: Wei Yang Acked-by: David Hildenbrand Reviewed-by: Lorenzo Stoakes Cc: Liam Howlett Cc: Ryan Roberts Cc: Wei Yang Cc: Zi Yan Signed-off-by: Andrew Morton --- mm/khugepaged.c | 35 +++++++++++++++++++++-------------- 1 file changed, 21 insertions(+), 14 deletions(-) diff --git a/mm/khugepaged.c b/mm/khugepaged.c index 1be61cd5440d2..68e487d537727 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; } @@ -1301,7 +1308,7 @@ 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 || -- 2.47.3