+ mm-khugepaged-guard-is_zero_pfn-calls-with-pte_present.patch added to mm-new branch

[Andrew Morton <akpm@linux-foundation.org>]

The patch titled
     Subject: mm/khugepaged: guard is_zero_pfn() calls with pte_present()
has been added to the -mm mm-new branch.  Its filename is
     mm-khugepaged-guard-is_zero_pfn-calls-with-pte_present.patch

This patch will shortly appear at
     https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/mm-khugepaged-guard-is_zero_pfn-calls-with-pte_present.patch

This patch will later appear in the mm-new branch at
    git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm

Note, mm-new is a provisional staging ground for work-in-progress
patches, and acceptance into mm-new is a notification for others take
notice and to finish up reviews.  Please do not hesitate to respond to
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------------------------------------------------------
From: Lance Yang <lance.yang@linux.dev>
Subject: mm/khugepaged: guard is_zero_pfn() calls with pte_present()
Date: Thu, 16 Oct 2025 11:36:43 +0800

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/20251016033643.10848-1-lance.yang@linux.dev
Signed-off-by: Lance Yang <lance.yang@linux.dev>
Suggested-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Reviewed-by: Dev Jain <dev.jain@arm.com>
Cc: Baolin Wang <baolin.wang@linux.alibaba.com>
Cc: Barry Song <baohua@kernel.org>
Cc: David Hildenbrand <david@redhat.com>
Cc: Liam Howlett <liam.howlett@oracle.com>
Cc: Mariano Pache <npache@redhat.com>
Cc: Ryan Roberts <ryan.roberts@arm.com>
Cc: Zi Yan <ziy@nvidia.com>
Cc: Wei Yang <richard.weiyang@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

 mm/khugepaged.c |   13 ++++++++-----
 1 file changed, 8 insertions(+), 5 deletions(-)

--- a/mm/khugepaged.c~mm-khugepaged-guard-is_zero_pfn-calls-with-pte_present
+++ a/mm/khugepaged.c
@@ -516,7 +516,7 @@ static void release_pte_pages(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_su
 	     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)) {
 				/*
 				 * ptl mostly unnecessary.
 				 */
@@ -794,7 +795,8 @@ static int __collapse_huge_page_copy(pte
 		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(struc
 				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 ||
_

Patches currently in -mm which might be from lance.yang@linux.dev are

mm-khugepaged-guard-is_zero_pfn-calls-with-pte_present.patch