From 1a2993227e8a27fc4cdea305bbdae82d290bc885 Mon Sep 17 00:00:00 2001 From: Kyle Zeng Date: Thu, 15 Oct 2026 18:15:49 -0700 Subject: [PATCH] mm/page_vma_mapped: continue past nonmatching huge PMDs A PTE-mapped large folio can straddle PMD boundaries after an unaligned mremap(). If the first part is discarded and replaced with an unrelated huge PMD, page_vma_mapped_walk() stops instead of looking for the rest of the folio in subsequent PMDs. This also affects migration entries. A walk that stops at an unrelated huge PMD can leave migration PTEs behind when migration or an anonymous folio split restores the mappings. Those entries do not hold mapping references and can outlive the source folio. This issue can lead to memory corruption and I have a reproducer for it. Skip the current PMD rather than ending the walk on a flag or PFN mismatch. Do the same if the PMD became nonpresent before the locked recheck. Release the PMD lock, clear the per-PMD state and advance with step_forward(), so the existing range limit controls termination. This covers present, migration and device-private huge PMDs while preserving the matching-PMD and split-to-PTE paths. Link: https://lore.kernel.org/20261016011549.61146-1-kylebot@openai.com Fixes: ace71a19cec5 ("mm: introduce page_vma_mapped_walk()") Signed-off-by: Kyle Zeng Signed-off-by: Andrew Morton Acked-by: David Hildenbrand (Arm) Assisted-by: LLM Cc: Lorenzo Stoakes Cc: Rik van Riel Cc: Liam R. Howlett Cc: Vlastimil Babka Cc: Harry Yoo Cc: Jann Horn Cc: Lance Yang Cc: --- mm/page_vma_mapped.c | 28 +++++++++++++++++++--------- 1 file changed, 19 insertions(+), 9 deletions(-) diff --git a/mm/page_vma_mapped.c b/mm/page_vma_mapped.c index 8408aee7571b..e6b6170c2f4f 100644 --- a/mm/page_vma_mapped.c +++ b/mm/page_vma_mapped.c @@ -265,28 +265,28 @@ bool page_vma_mapped_walk(struct page_vma_mapped_walk *pvmw) softleaf_t entry; if (!(pvmw->flags & PVMW_MIGRATION)) - return not_found(pvmw); + goto next_pmd; entry = softleaf_from_pmd(pmde); if (!check_pmd(softleaf_to_pfn(entry), pvmw)) - return not_found(pvmw); + goto next_pmd; return true; } else if (pmd_is_device_private_entry(pmde)) { softleaf_t entry; if (pvmw->flags & PVMW_MIGRATION) - return not_found(pvmw); + goto next_pmd; entry = softleaf_from_pmd(pmde); if (!check_pmd(softleaf_to_pfn(entry), pvmw)) - return not_found(pvmw); + goto next_pmd; return true; } else if (!pmd_present(pmde)) { - return not_found(pvmw); + goto next_pmd; } if (likely(pmd_trans_huge(pmde))) { if (pvmw->flags & PVMW_MIGRATION) - return not_found(pvmw); + goto next_pmd; if (!check_pmd(pmd_pfn(pmde), pvmw)) - return not_found(pvmw); + goto next_pmd; return true; } /* THP/device-private pmd was split under us: handle on pte level */ @@ -299,8 +299,7 @@ bool page_vma_mapped_walk(struct page_vma_mapped_walk *pvmw) (pvmw->nr_pages >= HPAGE_PMD_NR)) sync_with_folio_pmd_zap(mm, pvmw->pmd); - step_forward(pvmw, PMD_SIZE); - continue; + goto next_pmd; } if (!map_pte(pvmw, &pmde, &ptl)) { if (!pvmw->pte) @@ -343,6 +342,17 @@ bool page_vma_mapped_walk(struct page_vma_mapped_walk *pvmw) pvmw->ptl = ptl; } goto this_pte; +next_pmd: + /* + * A large folio can be PTE-mapped across PMDs, so a + * nonmatching PMD does not end the walk. + */ + if (pvmw->ptl) { + spin_unlock(pvmw->ptl); + pvmw->ptl = NULL; + } + pvmw->pmd = NULL; + step_forward(pvmw, PMD_SIZE); } while (pvmw->address < end); return false;