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 <kylebot@openai.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Acked-by: David Hildenbrand (Arm) <david@kernel.org>
Assisted-by: LLM
Cc: Lorenzo Stoakes <ljs@kernel.org>
Cc: Rik van Riel <riel@surriel.com>
Cc: Liam R. Howlett <liam@infradead.org>
Cc: Vlastimil Babka <vbabka@kernel.org>
Cc: Harry Yoo <harry@kernel.org>
Cc: Jann Horn <jannh@google.com>
Cc: Lance Yang <lance.yang@linux.dev>
Cc: <stable@vger.kernel.org>

1 files 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 @@ restart:
 				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 @@ restart:
 			    (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 @@ next_pte:
 			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;