mm/rmap: fix potential out-of-bounds page table access during batched unmap

As pointed out by David[1], the batched unmap logic in
try_to_unmap_one() may read past the end of a PTE table when a large
folio's PTE mappings are not fully contained within a single page
table.

While this scenario might be rare, an issue triggerable from userspace
must be fixed regardless of its likelihood.  This patch fixes the
out-of-bounds access by refactoring the logic into a new helper,
folio_unmap_pte_batch().

The new helper correctly calculates the safe batch size by capping the
scan at both the VMA and PMD boundaries.  To simplify the code, it also
supports partial batching (i.e., any number of pages from 1 up to the
calculated safe maximum), as there is no strong reason to special-case
for fully mapped folios.

Link: https://lkml.kernel.org/r/20250701143100.6970-1-lance.yang@linux.dev
Link: https://lkml.kernel.org/r/20250630011305.23754-1-lance.yang@linux.dev
Link: https://lkml.kernel.org/r/20250627062319.84936-1-lance.yang@linux.dev
Link: https://lore.kernel.org/linux-mm/a694398c-9f03-4737-81b9-7e49c857fcbe@redhat.com [1]
Fixes: 354dffd29575 ("mm: support batched unmap for lazyfree large folios during reclamation")
Signed-off-by: Lance Yang <lance.yang@linux.dev>
Suggested-by: David Hildenbrand <david@redhat.com>
Reported-by: David Hildenbrand <david@redhat.com>
Closes: https://lore.kernel.org/linux-mm/a694398c-9f03-4737-81b9-7e49c857fcbe@redhat.com
Suggested-by: Barry Song <baohua@kernel.org>
Acked-by: Barry Song <baohua@kernel.org>
Reviewed-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Acked-by: David Hildenbrand <david@redhat.com>
Reviewed-by: Harry Yoo <harry.yoo@oracle.com>
Cc: Baolin Wang <baolin.wang@linux.alibaba.com>
Cc: Chris Li <chrisl@kernel.org>
Cc: "Huang, Ying" <huang.ying.caritas@gmail.com>
Cc: Kairui Song <kasong@tencent.com>
Cc: Lance Yang <lance.yang@linux.dev>
Cc: Liam Howlett <liam.howlett@oracle.com>
Cc: Mingzhe Yang <mingzhe.yang@ly.com>
Cc: Rik van Riel <riel@surriel.com>
Cc: Ryan Roberts <ryan.roberts@arm.com>
Cc: Tangquan Zheng <zhengtangquan@oppo.com>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

1 files changed, 28 insertions, 18 deletions

diff --git a/mm/rmap.c b/mm/rmap.c
index fb63d9256f09..1320b88fab74 100644
--- a/mm/rmap.c
+++ b/mm/rmap.c
@@ -1845,23 +1845,32 @@ void folio_remove_rmap_pud(struct folio *folio, struct page *page,
 #endif
 }
 
-/* We support batch unmapping of PTEs for lazyfree large folios */
-static inline bool can_batch_unmap_folio_ptes(unsigned long addr,
-			struct folio *folio, pte_t *ptep)
+static inline unsigned int folio_unmap_pte_batch(struct folio *folio,
+			struct page_vma_mapped_walk *pvmw,
+			enum ttu_flags flags, pte_t pte)
 {
 	const fpb_t fpb_flags = FPB_IGNORE_DIRTY | FPB_IGNORE_SOFT_DIRTY;
-	int max_nr = folio_nr_pages(folio);
-	pte_t pte = ptep_get(ptep);
+	unsigned long end_addr, addr = pvmw->address;
+	struct vm_area_struct *vma = pvmw->vma;
+	unsigned int max_nr;
+
+	if (flags & TTU_HWPOISON)
+		return 1;
+	if (!folio_test_large(folio))
+		return 1;
 
+	/* We may only batch within a single VMA and a single page table. */
+	end_addr = pmd_addr_end(addr, vma->vm_end);
+	max_nr = (end_addr - addr) >> PAGE_SHIFT;
+
+	/* We only support lazyfree batching for now ... */
 	if (!folio_test_anon(folio) || folio_test_swapbacked(folio))
-		return false;
+		return 1;
 	if (pte_unused(pte))
-		return false;
-	if (pte_pfn(pte) != folio_pfn(folio))
-		return false;
+		return 1;
 
-	return folio_pte_batch(folio, addr, ptep, pte, max_nr, fpb_flags, NULL,
-			       NULL, NULL) == max_nr;
+	return folio_pte_batch(folio, addr, pvmw->pte, pte, max_nr, fpb_flags,
+			       NULL, NULL, NULL);
 }
 
 /*
@@ -2024,9 +2033,7 @@ static bool try_to_unmap_one(struct folio *folio, struct vm_area_struct *vma,
 			if (pte_dirty(pteval))
 				folio_mark_dirty(folio);
 		} else if (likely(pte_present(pteval))) {
-			if (folio_test_large(folio) && !(flags & TTU_HWPOISON) &&
-			    can_batch_unmap_folio_ptes(address, folio, pvmw.pte))
-				nr_pages = folio_nr_pages(folio);
+			nr_pages = folio_unmap_pte_batch(folio, &pvmw, flags, pteval);
 			end_addr = address + nr_pages * PAGE_SIZE;
 			flush_cache_range(vma, address, end_addr);
 
@@ -2206,13 +2213,16 @@ discard:
 			hugetlb_remove_rmap(folio);
 		} else {
 			folio_remove_rmap_ptes(folio, subpage, nr_pages, vma);
-			folio_ref_sub(folio, nr_pages - 1);
 		}
 		if (vma->vm_flags & VM_LOCKED)
 			mlock_drain_local();
-		folio_put(folio);
-		/* We have already batched the entire folio */
-		if (nr_pages > 1)
+		folio_put_refs(folio, nr_pages);
+
+		/*
+		 * If we are sure that we batched the entire folio and cleared
+		 * all PTEs, we can just optimize and stop right here.
+		 */
+		if (nr_pages == folio_nr_pages(folio))
 			goto walk_done;
 		continue;
 walk_abort: