Re: [PATCH] mm: fix address wraparound when scanning large folio PTEs

[Lance Yang <lance.yang@linux.dev>]

On Tue, Oct 13, 2026 at 02:30:37PM +0200, David Hildenbrand (Arm) wrote:
>numa_rebuild_large_mapping() and do_swap_page() can be tricked into
>generating an address wraparound, crashing the kernel. All that's required
>is to map a tail page of a large folio close enough to the start/end of
>the address space to e.g., make
>
>	unsigned long addr_start = addr - (nr << PAGE_SHIFT);
>
>to wrap around.
>
>Let's fix it by providing a better helper, folio_pte_scan_range(), that
>takes care of staying within VMA and page table boundaries while
>handling possible wraparounds.
>
>Reported-by: Kyle Zeng <kylebot@openai.com>
>Fixes: d2136d749d76 ("mm: support multi-size THP numa balancing")
>Fixes: 508758960b8d ("mm: swap: entirely map large folios found in swapcache")
>Cc: stable@vger.kernel.org
>Signed-off-by: David Hildenbrand (Arm) <david@kernel.org>
>---
> mm/memory.c | 43 +++++++++++++++++++++++++++++--------------
> 1 file changed, 29 insertions(+), 14 deletions(-)
>
>diff --git a/mm/memory.c b/mm/memory.c
>index 1f5d5f7d39cd2..a3823010aa981 100644
>--- a/mm/memory.c
>+++ b/mm/memory.c
>@@ -4933,6 +4933,27 @@ static void check_swap_exclusive(struct folio *folio, swp_entry_t entry,
> 	} while (--nr_pages);
> }
> 
>+/*
>+ * Get the address range in which a folio might be mapped, assuming its pages
>+ * are mapped consecutively and the folio page at the given idx is mapped at
>+ * the given address.
>+ */
>+static void folio_pte_scan_range(const struct folio *folio,
>+		const struct vm_area_struct *vma, unsigned long addr,
>+		unsigned long idx, unsigned long *start, unsigned long *end)
>+{
>+	/* Stay within the page table and VMA boundaries. */
>+	const unsigned long pmd_start = addr & PMD_MASK;
>+	const unsigned long pmd_end = pmd_start + PMD_SIZE;
>+	const unsigned long lower = max(pmd_start, vma->vm_start);
>+	const unsigned long upper = min(pmd_end - 1, vma->vm_end - 1) + 1;

That -1/+1 threw me for a sec ... Neat little trick!

>+	const unsigned long offset = idx << PAGE_SHIFT;
>+
>+	/* Avoid any address wraparounds. */
>+	*start = addr - min(addr - lower, offset);
>+	*end = addr + min(upper - addr, folio_size(folio) - offset);
>+}
>+
> /*
>  * We enter with either the VMA lock or the mmap_lock held (see
>  * FAULT_FLAG_VMA_LOCK), and pte mapped but not yet locked.
>@@ -5121,16 +5142,15 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
> 	address = vmf->address;
> 	ptep = vmf->pte;
> 	if (folio_test_large(folio) && folio_test_swapcache(folio)) {
>-		int nr = folio_nr_pages(folio);
>-		unsigned long idx = folio_page_idx(folio, page);
>-		unsigned long folio_start = address - idx * PAGE_SIZE;
>-		unsigned long folio_end = folio_start + nr * PAGE_SIZE;
>+		const int nr = folio_nr_pages(folio);
>+		const unsigned long idx = folio_page_idx(folio, page);
>+		unsigned long folio_start, folio_end;
> 		pte_t *folio_ptep;
> 		pte_t folio_pte;
> 
>-		if (unlikely(folio_start < max(address & PMD_MASK, vma->vm_start)))
>-			goto check_folio;
>-		if (unlikely(folio_end > pmd_addr_end(address, vma->vm_end)))
>+		folio_pte_scan_range(folio, vma, address, idx, &folio_start,
>+				     &folio_end);
>+		if (unlikely((folio_end - folio_start) >> PAGE_SHIFT != nr))
> 			goto check_folio;
> 
> 		folio_ptep = vmf->pte - idx;
>@@ -6280,16 +6300,11 @@ static void numa_rebuild_large_mapping(struct vm_fault *vmf, struct vm_area_stru
> 				       struct folio *folio, pte_t fault_pte,
> 				       bool ignore_writable, bool pte_write_upgrade)
> {
>-	int nr = pte_pfn(fault_pte) - folio_pfn(folio);
>+	const unsigned long idx = pte_pfn(fault_pte) - folio_pfn(folio);
> 	unsigned long start, end, addr = vmf->address;
>-	unsigned long addr_start = addr - (nr << PAGE_SHIFT);
>-	unsigned long pt_start = ALIGN_DOWN(addr, PMD_SIZE);
> 	pte_t *start_ptep;
> 
>-	/* Stay within the VMA and within the page table. */
>-	start = max3(addr_start, pt_start, vma->vm_start);
>-	end = min3(addr_start + folio_size(folio), pt_start + PMD_SIZE,
>-		   vma->vm_end);
>+	folio_pte_scan_range(folio, vma, addr, idx, &start, &end);
> 	start_ptep = vmf->pte - ((addr - start) >> PAGE_SHIFT);
> 
> 	/* Restore all PTEs' mapping of the large folio */
>
>---

Nothing jumped out at me, feel free to add:

Reviewed-by: Lance Yang <lance.yang@linux.dev>