[PATCH v4 1/1] mm/rmap: fix potential out-of-bounds page table access during batched unmap

[Harry Yoo]

On Tue, Jul 01, 2025 at 10:31:00PM +0800, Lance Yang wrote:
> From: Lance Yang <lance.yang at linux.dev>
> 
> 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.
> 
> [1] https://lore.kernel.org/linux-mm/a694398c-9f03-4737-81b9-7e49c857fcbe@redhat.com
> 
> Cc: <stable at vger.kernel.org>
> Reported-by: David Hildenbrand <david at redhat.com>
> Closes: https://lore.kernel.org/linux-mm/a694398c-9f03-4737-81b9-7e49c857fcbe@redhat.com
> Fixes: 354dffd29575 ("mm: support batched unmap for lazyfree large folios during reclamation")
> Suggested-by: Barry Song <baohua at kernel.org>
> Acked-by: Barry Song <baohua at kernel.org>
> Reviewed-by: Lorenzo Stoakes <lorenzo.stoakes at oracle.com>
> Acked-by: David Hildenbrand <david at redhat.com>
> Signed-off-by: Lance Yang <lance.yang at linux.dev>
> ---

LGTM,
Reviewed-by: Harry Yoo <harry.yoo at oracle.com>

With a minor comment below.

> diff --git a/mm/rmap.c b/mm/rmap.c
> index fb63d9256f09..1320b88fab74 100644
> --- a/mm/rmap.c
> +++ b/mm/rmap.c
> @@ -2206,13 +2213,16 @@ static bool try_to_unmap_one(struct folio *folio, struct vm_area_struct *vma,
>  			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;

Just a minor comment.

We should probably teach page_vma_mapped_walk() to skip nr_pages pages,
or just rely on next_pte: do { ... } while (pte_none(ptep_get(pvmw->pte)))
loop in page_vma_mapped_walk() to skip those ptes?

Taking different paths depending on (nr_pages == folio_nr_pages(folio))
doesn't seem sensible.

>  		continue;

-- 
Cheers,
Harry / Hyeonggon