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

[Harry Yoo <harry.yoo@oracle.com>]

On Mon, Jul 07, 2025 at 11:40:55PM +0800, Barry Song wrote:
> On Mon, Jul 7, 2025 at 1:40 PM Harry Yoo <harry.yoo@oracle.com> wrote:
> >
> > On Tue, Jul 01, 2025 at 10:31:00PM +0800, Lance Yang wrote:
> > > From: Lance Yang <lance.yang@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@vger.kernel.org>
> > > Reported-by: David Hildenbrand <david@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@kernel.org>
> > > Acked-by: Barry Song <baohua@kernel.org>
> > > Reviewed-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
> > > Acked-by: David Hildenbrand <david@redhat.com>
> > > Signed-off-by: Lance Yang <lance.yang@linux.dev>
> > > ---
> >
> > LGTM,
> > Reviewed-by: Harry Yoo <harry.yoo@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 teachhttps://lore.kernel.org/linux-mm/5db6fb4c-079d-4237-80b3-637565457f39@redhat.com/() 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.
> 
> Hi Harry,

Hi Lance and Barry.
 
> I believe we've already had this discussion here:
> https://lore.kernel.org/linux-mm/5db6fb4c-079d-4237-80b3-637565457f39@redhat.com/
>
> My main point is that nr_pages = folio_nr_pages(folio) is the
> typical/common case.
> Also, modifying page_vma_mapped_walk() feels like a layering violation.

Agreed. Perhaps it's not worth the trouble, nevermind :)

The patch looks good to me as-is.

-- 
Cheers,
Harry / Hyeonggon

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