[PATCH v2 1/1] mm/rmap: fix potential out-of-bounds page table access during batched unmap

[Barry Song]

On Fri, Jun 27, 2025 at 7:15 PM Lance Yang <lance.yang at linux.dev> wrote:
>
>
>
> On 2025/6/27 14:55, Barry Song wrote:
> > On Fri, Jun 27, 2025 at 6:52 PM Barry Song <21cnbao at gmail.com> wrote:
> >>
> >> On Fri, Jun 27, 2025 at 6:23 PM Lance Yang <ioworker0 at gmail.com> wrote:
> >>>
> >>> From: Lance Yang <lance.yang at linux.dev>
> >>>
> >>> As pointed out by David[1], the batched unmap logic in try_to_unmap_one()
> >>> can read past the end of a PTE table if a large folio is mapped starting at
> >>> the last entry of that table. It would be quite rare in practice, as
> >>> MADV_FREE typically splits the large folio ;)
> >>>
> >>> So let's fix the potential out-of-bounds read by refactoring the logic into
> >>> a new helper, folio_unmap_pte_batch().
> >>>
> >>> The new helper now correctly calculates the safe number of pages to scan by
> >>> limiting the operation to the boundaries of the current VMA and the PTE
> >>> table.
> >>>
> >>> In addition, the "all-or-nothing" batching restriction is removed to
> >>> support partial batches. The reference counting is also cleaned up to use
> >>> folio_put_refs().
> >>>
> >>> [1] https://lore.kernel.org/linux-mm/a694398c-9f03-4737-81b9-7e49c857fcbe@redhat.com
> >>>
> >>
> >> What about ?
> >>
> >> 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 spans across two PMDs,
> >> particularly after being remapped with mremap(). This patch fixes the
> >> potential out-of-bounds access by capping the batch at vm_end and the PMD
> >> boundary.
> >>
> >> It also refactors the logic into a new helper, folio_unmap_pte_batch(),
> >> which supports batching between 1 and folio_nr_pages. This improves code
> >> clarity. Note that such cases are rare in practice, as MADV_FREE typically
> >> splits large folios.
> >
> > Sorry, I meant that MADV_FREE typically splits large folios if the specified
> > range doesn't cover the entire folio.
>
> Hmm... I got it wrong as well :( It's the partial coverage that triggers
> the split.
>
> how about this revised version:
>
> 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 spans across two
> PMDs, particularly after being remapped with mremap(). This patch fixes
> the potential out-of-bounds access by capping the batch at vm_end and the
> PMD boundary.
>
> It also refactors the logic into a new helper, folio_unmap_pte_batch(),
> which supports batching between 1 and folio_nr_pages. This improves code
> clarity. Note that such boundary-straddling cases are rare in practice, as
> MADV_FREE will typically split a large folio if the advice range does not
> cover the entire folio.

I assume the out-of-bounds access must be fixed, even though it is very
unlikely. It might occur after a large folio is marked with MADV_FREE and
then remapped to an unaligned address, potentially crossing two PTE tables.

A batch size between 2 and nr_pages - 1 is practically rare, as we typically
split large folios when MADV_FREE does not cover the entire folio range.
Cases where a batch of size 2 or nr_pages - 1 occurs may only happen if a
large folio is partially unmapped after being marked MADV_FREE, which is
quite an unusual pattern in userspace.

Let's wait for David's feedback before preparing a new version :-)

Thanks
Barry