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

[Lance Yang]

On 2025/6/27 14:55, Barry Song wrote:
> On Fri, Jun 27, 2025 at 6:52 PM Barry Song <21cnbao@xxxxxxxxx> wrote:
> > On Fri, Jun 27, 2025 at 6:23 PM Lance Yang <ioworker0@xxxxxxxxx> wrote:
> > > From: Lance Yang <lance.yang@xxxxxxxxx>
> > >
> > > 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@xxxxxxxxxx
> >
> > 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.