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

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Message-ID: <CAGsJ_4xQW3O=-VoC7aTCiwU4NZnK0tNsG1faAUgLvf4aZSm8Eg@mail.gmail.com>
Date: Fri, 27 Jun 2025 18:52:43 +1200
From: Barry Song <21cnbao@...il.com>
To: Lance Yang <ioworker0@...il.com>
Cc: akpm@...ux-foundation.org, david@...hat.com, baolin.wang@...ux.alibaba.com, 
	chrisl@...nel.org, kasong@...cent.com, linux-arm-kernel@...ts.infradead.org, 
	linux-kernel@...r.kernel.org, linux-mm@...ck.org, 
	linux-riscv@...ts.infradead.org, lorenzo.stoakes@...cle.com, 
	ryan.roberts@....com, v-songbaohua@...o.com, x86@...nel.org, 
	huang.ying.caritas@...il.com, zhengtangquan@...o.com, riel@...riel.com, 
	Liam.Howlett@...cle.com, vbabka@...e.cz, harry.yoo@...cle.com, 
	mingzhe.yang@...com, stable@...r.kernel.org, 
	Lance Yang <lance.yang@...ux.dev>
Subject: Re: [PATCH v2 1/1] mm/rmap: fix potential out-of-bounds page table
 access during batched unmap

On Fri, Jun 27, 2025 at 6:23 PM Lance Yang <ioworker0@...il.com> wrote:
>
> From: Lance Yang <lance.yang@...ux.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.

Thanks
Barry