linux-kernel - Re: [PATCH v3 1/1] mm/rmap: fix potential out-of-bounds page table access during batched unmap

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Message-ID: <330f29ee-ba55-4ae6-a695-ddaba58d5cb8@redhat.com>
Date: Tue, 1 Jul 2025 16:03:06 +0200
From: David Hildenbrand <david@...hat.com>
To: Lance Yang <ioworker0@...il.com>, akpm@...ux-foundation.org,
 21cnbao@...il.com
Cc: 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,
 Barry Song <baohua@...nel.org>, Lance Yang <lance.yang@...ux.dev>
Subject: Re: [PATCH v3 1/1] mm/rmap: fix potential out-of-bounds page table
 access during batched unmap

On 30.06.25 03:13, Lance Yang wrote:
> From: Lance Yang <lance.yang@...ux.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
> 
> Fixes: 354dffd29575 ("mm: support batched unmap for lazyfree large folios during reclamation")
> Cc: <stable@...r.kernel.org>
> Acked-by: Barry Song <baohua@...nel.org>
> Suggested-by: David Hildenbrand <david@...hat.com>

Realized this now: This should probably be a "Reported-by:" with the 
"Closes:" and and a link to my mail.

-- 
Cheers,

David / dhildenb