linux-kernel - Re: [PATCH v5 1/1] mm/rmap: fix soft-dirty and uffd-wp bit loss when remapping zero-filled mTHP subpage to shared zeropage

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [day] [month] [year] [list]

Message-ID: <aN3SDmKeVqIXwGfV@hyeyoo>
Date: Thu, 2 Oct 2025 10:14:54 +0900
From: Harry Yoo <harry.yoo@...cle.com>
To: Lance Yang <lance.yang@...ux.dev>
Cc: akpm@...ux-foundation.org, david@...hat.com, lorenzo.stoakes@...cle.com,
        peterx@...hat.com, ziy@...dia.com, baolin.wang@...ux.alibaba.com,
        baohua@...nel.org, ryan.roberts@....com, dev.jain@....com,
        npache@...hat.com, riel@...riel.com, Liam.Howlett@...cle.com,
        vbabka@...e.cz, jannh@...gle.com, matthew.brost@...el.com,
        joshua.hahnjy@...il.com, rakie.kim@...com, byungchul@...com,
        gourry@...rry.net, ying.huang@...ux.alibaba.com, apopple@...dia.com,
        usamaarif642@...il.com, yuzhao@...gle.com,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org, ioworker0@...il.com,
        stable@...r.kernel.org
Subject: Re: [PATCH v5 1/1] mm/rmap: fix soft-dirty and uffd-wp bit loss when
 remapping zero-filled mTHP subpage to shared zeropage

On Tue, Sep 30, 2025 at 04:10:40PM +0800, Lance Yang wrote:
> From: Lance Yang <lance.yang@...ux.dev>
> 
> When splitting an mTHP and replacing a zero-filled subpage with the shared
> zeropage, try_to_map_unused_to_zeropage() currently drops several important
> PTE bits.
> 
> For userspace tools like CRIU, which rely on the soft-dirty mechanism for
> incremental snapshots, losing the soft-dirty bit means modified pages are
> missed, leading to inconsistent memory state after restore.
> 
> As pointed out by David, the more critical uffd-wp bit is also dropped.
> This breaks the userfaultfd write-protection mechanism, causing writes
> to be silently missed by monitoring applications, which can lead to data
> corruption.
> 
> Preserve both the soft-dirty and uffd-wp bits from the old PTE when
> creating the new zeropage mapping to ensure they are correctly tracked.
> 
> Cc: <stable@...r.kernel.org>
> Fixes: b1f202060afe ("mm: remap unused subpages to shared zeropage when splitting isolated thp")
> Suggested-by: David Hildenbrand <david@...hat.com>
> Suggested-by: Dev Jain <dev.jain@....com>
> Acked-by: David Hildenbrand <david@...hat.com>
> Reviewed-by: Dev Jain <dev.jain@....com>
> Signed-off-by: Lance Yang <lance.yang@...ux.dev>
> ---

Looks good to me,
Reviewed-by: Harry Yoo <harry.yoo@...cle.com>

-- 
Cheers,
Harry / Hyeonggon