linux-kernel - Re: [PATCH mm-new v3 1/1] mm/khugepaged: guard is_zero_pfn() calls with pte

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Message-ID: <81fc5322-6ab4-4830-9b2e-15d834b461c2@redhat.com>
Date: Mon, 20 Oct 2025 19:14:38 +0200
From: David Hildenbrand <david@...hat.com>
To: Lance Yang <lance.yang@...ux.dev>, akpm@...ux-foundation.org,
 lorenzo.stoakes@...cle.com
Cc: ziy@...dia.com, baolin.wang@...ux.alibaba.com, Liam.Howlett@...cle.com,
 npache@...hat.com, ryan.roberts@....com, dev.jain@....com,
 baohua@...nel.org, ioworker0@...il.com, linux-kernel@...r.kernel.org,
 linux-mm@...ck.org, Wei Yang <richard.weiyang@...il.com>
Subject: Re: [PATCH mm-new v3 1/1] mm/khugepaged: guard is_zero_pfn() calls
 with pte_present()

On 20.10.25 17:11, Lance Yang wrote:
> From: Lance Yang <lance.yang@...ux.dev>
> 
> A non-present entry, like a swap PTE, contains completely different data
> (swap type and offset). pte_pfn() doesn't know this, so if we feed it a
> non-present entry, it will spit out a junk PFN.
> 
> What if that junk PFN happens to match the zeropage's PFN by sheer
> chance? While really unlikely, this would be really bad if it did.
> 
> So, let's fix this potential bug by ensuring all calls to is_zero_pfn()
> in khugepaged.c are properly guarded by a pte_present() check.
> 
> Suggested-by: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
> Reviewed-by: Nico Pache <npache@...hat.com>
> Reviewed-by: Dev Jain <dev.jain@....com>
> Reviewed-by: Baolin Wang <baolin.wang@...ux.alibaba.com>
> Reviewed-by: Wei Yang <richard.weiyang@...il.com>
> Signed-off-by: Lance Yang <lance.yang@...ux.dev>
> ---

Works for me!

Acked-by: David Hildenbrand <david@...hat.com>

-- 
Cheers

David / dhildenb