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

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Message-ID: <20251016093310.rilvenglgnr65ojq@master>
Date: Thu, 16 Oct 2025 09:33:10 +0000
From: Wei Yang <richard.weiyang@...il.com>
To: Lance Yang <lance.yang@...ux.dev>
Cc: akpm@...ux-foundation.org, david@...hat.com, lorenzo.stoakes@...cle.com,
	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
Subject: Re: [PATCH mm-new 1/1] mm/khugepaged: guard is_zero_pfn() calls with
 pte_present()

On Thu, Oct 16, 2025 at 11:36:43AM +0800, 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.
>

Does it more like to guard pte_pfn() with pte_present()?

-- 
Wei Yang
Help you, Help me