linux-kernel - Re: [PATCH v2] arm64/mm: pmd_mkinvalid() must handle swap pmds

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Message-ID: <e0624ca9-b321-479e-9b64-59a4cd242f6d@arm.com>
Date: Wed, 1 May 2024 09:05:17 +0100
From: Ryan Roberts <ryan.roberts@....com>
To: Catalin Marinas <catalin.marinas@....com>, Will Deacon <will@...nel.org>,
 Mark Rutland <mark.rutland@....com>,
 Anshuman Khandual <anshuman.khandual@....com>,
 Andrew Morton <akpm@...ux-foundation.org>, Zi Yan <zi.yan@...rutgers.edu>,
 "Aneesh Kumar K.V" <aneesh.kumar@...nel.org>
Cc: linux-arm-kernel@...ts.infradead.org, linux-mm@...ck.org,
 linux-kernel@...r.kernel.org, stable@...r.kernel.org
Subject: Re: [PATCH v2] arm64/mm: pmd_mkinvalid() must handle swap pmds

On 30/04/2024 18:57, Catalin Marinas wrote:
> On Tue, 30 Apr 2024 14:31:38 +0100, Ryan Roberts wrote:
>> __split_huge_pmd_locked() can be called for a present THP, devmap or
>> (non-present) migration entry. It calls pmdp_invalidate()
>> unconditionally on the pmdp and only determines if it is present or not
>> based on the returned old pmd.
>>
>> But arm64's pmd_mkinvalid(), called by pmdp_invalidate(),
>> unconditionally sets the PMD_PRESENT_INVALID flag, which causes future
>> pmd_present() calls to return true - even for a swap pmd. Therefore any
>> lockless pgtable walker could see the migration entry pmd in this state
>> and start interpretting the fields (e.g. pmd_pfn()) as if it were
>> present, leading to BadThings (TM). GUP-fast appears to be one such
>> lockless pgtable walker.
>>
>> [...]
> 
> Applied to arm64 (for-next/fixes), thanks! It should land in 6.9-rc7. I
> removed the debug/test code, please send it as a separate patch for
> 6.10.

Thanks Catalin! I'm guessing this will turn up in today's linux-next, so if I
send the tests today and Andrew puts them straight in mm-unstable (which will
goto linux-next) there is no risk that the tests are there without the fix? Or
do I need to hold off until the fix is in v6.9-rc7?

> 
> [1/1] arm64/mm: pmd_mkinvalid() must handle swap pmds
>       https://git.kernel.org/arm64/c/e783331c7720
>