linux-kernel - Re: [PATCH v3] mm: Fix race between __split_huge_pmd

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Date: Thu, 2 May 2024 14:47:05 +0100
From: Ryan Roberts <ryan.roberts@....com>
To: David Hildenbrand <david@...hat.com>,
 Andrew Morton <akpm@...ux-foundation.org>,
 Catalin Marinas <catalin.marinas@....com>, Will Deacon <will@...nel.org>,
 Mark Rutland <mark.rutland@....com>,
 Anshuman Khandual <anshuman.khandual@....com>, Zi Yan
 <zi.yan@...rutgers.edu>, "Aneesh Kumar K.V" <aneesh.kumar@...nel.org>,
 Jonathan Corbet <corbet@....net>, Nicholas Piggin <npiggin@...il.com>,
 Christophe Leroy <christophe.leroy@...roup.eu>,
 "Naveen N. Rao" <naveen.n.rao@...ux.ibm.com>,
 Christian Borntraeger <borntraeger@...ux.ibm.com>,
 Sven Schnelle <svens@...ux.ibm.com>, "David S. Miller"
 <davem@...emloft.net>, Andreas Larsson <andreas@...sler.com>,
 Dave Hansen <dave.hansen@...ux.intel.com>, Andy Lutomirski
 <luto@...nel.org>, Peter Zijlstra <peterz@...radead.org>,
 Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>,
 Borislav Petkov <bp@...en8.de>
Cc: linux-arm-kernel@...ts.infradead.org, linux-mm@...ck.org,
 linux-kernel@...r.kernel.org, stable@...r.kernel.org,
 Felix Kuehling <Felix.Kuehling@....com>
Subject: Re: [PATCH v3] mm: Fix race between __split_huge_pmd_locked() and
 GUP-fast

On 02/05/2024 14:08, David Hildenbrand wrote:
> On 01.05.24 16:33, 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. This is a problem for the migration entry
>> case because pmd_mkinvalid(), called by pmdp_invalidate() must only be
>> called for a present pmd.
>>
>> On arm64 at least, pmd_mkinvalid() will mark the pmd such that any
>> future call to pmd_present() will return true. And therefore any
>> lockless pgtable walker could see the migration entry pmd in this state
>> and start interpretting the fields as if it were present, leading to
>> BadThings (TM). GUP-fast appears to be one such lockless pgtable walker.
>>
>> x86 does not suffer the above problem, but instead pmd_mkinvalid() will
>> corrupt the offset field of the swap entry within the swap pte. See link
>> below for discussion of that problem.
> 
> Could that explain:
> 
> https://lore.kernel.org/all/YjoGbhreg8lGCGIJ@linutronix.de/
> 
> Where the PFN of a migration entry might have been corrupted?

Ahh interesting! Yes, it seems to fit...

> 
> Ccing Felix

Are you able to reliably reproduce the bug, Felix? If so, would you mind trying
with this patch to see if it goes away?

> 
> 
> Patch itself looks good to me
> 
> Acked-by: David Hildenbrand <david@...hat.com>

Thanks!