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

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Date: Wed, 1 May 2024 11:04:44 +0100
From: Catalin Marinas <catalin.marinas@....com>
To: Ryan Roberts <ryan.roberts@....com>
Cc: 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>,
	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 Wed, May 01, 2024 at 09:05:17AM +0100, Ryan Roberts wrote:
> 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?

It looks like we don't push for-next/fixes to linux-next, it's
short-lived usually, it ends up upstream quickly. I can send the pull
request later today, should turn up in mainline by tomorrow. You can add
a note to your patch for Andrew that it will fail on arm64 until the fix
ends up upstream. It's a matter of a couple of days anyway.

-- 
Catalin