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

[Catalin Marinas <catalin.marinas@arm.com>]

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