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

[Ryan Roberts]

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
>