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

[Ryan Roberts]

On 30/04/2024 14:55, Will Deacon wrote:
> On Tue, Apr 30, 2024 at 02:31:38PM +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.
>>
>> While the obvious fix is for core-mm to avoid such calls for non-present
>> pmds (pmdp_invalidate() will also issue TLBI which is not necessary for
>> this case either), all other arches that implement pmd_mkinvalid() do it
>> in such a way that it is robust to being called with a non-present pmd.
>> So it is simpler and safer to make arm64 robust too. This approach means
>> we can even add tests to debug_vm_pgtable.c to validate the required
>> behaviour.
>>
>> This is a theoretical bug found during code review. I don't have any
>> test case to trigger it in practice.
>>
>> Cc: stable at vger.kernel.org
>> Fixes: 53fa117bb33c ("arm64/mm: Enable THP migration")
>> Signed-off-by: Ryan Roberts <ryan.roberts at arm.com>
>> ---
>>
>> Hi all,
>>
>> v1 of this fix [1] took the approach of fixing core-mm to never call
>> pmdp_invalidate() on a non-present pmd. But Zi Yan highlighted that only arm64
>> suffers this problem; all other arches are robust. So his suggestion was to
>> instead make arm64 robust in the same way and add tests to validate it. Despite
>> my stated reservations in the context of the v1 discussion, having thought on it
>> for a bit, I now agree with Zi Yan. Hence this post.
>>
>> Andrew has v1 in mm-unstable at the moment, so probably the best thing to do is
>> remove it from there and have this go in through the arm64 tree? Assuming there
>> is agreement that this approach is right one.
>>
>> This applies on top of v6.9-rc5. Passes all the mm selftests on arm64.
>>
>> [1] https://lore.kernel.org/linux-mm/20240425170704.3379492-1-ryan.roberts@arm.com/
>>
>> Thanks,
>> Ryan
>>
>>
>>  arch/arm64/include/asm/pgtable.h | 12 +++++--
>>  mm/debug_vm_pgtable.c            | 61 ++++++++++++++++++++++++++++++++
>>  2 files changed, 71 insertions(+), 2 deletions(-)
>>
>> diff --git a/arch/arm64/include/asm/pgtable.h b/arch/arm64/include/asm/pgtable.h
>> index afdd56d26ad7..7d580271a46d 100644
>> --- a/arch/arm64/include/asm/pgtable.h
>> +++ b/arch/arm64/include/asm/pgtable.h
>> @@ -511,8 +511,16 @@ static inline int pmd_trans_huge(pmd_t pmd)
>>
>>  static inline pmd_t pmd_mkinvalid(pmd_t pmd)
>>  {
>> -	pmd = set_pmd_bit(pmd, __pgprot(PMD_PRESENT_INVALID));
>> -	pmd = clear_pmd_bit(pmd, __pgprot(PMD_SECT_VALID));
>> +	/*
>> +	 * If not valid then either we are already present-invalid or we are
>> +	 * not-present (i.e. none or swap entry). We must not convert
>> +	 * not-present to present-invalid. Unbelievably, the core-mm may call
>> +	 * pmd_mkinvalid() for a swap entry and all other arches can handle it.
>> +	 */
>> +	if (pmd_valid(pmd)) {
>> +		pmd = set_pmd_bit(pmd, __pgprot(PMD_PRESENT_INVALID));
>> +		pmd = clear_pmd_bit(pmd, __pgprot(PMD_SECT_VALID));
>> +	}
>>
>>  	return pmd;
>>  }
> 
> Acked-by: Will Deacon <will at kernel.org>

Thanks

> 
> But it might be worth splitting the tests from the fix to make backporting
> easier.

Yes good point. I'll leave this hanging for today to see if any more comments
come in, and will re-post tomorrow as 2 patches. I assume we need to go fast to
catch 6.9.

> 
> Catalin -- I assume you'll pick this up, but please shout if you want me
> to take it instead.
> 
> Will