Re: [PATCH v3] mm: Fix race between __split_huge_pmd_locked() and GUP-fast

[Ryan Roberts <ryan.roberts@arm.com>]

On 02/05/2024 02:27, Zi Yan wrote:
> On 1 May 2024, at 10:33, 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.
>>
>> Fix all of this by only calling pmdp_invalidate() for a present pmd. And
>> for good measure let's add a warning to all implementations of
>> pmdp_invalidate[_ad](). I've manually reviewed all other
>> pmdp_invalidate[_ad]() call sites and believe all others to be
>> conformant.
>>
>> This is a theoretical bug found during code review. I don't have any
>> test case to trigger it in practice.
>>
>> Cc: stable@vger.kernel.org
>> Link: https://lore.kernel.org/all/0dd7827a-6334-439a-8fd0-43c98e6af22b@arm.com/
>> Fixes: 84c3fc4e9c56 ("mm: thp: check pmd migration entry in common path")
>> Signed-off-by: Ryan Roberts <ryan.roberts@arm.com>
>> ---
>>
>> Right v3; this goes back to the original approach in v1 to fix core-mm rather
>> than push the fix into arm64, since we discovered that x86 can't handle
>> pmd_mkinvalid() being called for non-present pmds either.
>>
>> I'm pulling in more arch maintainers because this version adds some warnings in
>> arch code to help spot incorrect usage.
>>
>> Although Catalin had already accepted v2 (fixing arm64) [2] into for-next/fixes,
>> he's agreed to either remove or revert it.
>>
>>
>> Changes since v1 [1]
>> ====================
>>
>>   - Improve pmdp_mkinvalid() docs to make it clear it can only be called for
>>     present pmd (per JohnH, Zi Yan)
>>   - Added warnings to arch overrides of pmdp_invalidate[_ad]() (per Zi Yan)
>>   - Moved comment next to new location of pmpd_invalidate() (per Zi Yan)
>>
>>
>> [1] https://lore.kernel.org/linux-mm/20240425170704.3379492-1-ryan.roberts@arm.com/
>> [2] https://lore.kernel.org/all/20240430133138.732088-1-ryan.roberts@arm.com/
>>
>> Thanks,
>> Ryan
>>
>>
>>  Documentation/mm/arch_pgtable_helpers.rst |  6 ++-
>>  arch/powerpc/mm/book3s64/pgtable.c        |  1 +
>>  arch/s390/include/asm/pgtable.h           |  4 +-
>>  arch/sparc/mm/tlb.c                       |  1 +
>>  arch/x86/mm/pgtable.c                     |  2 +
>>  mm/huge_memory.c                          | 49 ++++++++++++-----------
>>  mm/pgtable-generic.c                      |  2 +
>>  7 files changed, 39 insertions(+), 26 deletions(-)
> 
> The changes in Documentation/mm and mm/* look good to me. Thanks.
> Reviewed-by: Zi Yan <ziy@nvidia.com>

Thanks!

> 
> I wonder if making Documentation/mm and mm/* changes in a separate patch
> would be better, since you will not need acks from arch maintainers and
> get the patch in quicker.

Yeah maybe - I considered that, but then decided I'm literally just adding a
debug warning to the arch code so it shouldn't be too controversial. Perhaps
wait a few days for acks and if nothing turns up then I'll re-post with it split?

> 
> 
> --
> Best Regards,
> Yan, Zi