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

[Zi Yan <ziy@nvidia.com>]

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On 2 May 2024, at 3:33, Ryan Roberts wrote:

> 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?
Sure, sounds good to me. Thanks.

--
Best Regards,
Yan, Zi

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