Re: [PATCH v1] mm: Fix race between __split_huge_pmd_locked() and GUP-fast

[Zi Yan <ziy@nvidia.com>]

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On 27 Apr 2024, at 0:25, John Hubbard wrote:

> On 4/26/24 7:53 AM, Zi Yan wrote:
>
> Hi Zi (and Ryan)!
>
>>>>>> 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.
>>>>>
>>>>> Could you please explain how bad things might happen ?
>>>>
>>>> See 2 places where pmdp_get_lockless() is called in gup.c, without the PTL.
>>>> These could both return the swap pte for which pmd_mkinvalid() has been called.
>>>> In both cases, this would lead to the pmd_present() check eroneously returning
>>>> true, eventually causing incorrect interpretation of the pte fields. e.g.:
>>>>
>>>> gup_pmd_range()
>>>>    pmd_t pmd = pmdp_get_lockless(pmdp);
>>>>    gup_huge_pmd(pmd, ...)
>>>>      page = nth_page(pmd_page(orig), (addr & ~PMD_MASK) >> PAGE_SHIFT);
>>>>
>>>> page is guff.
>>>>
>>>> Let me know what you think!
>>
>> Add JohnH to check GUP code.
>
> Ryan is correct about this behavior.
>
> By the way, remember that gup is not the only lockless page table
> walker: there is also the CPU hardware itself, which inconveniently
> refuses to bother with taking page table locks. :)
>
> So if we have code that can make a non-present PTE appear to be present
> to any of these page walkers, whether software or hardware, it's a
> definitely Not Good and will lead directly to bugs.

This issue does not bother hardware, because the PTE_VALID/PMD_SECT_VALID
is always unset and hardware always sees this PMD as invalid. It is a pure
software issue, since for THP splitting, we do not want hardware to access
the page but still allow kernel to user pmd_page() to get the pfn, so
pmd_present() returns true even if PTE_VALID/PMD_SECT_VALID is unset by
setting and checking PMD_PRESENT_INVALID bit. pmd_mkinvalid() sets
PMD_PRESENT_INVALID, turning a migration entry from !pmd_present() to
pmd_present(), while it is always a invalid PMD to hardware.

>
> Since I had to study this patch and discussion a bit in order to
> respond, I'll go ahead and also reply to the original patch with review
> comments.
>
>
> thanks,
> -- 
> John Hubbard
> NVIDIA


--
Best Regards,
Yan, Zi

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