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

[Anshuman Khandual <anshuman.khandual@arm.com>]

On 4/27/24 20:37, Zi Yan wrote:
> 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.

Agreed, this is not a HW issue at all, MMU sees such an entry as invalid
even if pmd_present() returns true.