Re: [PATCH] arm64/mm: restrict transparent contpte mappings to valid PTEs

["Zhao Gongyi" <zhaogongyi@bytedance.com>]

On 10/14/26 5:41 PM, David Hildenbrand (Arm) wrote:
> CCing Ryan:
> 
> https://lore.kernel.org/r/20261013093600.54995-1-zhaogongyi@bytedance.com
> 
> On 10/13/26 11:36, Zhao Gongyi wrote:
>> The transparent contpte layer treats PTE_CONT as software state: when a
>> PTE has PTE_CONT set, the contpte helpers may align the operation to a
>> CONT_PTE_SIZE block and operate on the whole block. That invariant only
>> makes sense for valid mappings. Explicit users such as hugetlb manage
>> their contiguous encodings separately.
>>
>> contpte_set_ptes() currently sets PTE_CONT from alignment alone. Batched
>> set_ptes() users can therefore install present-invalid PTEs with
>> PTE_CONT set, for example mprotect(PROT_NONE), NUMA hinting through
>> modify_prot_commit_ptes(), fork() copying a PROT_NONE large folio, or a
>> PMD split installing PAGE_NONE PTEs.
>>
>> This is not a hardware programming problem: invalid descriptors are not
>> translated, so the hardware ignores descriptor bits such as PTE_CONT. It
>> is a software-state problem. Some arm64 fast paths intentionally test
>> pte_cont() directly instead of pte_valid_cont(), for example commit
>> 9970a9a27ffc ("arm64: mm: implement the architecture-specific
>> test_and_clear_young_ptes()"). Fixing the producer side preserves these
>> fast paths and covers all consumers.
>>
>> In the observed failure, mprotect(PROT_NONE) installed a full
>> CONT_PTE_SIZE range of present-invalid PTEs with PTE_CONT set.
>> MADV_GUARD_INSTALL then zapped one PTE. The nr == 1 clear_full_ptes()
>> path only unfolds pte_valid_cont() entries, so the neighbouring
>> present-invalid entries kept PTE_CONT while the target entry became a
>> guard marker.
> 
> Hm, is there a good reason why we cannot just use the cont-pte bit for all
> *present* PTEs, even if HW would not be prepared to benefit (!pte_valid())?
> 
> I guess we really only have to make sure to shatter properly, right?
> 


Hi David,

Right, there is no architectural restriction preventing PTE_CONT from
being used as software grouping state for present-invalid PTEs. For this
particular failure, making contpte_try_unfold() recognize something like

         pte_present(pte) && pte_cont(pte)

would make clear_full_ptes(nr == 1) unfold the block before replacing one
entry with the guard marker.

My reason for preferring the producer-side fix is that the existing
transparent contpte helpers mostly use pte_valid_cont() as the membership
test, including contpte_try_unfold(), ptep_get[_lockless](),
pte_batch_hint() and contpte_is_consistent(), while some range fast paths
test pte_cont() directly.

If present-invalid PTEs are also considered members of a transparent
contpte block, I think we should audit the single-PTE modification paths,
rather than changing only this clear path. We would likely also need a
pte_present_cont() helper: the pte_present() guard matters because the
PTE_CONT bit position is part of the swap offset for non-present entries.

There is no hardware benefit while the descriptors are invalid, and
set_ptes() can recreate the contiguous encoding when an eligible range is
made valid again. For a stable fix, I therefore preferred to stop
contpte_set_ptes() from creating present-invalid transparent contpte
mappings, which also matches the pte_valid_cont() contract used by most
of the existing helpers.

I agree that saying this invariant "only makes sense" for valid mappings
is too strong. I will reword that to describe the current helper
contract instead.

If preserving the software grouping across PROT_NONE is the intended
semantics, I can rework this as a broader unfold-path audit instead.

Thanks,
Gongyi