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

[Lance Yang <lance.yang@linux.dev>]

On 2026/10/14 16:38, Baolin Wang wrote:
> 
> 
> On 10/14/26 2:46 PM, Lance Yang wrote:
>>
>>
>> On 2026/10/14 14:31, Zhao Gongyi wrote:
>>> On 10/13/26 11:39 PM, Lance Yang wrote:
>>>>>
>>>>> +    /*
>>>>> +     * Keep the producer-side invariant documented in pgtable.h 
>>>>> true even
>>>>> +     * if a direct caller passes an invalid descriptor with 
>>>>> PTE_CONT set.
>>>>> +     */
>>>>> +    if (!pte_valid(pte))
>>>>> +        return __set_ptes(mm, addr, ptep, pte_mknoncont(pte), nr);
>>>>
>>>> set_ptes() is the only in-tree caller, and it already clears PTE_CONT
>>>> before we get here ... no? Why not fold !pte_valid(pte) into the
>>>> !mm_is_user(mm) check below and return __set_ptes() directly?
>>>>
>>>>     if (!mm_is_user(mm) || !pte_valid(pte))
>>>>         return __set_ptes(mm, addr, ptep, pte, nr);
>>>>
>>>> If a direct caller does pass a !pte_valid PTE with PTE_CONT set, I'd
>>>> rather add a VM_WARN_ON() to catch that. Silently clearing the bit 
>>>> would
>>>> hide a caller bug :)
>>>>
>>>
>>> Hi Lance,
>>>
>>> Thanks for taking a look.
>>
>> Cheers!
>>
>>> Yes, you are right. The public set_ptes() wrapper already strips 
>>> PTE_CONT
>>> before calling contpte_set_ptes(), so the pte_mknoncont() in the invalid
>>> path is unnecessary for the current in-tree path.
>>>
>>> The issue I was trying to avoid is not that the incoming pte already has
>>> PTE_CONT set, but that contpte_set_ptes() rebuilds the pte from pfn/prot
>>> and then adds PTE_CONT again based on alignment. For an invalid 
>>> descriptor,
>>> that transparent contiguous encoding should be skipped entirely.
>>>
>>> So your suggested shape makes sense to me. I will change this to return
>>> through __set_ptes() directly for invalid descriptors, and add a warning
>>> for any unexpected direct caller that passes an invalid PTE with 
>>> PTE_CONT
>>> set, e.g.:
>>>
>>>     if (!mm_is_user(mm) || !pte_valid(pte)) {
>>>         VM_WARN_ON_ONCE(!pte_valid(pte) && pte_cont(pte));
> 
> mprotect() can trigger the warning here when setting PAGE_NONE for a 
> batch of PTEs?

Nope. set_ptes() clears PTE_CONT before calling contpte_set_ptes().
PAGE_NONE makes !pte_valid(pte) true, but pte_cont(pte) is false,
so the warning won't fire :)

>>>         return __set_ptes(mm, addr, ptep, pte, nr);
> 
> I am trying to convince myself that the 'nr' passed in is always 
> CONT_PTE_SIZE aligned, but it seems OK.

Still nope :)

IIRC, any partially covered contpte block should already be unfolded
by the time we get here ...

We're just installing PTEs without PTE_CONT into cleared entries,
so nr doesn't need to be aligned to CONT_PTES.

Cheers, Lance