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

[Lance Yang]

On 2026/10/14 18:03, Dev Jain wrote:
> 
> 
> On 14/10/26 12:01 pm, 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.
>>
>> 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));
> 
> set_ptes does pte_mknoncont(), so the warning will *never* fire I think?

YES. Just a sanity check to catch a future direct caller passing
a !pte_valid PTE with PTE_CONT set, rather than silently clearing
the bit :)

Cheers, Lance

> 
>> 		return __set_ptes(mm, addr, ptep, pte, nr);
>> 	}
>>
>> That keeps the fix focused on not creating new invalid contpte entries,
>> while still catching a bad direct caller instead of silently sanitizing it.
>>
>> Thanks,
>> Zhao
>