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

[Zhao Gongyi]

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));
		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