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

[Dev Jain]

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?


> 		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