Re: [PATCH -next 3/4] arm64: mm: add support for page table check

[Tong Tiangen <tongtiangen@huawei.com>]

在 2022/3/18 3:00, Catalin Marinas 写道:
> On Thu, Mar 17, 2022 at 02:12:02PM +0000, Tong Tiangen wrote:
>> @@ -628,6 +647,25 @@ static inline unsigned long pmd_page_vaddr(pmd_t pmd)
>>   #define pud_leaf(pud)		pud_sect(pud)
>>   #define pud_valid(pud)		pte_valid(pud_pte(pud))
>>   
>> +#ifdef CONFIG_PAGE_TABLE_CHECK
>> +static inline bool pte_user_accessible_page(pte_t pte)
>> +{
>> +	return (pte_val(pte) & PTE_VALID) && (pte_val(pte) & PTE_USER);
>> +}
> 
> There is another class of user mappings, execute-only, that have both
> PTE_USER and PTE_UXN cleared. So this logic should be:
> 
> 	pte_valid(pte) && (pte_user(pte) || pte_user_exec(pte))
> 
> with pte_user() as:
> 
> #define pte_user(pte)	(!!(pte_val(pte) & PTE_USER))

Good suggestion, the PTC(page table check) can cover UXN page and 
pte_user(pte) helper is required.

> 
> Do we care about PROT_NONE mappings here? They have the valid bit
> cleared but pte_present() is true.
>

PTC will not check this special type(PROT_NONE) of page.

>> +static inline bool pmd_user_accessible_page(pmd_t pmd)
>> +{
>> +	return pmd_leaf(pmd) && (pmd_val(pmd) & PTE_VALID) &&
>> +		(pmd_val(pmd) & PTE_USER);
>> +}
> 
> pmd_leaf() implies valid, so you can skip it if that's the aim.

PTC only checks whether the memory block corresponding to the pmd_leaf 
type can access, for !pmd_leaf, PTC checks at the pte level. So i think 
this is necessary.

> 
> Similar comment to the pte variant on execute-only and PROT_NONE
> mappings

Same considerations as above.

Thanks.
Tong
>