# Re: \[PATCH v28 09/32\] x86/mm: Introduce \_PAGE\_COW

- From: Yu, Yu-cheng \<hidden\>
- Date: 2021-08-17 18:24:45 
- List: linux-arch
- Page: https://ratatoskr.run/linux-arch/2021/08/1480698
- Thread: https://ratatoskr.run/linux-doc/2021/07/1455208/t.md

```text
On 8/16/2021 3:43 AM, Borislav Petkov wrote:
> On Thu, Jul 22, 2021 at 01:51:56PM -0700, Yu-cheng Yu wrote:
>> @@ -153,13 +178,23 @@ static inline int pud_young(pud_t pud)
>>   
>>   static inline int pte_write(pte_t pte)
>>   {
>> -	return pte_flags(pte) & _PAGE_RW;
>> +	/*
>> +	 * Shadow stack pages are always writable - but not by normal
>> +	 * instructions, and only by shadow stack operations.  Therefore,
>> +	 * the W=0,D=1 test with pte_shstk().
>> +	 */
>> +	return (pte_flags(pte) & _PAGE_RW) || pte_shstk(pte);
>
> Well, this is weird: if some kernel code queries a shstk page and this
> here function says it is writable but then goes and tries to write into
> it and that write fails, then it'll confuse the user.
>
> IOW, from where I'm standing, that should be:
>
> 	return (pte_flags(pte) & _PAGE_RW) && !pte_shstk(pte);
>
> as in, a writable page is one which has _PAGE_RW and it is *not* a
> shadow stack page because latter is special and not really writable.
> > Hmmm?
>

Indeed, this can be looked at in a few ways.  We can visualize pte_write() as 'CPU can write to it with MOV' or 'CPU can write to it with any opcodes'.  Depending on whatever pte_write() is, copy-on-write code can be adjusted accordingly.

Yu-cheng
```