# Re: \[PATCH v19 08/25\] x86/mm: Introduce \_PAGE\_COW

- From: Yu, Yu-cheng \<hidden\>
- Date: 2021-02-05 18:45:56 
- List: linux-doc
- Page: https://ratatoskr.run/linux-doc/2021/02/6260163
- Thread: https://ratatoskr.run/linux-doc/2021/02/6257763/t.md

```text
On 2/4/2021 12:27 PM, Dave Hansen wrote:
> On 2/4/21 12:19 PM, Kees Cook wrote:
>>> (e) A page where the processor observed a Write=1 PTE, started a write, set
>>>      Dirty=1, but then observed a Write=0 PTE.  That's possible today, but
>>>      will not happen on processors that support shadow stack.
>> What happens for "e" with/without CET? It sounds like direct writes to
>> such pages will be (correctly) rejected by the MMU?
>
> A page fault would be generated regardless of CET support.
>
> If CET were not around, the fault would be reported as a present, write
> fault.
>
> If this happened and CET were around (which shouldn't happen in
> practice, it means we have a hardware issue) a page fault exception is
> generated.

Thanks for the clarification.  With or without CET, direct write to Write=0, Dirty=1 PTE triggers page fault.

> Yu-cheng, I'm not sure there's enough debugging around to
> tell us if this happens.  Would we even notice?

That potential hardware issue is, on a CET-capable system, a processor writes to a Write=1, Dirty=0 page, and then observes the PTE is Write=0, Dirty=1.  Let me think about it...

Thanks!

--
Yu-cheng
```