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

- From: Dave Hansen \<hidden\>
- Date: 2021-02-04 20:30:56 
- List: linux-doc
- Page: https://ratatoskr.run/linux-doc/2021/02/6259145
- Thread: https://ratatoskr.run/linux-doc/2021/02/6257763/t.md

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