# Re: \[PATCH v19 14/25\] x86/mm: Update maybe\_mkwrite() for shadow stack

- From: Kees Cook \<hidden\>
- Date: 2021-02-04 20:23:20 
- List: linux-doc
- Page: https://ratatoskr.run/linux-doc/2021/02/6259130
- Thread: https://ratatoskr.run/linux-doc/2021/02/6257763/t.md

```text
On Wed, Feb 03, 2021 at 02:55:36PM -0800, Yu-cheng Yu wrote:
> When serving a page fault, maybe_mkwrite() makes a PTE writable if its vma
> has VM_WRITE.
> 
> A shadow stack vma has VM_SHSTK.  Its PTEs have _PAGE_DIRTY, but not
> _PAGE_WRITE.  In fork(), _PAGE_DIRTY is cleared to effect copy-on-write,
> and in page fault, _PAGE_DIRTY is restored and the shadow stack page is
> writable again.
> 
> Update maybe_mkwrite() by introducing arch_maybe_mkwrite(), which sets
> _PAGE_DIRTY for a shadow stack PTE.
> 
> Apply the same changes to maybe_pmd_mkwrite().
> 
> Signed-off-by: Yu-cheng Yu <redacted>

Reviewed-by: Kees Cook <redacted>

-- 
Kees Cook
```