# Re: \[PATCH v23 15/28\] x86/mm: Update maybe\_mkwrite() for shadow stack

- From: Borislav Petkov \<bp@alien8.de\>
- Date: 2021-03-17 15:57:16 
- List: linux-doc
- Page: https://ratatoskr.run/linux-doc/2021/03/6305272
- Thread: https://ratatoskr.run/linux-doc/2021/03/6303369/t.md

```text
On Tue, Mar 16, 2021 at 08:10:41AM -0700, 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,

						  to cause

> and in page fault, _PAGE_DIRTY is restored and the shadow stack page is

      in the page fault handler...

> 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>
> ---
>  arch/x86/Kconfig        |  4 ++++
>  arch/x86/mm/pgtable.c   | 18 ++++++++++++++++++
>  include/linux/mm.h      |  2 ++
>  include/linux/pgtable.h | 24 ++++++++++++++++++++++++
>  mm/huge_memory.c        |  2 ++
>  5 files changed, 50 insertions(+)

Looks straightforward to me but I guess it needs a mm person's ack.

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette
```