# Re: \[PATCH v19 16/25\] mm: Add guard pages around a shadow stack.

- From: Kees Cook \<hidden\>
- Date: 2021-02-04 20:25:02 
- List: linux-arch
- Page: https://ratatoskr.run/linux-arch/2021/02/6259134
- Thread: https://ratatoskr.run/linux-doc/2021/02/6257763/t.md

```text
On Wed, Feb 03, 2021 at 02:55:38PM -0800, Yu-cheng Yu wrote:
> INCSSP(Q/D) increments shadow stack pointer and 'pops and discards' the
> first and the last elements in the range, effectively touches those memory
> areas.
> 
> The maximum moving distance by INCSSPQ is 255 * 8 = 2040 bytes and
> 255 * 4 = 1020 bytes by INCSSPD.  Both ranges are far from PAGE_SIZE.
> Thus, putting a gap page on both ends of a shadow stack prevents INCSSP,
> CALL, and RET from going beyond.
> 
> Signed-off-by: Yu-cheng Yu <redacted>

Yay guard pages! :)

Reviewed-by: Kees Cook <redacted>

-- 
Kees Cook
```