# arm64 physmap (was Re: \[kernel-hardening\] \[PATCH 4/6\] Protectable Memory)

- From: Kees Cook \<hidden\>
- Date: 2018-02-14 19:48:38 
- List: linux-arm-kernel
- Page: https://ratatoskr.run/linux-arm-kernel/2018/02/5197652
- Thread: https://ratatoskr.run/linux-security-module/2018/01/5184248/t.md

```text
On Wed, Feb 14, 2018 at 11:06 AM, Laura Abbott [ref] wrote:
> fixed. Modules yes are not fully protected. The conclusion from past
> experience has been that we cannot safely break down larger page sizes
> at runtime like x86 does. We could theoretically
> add support for fixing up the alias if PAGE_POISONING is enabled but
> I don't know who would actually use that in production. Performance
> is very poor at that point.

XPFO forces 4K pages on the physmap[1] for similar reasons. I have no
doubt about performance changes, but I'd be curious to see real
numbers. Did anyone do benchmarks on just the huge/4K change? (Without
also the XPFO overhead?)

If this, XPFO, and PAGE_POISONING all need it, I think we have to
start a closer investigation. :)

-Kees

[1] http://www.openwall.com/lists/kernel-hardening/2017/09/07/13

-- 
Kees Cook
Pixel Security
```