Re: [RFC PATCH v3 1/2] Add support for eXclusive Page Frame Ownership (XPFO)

[Kees Cook]

On Fri, Nov 4, 2016 at 7:45 AM, Juerg Haefliger <juerg.haefliger@xxxxxxx> wrote:
> This patch adds support for XPFO which protects against 'ret2dir' kernel
> attacks. The basic idea is to enforce exclusive ownership of page frames
> by either the kernel or userspace, unless explicitly requested by the
> kernel. Whenever a page destined for userspace is allocated, it is
> unmapped from physmap (the kernel's page table). When such a page is
> reclaimed from userspace, it is mapped back to physmap.
>
> Additional fields in the page_ext struct are used for XPFO housekeeping.
> Specifically two flags to distinguish user vs. kernel pages and to tag
> unmapped pages and a reference counter to balance kmap/kunmap operations
> and a lock to serialize access to the XPFO fields.

Thanks for keeping on this! I'd really like to see it land and then
get more architectures to support it.

> Known issues/limitations:
>   - Only supports x86-64 (for now)
>   - Only supports 4k pages (for now)
>   - There are most likely some legitimate uses cases where the kernel needs
>     to access userspace which need to be made XPFO-aware
>   - Performance penalty

In the Kconfig you say "slight", but I'm curious what kinds of
benchmarks you've done and if there's a more specific cost we can
declare, just to give people more of an idea what the hit looks like?
(What workloads would trigger a lot of XPFO unmapping, for example?)

Thanks!

-Kees

-- 
Kees Cook
Nexus Security