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

On Fri, Nov 4, 2016 at 7:45 AM, Juerg Haefliger [off-list ref] 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

--
To unsubscribe, send a message with 'unsubscribe linux-mm' in
the body to majordomo@kvack.org.  For more info on Linux MM,
see: http://www.linux-mm.org/ .
Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>