Re: [kernel-hardening] [RFC PATCH v2 0/3] Add support for eXclusive Page Frame Ownership (XPFO)

[Mark Rutland]

Hi,

On Wed, Sep 14, 2016 at 09:18:58AM +0200, Juerg Haefliger wrote:

> This patch series 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.

> 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
> 
> Reference paper by the original patch authors:
>   http://www.cs.columbia.edu/~vpk/papers/ret2dir.sec14.pdf

Just to check, doesn't DEBUG_RODATA ensure that the linear mapping is
non-executable on x86_64 (as it does for arm64)?

For both arm64 and x86_64, DEBUG_RODATA is mandatory (or soon to be so).
Assuming that implies a lack of execute permission for x86_64, that
should provide a similar level of protection against erroneously
branching to addresses in the linear map, without the complexity and
overhead of mapping/unmapping pages.

So to me it looks like this approach may only be useful for
architectures without page-granular execute permission controls.

Is this also intended to protect against erroneous *data* accesses to
the linear map?

Am I missing something?

Thanks,
Mark.