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

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.

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