[kernel-hardening] Re: [RFC v2][PATCH 04/11] x86: Implement __arch_rare_write_begin/unmap()

Andy Lutomirski luto at kernel.org
Sat Apr 8 08:20:03 PDT 2017

On Sat, Apr 8, 2017 at 12:33 AM, Daniel Micay <danielmicay at gmail.com> wrote:
> The
> submitted code is aimed at rare writes to globals, but this feature is
> more than that and design decisions shouldn't be based on just the
> short term.

Then, if you disagree with a proposed design, *explain why* in a
standalone manner.  Say what future uses a different design would

> I actually care a lot more about 64-bit ARM support than I do x86, but
> using a portable API for pax_open_kernel (for the simple uses at
> least) is separate from choosing the underlying implementation. There
> might not be a great way to do it on the architectures I care about
> but that doesn't need to hinder x86. It's really not that much code...
> A weaker/slower implementation for x86 also encourages the same
> elsewhere.

No one has explained how CR0.WP is weaker or slower than my proposal.
Here's what I'm proposing:

At boot, choose a random address A.  Create an mm_struct that has a
single VMA starting at A that represents the kernel's rarely-written
section.  Compute O = (A - VA of rarely-written section).  To do a
rare write, use_mm() the mm, write to (VA + O), then unuse_mm().

This should work on any arch that has an MMU that allows this type of
aliasing and that doesn't have PA-based protections on the
rarely-written section.

It'll be considerably slower than CR0.WP on a current x86 kernel, but,
with PCID landed, it shouldn't be much slower.  It has the added
benefit that writes to non-rare-write data using the rare-write
primitive will fail.


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