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

[Ingo Molnar <mingo@kernel.org>]

* Andy Lutomirski <luto@kernel.org> wrote:

> On Sat, Apr 8, 2017 at 12:33 AM, Daniel Micay <danielmicay@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
> have.
> 
> > 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().

BTW., note that this is basically a pagetable based protection key variant.

> 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.

... which is a security advantage of the use_mm() based design you suggest.

Thanks,

	Ingo