Messages in this thread | | | Date | Sun, 9 Apr 2017 12:53:36 +0200 | From | Ingo Molnar <> | Subject | Re: [kernel-hardening] Re: [RFC v2][PATCH 04/11] x86: Implement __arch_rare_write_begin/unmap() |
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* 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
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