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Date: Sun, 9 Apr 2017 12:53:36 +0200
From: Ingo Molnar <mingo@...nel.org>
To: Andy Lutomirski <luto@...nel.org>
Cc: Daniel Micay <danielmicay@...il.com>,
Mathias Krause <minipli@...glemail.com>,
Thomas Gleixner <tglx@...utronix.de>,
Kees Cook <keescook@...omium.org>,
"kernel-hardening@...ts.openwall.com" <kernel-hardening@...ts.openwall.com>,
Mark Rutland <mark.rutland@....com>,
Hoeun Ryu <hoeun.ryu@...il.com>, PaX Team <pageexec@...email.hu>,
Emese Revfy <re.emese@...il.com>,
Russell King <linux@...linux.org.uk>, X86 ML <x86@...nel.org>,
"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
"linux-arm-kernel@...ts.infradead.org" <linux-arm-kernel@...ts.infradead.org>,
Peter Zijlstra <peterz@...radead.org>
Subject: Re: Re: [RFC v2][PATCH 04/11] x86: Implement
__arch_rare_write_begin/unmap()
* Andy Lutomirski <luto@...nel.org> wrote:
> On Sat, Apr 8, 2017 at 12:33 AM, Daniel Micay <danielmicay@...il.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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