Date: Sat, 8 Apr 2017 08:20:03 -0700 From: Andy Lutomirski <luto@...nel.org> To: Daniel Micay <danielmicay@...il.com> Cc: Andy Lutomirski <luto@...nel.org>, 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() 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(). 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. --Andy
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