linux-kernel - Re: [PATCH] x86/fpu: move FPU state into separate cache

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Message-ID: <ea0a7663-7318-6086-1cb8-25fd9a252f83@zytor.com>
Date:   Wed, 29 Mar 2017 13:45:32 -0700
From:   "H. Peter Anvin" <hpa@...or.com>
To:     Kees Cook <keescook@...omium.org>, linux-kernel@...r.kernel.org
Cc:     Rik van Riel <riel@...hat.com>, Andy Lutomirski <luto@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, x86@...nel.org,
        Paolo Bonzini <pbonzini@...hat.com>,
        Radim Krčmář <rkrcmar@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Yu-cheng Yu <yu-cheng.yu@...el.com>,
        Masahiro Yamada <yamada.masahiro@...ionext.com>,
        Borislav Petkov <bp@...e.de>,
        Christian Borntraeger <borntraeger@...ibm.com>,
        Thomas Garnier <thgarnie@...gle.com>,
        Brian Gerst <brgerst@...il.com>,
        He Chen <he.chen@...ux.intel.com>,
        Mathias Krause <minipli@...glemail.com>,
        Fenghua Yu <fenghua.yu@...el.com>,
        Piotr Luc <piotr.luc@...el.com>, Kyle Huey <me@...ehuey.com>,
        Len Brown <len.brown@...el.com>, kvm@...r.kernel.org,
        kernel-hardening@...ts.openwall.com
Subject: Re: [PATCH] x86/fpu: move FPU state into separate cache

On 03/29/17 13:39, Kees Cook wrote:
> This removes ARCH_WANTS_DYNAMIC_TASK_STRUCT from x86, leaving only s390
> still defining this config.
> 
> In order to support future structure layout randomization of the
> task_struct, none of the structure fields are allowed to have a specific
> position or dynamic size. To enable randomization of task_struct on
> x86, the FPU state must be moved to its own dynamically sized cache,
> and dereferenced from the task_struct.
> 
> This change is nearly identical to what was done in grsecurity to support
> structure layout randomization. Hopefully I found all the needed changes.
> This passes allyesconfig, and boot tests.

Is this really what we want to happen?  It seems much more sane to
simply make them adjacent; they don't need to be part of the same
structure (in practice, there are three objects: thread_info,
task_struct, and the FPU state.)

	-hpa