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

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Message-ID: <ca888ec2-e2e6-3600-3e39-c18e61e0c735@linux.intel.com>
Date:   Wed, 29 Mar 2017 14:19:37 -0700
From:   Dave Hansen <dave.hansen@...ux.intel.com>
To:     Kees Cook <keescook@...omium.org>, "H. Peter Anvin" <hpa@...or.com>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Rik van Riel <riel@...hat.com>,
        Andy Lutomirski <luto@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "x86@...nel.org" <x86@...nel.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Radim Krčmář <rkrcmar@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        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 <kvm@...r.kernel.org>,
        "kernel-hardening@...ts.openwall.com" 
        <kernel-hardening@...ts.openwall.com>
Subject: Re: [PATCH] x86/fpu: move FPU state into separate cache

On 03/29/2017 02:09 PM, Kees Cook wrote:
> They're adjacent already, which poses a problem for the struct layout
> randomization plugin, since adjacency may no longer be true (after
> layout randomization). This adjacency (or not) isn't really the
> problem: it's that FPU state size is only known at runtime. Another
> solution would be to have FPU state be a fixed size...

We don't want that.  It varies from a couple hundred bytes to ~3k on
newer CPUs.  We don't want to eat an extra 2.5k per task on the older
processors.