Subject: Re: [PATCH] x86/fpu: move FPU state into separate cache
To: Kees Cook <keescook@chromium.org>, "H. Peter Anvin" <hpa@zytor.com>
References: <20170329203908.GA39222@beast>
 <ea0a7663-7318-6086-1cb8-25fd9a252f83@zytor.com>
 <CAGXu5jLQfS6gK2MyetWPjyJDOg8NdACXsPXLt7OasQE03VUwPg@mail.gmail.com>
Cc: LKML <linux-kernel@vger.kernel.org>, Rik van Riel <riel@redhat.com>,
        Andy Lutomirski <luto@kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>, Ingo Molnar <mingo@redhat.com>,
        "x86@kernel.org" <x86@kernel.org>, Paolo Bonzini <pbonzini@redhat.com>,
        =?UTF-8?B?UmFkaW0gS3LEjW3DocWZ?= <rkrcmar@redhat.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Yu-cheng Yu <yu-cheng.yu@intel.com>,
        Masahiro Yamada <yamada.masahiro@socionext.com>,
        Borislav Petkov <bp@suse.de>,
        Christian Borntraeger <borntraeger@de.ibm.com>,
        Thomas Garnier <thgarnie@google.com>, Brian Gerst <brgerst@gmail.com>,
        He Chen <he.chen@linux.intel.com>,
        Mathias Krause <minipli@googlemail.com>,
        Fenghua Yu <fenghua.yu@intel.com>, Piotr Luc <piotr.luc@intel.com>,
        Kyle Huey <me@kylehuey.com>, Len Brown <len.brown@intel.com>,
        KVM <kvm@vger.kernel.org>,
        "kernel-hardening@lists.openwall.com" 
        <kernel-hardening@lists.openwall.com>
From: Dave Hansen <dave.hansen@linux.intel.com>
Message-ID: <ca888ec2-e2e6-3600-3e39-c18e61e0c735@linux.intel.com>
Date: Wed, 29 Mar 2017 14:19:37 -0700
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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.