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From: Linus Torvalds <torvalds@linux-foundation.org>
Date: Wed, 29 Mar 2017 14:30:02 -0700
Message-ID: <CA+55aFxGEQroSYmSgEQefNUhJfmvMRg8XAy3TrtYoiLwUQNwKA@mail.gmail.com>
Subject: Re: [PATCH] x86/fpu: move FPU state into separate cache
To: Kees Cook <keescook@chromium.org>
Cc: "H. Peter Anvin" <hpa@zytor.com>, 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>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        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>
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On Wed, Mar 29, 2017 at 2:09 PM, Kees Cook <keescook@chromium.org> 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). T

What?

The layout randomization can't change anything, if you just make the
adjacency be done explicitly instead of by having the thing be a fixed
member.

The trivial model might be to just declare the fpu part as an unsized
array at the end:

        /* Floating point and extended processor state */
        struct fpu              fpu[];

because there is no way in hell that any randomization code can move
those kinds of unsized arrays around. If it does, the gcc plugin is
such unbelievable garbage that it would be insane to depend on such
shit in the first place.

                    Linus