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Date: Wed, 29 Mar 2017 14:09:15 -0700
From: Kees Cook <keescook@...omium.org>
To: "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>, 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 <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 Wed, Mar 29, 2017 at 1:45 PM, H. Peter Anvin <hpa@...or.com> wrote:
> 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.)

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...

-Kees

-- 
Kees Cook
Pixel Security

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