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Date: Wed, 29 Mar 2017 14:09:15 -0700
From: Kees Cook <>
To: "H. Peter Anvin" <>
Cc: LKML <>, Rik van Riel <>, 
	Andy Lutomirski <>, Thomas Gleixner <>, Ingo Molnar <>, 
	"" <>, Paolo Bonzini <>, 
	Radim Krčmář <>, 
	Peter Zijlstra <>, Dave Hansen <>, 
	Yu-cheng Yu <>, Masahiro Yamada <>, 
	Borislav Petkov <>, Christian Borntraeger <>, 
	Thomas Garnier <>, Brian Gerst <>, 
	He Chen <>, Mathias Krause <>, 
	Fenghua Yu <>, Piotr Luc <>, Kyle Huey <>, 
	Len Brown <>, KVM <>, 
	"" <>
Subject: Re: [PATCH] x86/fpu: move FPU state into separate cache

On Wed, Mar 29, 2017 at 1:45 PM, H. Peter Anvin <> 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 Cook
Pixel Security

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