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Date: Mon, 21 Aug 2017 08:57:58 -0700
From: Thomas Garnier <thgarnie@...gle.com>
To: Peter Zijlstra <peterz@...radead.org>
Cc: Ingo Molnar <mingo@...nel.org>, Herbert Xu <herbert@...dor.apana.org.au>, 
	"David S . Miller" <davem@...emloft.net>, Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>, 
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	Matthias Kaehlcke <mka@...omium.org>, Boris Ostrovsky <boris.ostrovsky@...cle.com>, 
	Juergen Gross <jgross@...e.com>, Paolo Bonzini <pbonzini@...hat.com>, 
	Radim Krčmář <rkrcmar@...hat.com>, 
	Joerg Roedel <joro@...tes.org>, Tom Lendacky <thomas.lendacky@....com>, 
	Andy Lutomirski <luto@...nel.org>, Borislav Petkov <bp@...e.de>, Brian Gerst <brgerst@...il.com>, 
	"Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>, "Rafael J . Wysocki" <rjw@...ysocki.net>, 
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	Chris Metcalf <cmetcalf@...lanox.com>, Andrew Morton <akpm@...ux-foundation.org>, 
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	Christopher Li <sparse@...isli.org>, "Rafael J . Wysocki" <rafael.j.wysocki@...el.com>, 
	Lukas Wunner <lukas@...ner.de>, Mika Westerberg <mika.westerberg@...ux.intel.com>, 
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	Michal Hocko <mhocko@...e.com>, Matthew Wilcox <mawilcox@...rosoft.com>, 
	"H . J . Lu" <hjl.tools@...il.com>, Paul Bolle <pebolle@...cali.nl>, Rob Landley <rob@...dley.net>, 
	Baoquan He <bhe@...hat.com>, Daniel Micay <danielmicay@...il.com>, 
	"the arch/x86 maintainers" <x86@...nel.org>, Linux Crypto Mailing List <linux-crypto@...r.kernel.org>, 
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	linux-arch <linux-arch@...r.kernel.org>, linux-sparse@...r.kernel.org, 
	Kernel Hardening <kernel-hardening@...ts.openwall.com>, 
	Linus Torvalds <torvalds@...ux-foundation.org>, Borislav Petkov <bp@...en8.de>
Subject: Re: x86: PIE support and option to extend KASLR randomization

On Mon, Aug 21, 2017 at 7:31 AM, Peter Zijlstra <peterz@...radead.org> wrote:
> On Tue, Aug 15, 2017 at 07:20:38AM -0700, Thomas Garnier wrote:
>> On Tue, Aug 15, 2017 at 12:56 AM, Ingo Molnar <mingo@...nel.org> wrote:
>
>> > Have you considered a kernel with -mcmodel=small (or medium) instead of -fpie
>> > -mcmodel=large? We can pick a random 2GB window in the (non-kernel) canonical
>> > x86-64 address space to randomize the location of kernel text. The location of
>> > modules can be further randomized within that 2GB window.
>>
>> -model=small/medium assume you are on the low 32-bit. It generates
>> instructions where the virtual addresses have the high 32-bit to be
>> zero.
>
> That's a compiler fail, right? Because the SDM states that for "CALL
> rel32" the 32bit displacement is sign extended on x86_64.
>

That's different than what I expected at first too.

Now, I think I have an alternative of using mcmodel=large. I could use
-fPIC and ensure modules are never far away from the main kernel
(moving the module section start close to the random kernel end). I
looked at it and that seems possible but will require more work. I
plan to start with the mcmodel=large support and add this mode in a
way that could benefit classic KASLR (without -fPIC) because it
randomize where modules start based on the kernel.

-- 
Thomas

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