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Date: Mon, 21 Aug 2017 16:31:17 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: Thomas Garnier <thgarnie@...gle.com>
Cc: Ingo Molnar <mingo@...nel.org>,
	Herbert Xu <herbert@...dor.apana.org.au>,
	"David S . Miller" <davem@...emloft.net>,
	Thomas Gleixner <tglx@...utronix.de>,
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	"H . J . Lu" <hjl.tools@...il.com>, Paul Bolle <pebolle@...cali.nl>,
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	Linus Torvalds <torvalds@...ux-foundation.org>,
	Borislav Petkov <bp@...en8.de>
Subject: Re: x86: PIE support and option to extend KASLR randomization

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.

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