From: Peter Zijlstra <peterz@infradead.org>
Subject: Re: x86: PIE support and option to extend KASLR randomization
Date: Mon, 21 Aug 2017 16:31:17 +0200
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Cc: Ingo Molnar <mingo@kernel.org>,
	Herbert Xu <herbert@gondor.apana.org.au>,
	"David S . Miller" <davem@davemloft.net>,
	Thomas Gleixner <tglx@linutronix.de>,
	Ingo Molnar <mingo@redhat.com>, "H . Peter Anvin" <hpa@zytor.com>,
	Josh Poimboeuf <jpoimboe@redhat.com>, Arnd Bergmann <arnd@arndb.de>,
	Matthias Kaehlcke <mka@chromium.org>,
	Boris Ostrovsky <boris.ostrovsky@oracle.com>,
	Juergen Gross <jgross@suse.com>,
	Paolo Bonzini <pbonzini@redhat.com>,
	Radim =?utf-8?B?S3LEjW3DocWZ?= <rkrcmar@redhat.com>,
	Joerg Roedel <joro@8bytes.org>,
	Tom Lendacky <thomas.lendacky@amd.com>,
	Andy Lutomirski <luto@kernel.org>, Borislav Petkov <bp@suse.de>,
	Brian Gerst <brgerst@gmail.com>,
	"Kirill A . Shutemov" <kirill.shutemov@linux.intel.com>,
	"Rafael J . Wysocki" <rjw@rjwysocki.net>,
	Len Brown <len.brown@intel.com>, Pavel Machek <pavel@ucw.cz>,
	Tejun Heo <tj@kernel.org>, Christoph Lameter <cl@l
To: Thomas Garnier <thgarnie@google.com>
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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@kernel.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.