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Date: Sun, 27 Aug 2017 18:26:05 -0700
From: "H. Peter Anvin" <hpa@...or.com>
To: Peter Zijlstra <peterz@...radead.org>,
        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>, Ingo Molnar <mingo@...hat.com>,
        Josh Poimboeuf <jpoimboe@...hat.com>, Arnd Bergmann <arnd@...db.de>,
        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>,
        Len Brown <len.brown@...el.com>, Pavel Machek <pavel@....cz>,
        Tejun Heo <tj@...nel.org>, Christoph Lameter <cl@...ux.com>,
        Paul Gortmaker <paul.gortmaker@...driver.com>,
        Chris Metcalf <cmetcalf@...lanox.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        "Paul E . McKenney" <paulmck@...ux.vnet.ibm.com>,
        Nicolas Pitre <nicolas.pitre@...aro.org>,
        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>,
        Dou Liyang <douly.fnst@...fujitsu.com>,
        Daniel Borkmann <daniel@...earbox.net>,
        Alexei Starovoitov <ast@...nel.org>,
        Masahiro Yamada <yamada.masahiro@...ionext.com>,
        Markus Trippelsdorf <markus@...ppelsdorf.de>,
        Steven Rostedt <rostedt@...dmis.org>,
        Kees Cook <keescook@...omium.org>, Rik van Riel <riel@...hat.com>,
        David Howells <dhowells@...hat.com>, Waiman Long <longman@...hat.com>,
        Kyle Huey <me@...ehuey.com>, Peter Foley <pefoley2@...oley.com>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        Catalin Marinas <catalin.marinas@....com>,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        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@...r.kernel.org, LKML <linux-kernel@...r.kernel.org>,
        xen-devel@...ts.xenproject.org, kvm list <kvm@...r.kernel.org>,
        Linux PM list <linux-pm@...r.kernel.org>,
        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 08/21/17 07:31, Peter Zijlstra 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.
> 

No.  It is about whether you can do something like:

	movl $variable, %eax		/* rax = &variable; */

or

	addl %ecx,variable(,%rsi,4)	/* variable[rsi] += ecx */

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