linux-kernel - Re: [PATCH 3/3] x86: kernel base offset ASLR

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Date:	Fri, 05 Apr 2013 13:29:03 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Yinghai Lu <yinghai@...nel.org>
CC:	Kees Cook <keescook@...omium.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	kernel-hardening@...ts.openwall.com,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	the arch/x86 maintainers <x86@...nel.org>,
	Jarkko Sakkinen <jarkko.sakkinen@...el.com>,
	Matthew Garrett <mjg@...hat.com>,
	Matt Fleming <matt.fleming@...el.com>,
	Eric Northup <digitaleric@...gle.com>,
	Dan Rosenberg <drosenberg@...curity.com>,
	Julien Tinnes <jln@...gle.com>, Will Drewry <wad@...omium.org>
Subject: Re: [PATCH 3/3] x86: kernel base offset ASLR

On 04/05/2013 01:19 PM, Yinghai Lu wrote:
> On Fri, Apr 5, 2013 at 1:05 PM, H. Peter Anvin <hpa@...or.com> wrote:
> 
>> That makes zero difference, since the issue at hand is the *virtual*
>> addresses the kernel are running at.  Currently, the 64-bit kernel
>> always runs at 0xffffffff81000000 virtual.  We can't run out of
>> arbitrary bits of the 64-bit address space because of the memory model.
> 
> Not sure if I understand this.
> 
> when bzImage64 is loaded high,  the kernel high address 0xffffffff81000000
> is pointed to phys address above 4G without problem.
> 

That' s the problem.

KASLR is about randomizing the *virtual* address space, not the
*physical* address space.  On 32 bits those are connected (which is a
problem all on its own), on 64 bits not so much.

>>
>> Furthermore, dealing with the boot loaders means dealing with the boot
>> loader maintainers, which can be insanely painful.  Consider that Grub2,
>> 10 years after this was implemented, still can't load more than one
>> initramfs component.
> 
> syslinux and pxelinux could do that?
> 

Yes, they can.

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