linux-kernel - Re: [resend Patch v3 1/2] kaslr: check if kernel location is changed

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Message-ID: <20141013172242.GB9777@redhat.com>
Date:	Mon, 13 Oct 2014 13:22:42 -0400
From:	Vivek Goyal <vgoyal@...hat.com>
To:	"H. Peter Anvin" <hpa@...or.com>
Cc:	Baoquan He <bhe@...hat.com>, Kees Cook <keescook@...omium.org>,
	linux-kernel@...r.kernel.org, tglx@...utronix.de, mingo@...hat.com,
	x86@...nel.org, ak@...ux.intel.com, ebiederm@...ssion.com,
	kexec@...ts.infradead.org, whissi@...ssi.de,
	kumagai-atsushi@....nes.nec.co.jp, stable@...r.kernel.org
Subject: Re: [resend Patch v3 1/2] kaslr: check if kernel location is changed

On Mon, Oct 13, 2014 at 08:43:00AM -0700, H. Peter Anvin wrote:
> On 10/13/2014 08:19 AM, Vivek Goyal wrote:
> >>>
> >>> This really shouldn't have happened this way on x86-64.  It has to happen
> >>> this way on i386, but I worry that this may be a serious misdesign in kaslr
> >>> on x86-64.  I'm also wondering if there is any other fallout of this?
> >>
> >> I agree. On x86_64, we should stick to previous design and this new
> >> logic of performing relocations does not sound very clean and makes
> >> things very confusing.
> >>
> >> I am wondering that why couldn't we simply adjust page tables in case of
> >> kaslr on x86_64, instead of performing relocations.
> > 
> > Well, IIUC, if virtual addresses are shifted w.r.t what virtual address
> > kernel was compiled for, then relocation will have to be done.
> > 
> > So question will be if physical address shift is enough for kaslr or
> > virtual address shift is necessary.
> > 
> 
> I would assume that without a virtual address shift kaslr is pretty darn
> pointless.  Without the physical address shift the 1:1 map can be used,
> and again, kaslr becomes pointless.  However, there is absolutely no
> reason why they should be coupled.  They can, in fact, be independently
> randomized.

Agreed. On x86_64, we should be able to randomize virtual address space
and physical address space independently. And in that case whole of
the physical memory should be available for a possible location for
kernel. (As opposed to a small limit (I guess 1GB) now)

Thanks
Vivek
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