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

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Date:	Mon, 13 Oct 2014 08:43:00 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Vivek Goyal <vgoyal@...hat.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 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.

	-hpa


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