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

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Message-ID: <543907B5.7060001@zytor.com>
Date:	Sat, 11 Oct 2014 03:34:29 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Baoquan He <bhe@...hat.com>, Vivek Goyal <vgoyal@...hat.com>,
	Kees Cook <keescook@...omium.org>
CC:	linux-kernel@...r.kernel.org, tglx@...utronix.de, mingo@...hat.com,
	x86@...nel.org, keescook@...omium.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 10/10/2014 08:14 PM, Baoquan He wrote:
> On 10/08/14 at 03:27pm, Vivek Goyal wrote:
>> On Wed, Oct 08, 2014 at 08:09:59AM -0700, H. Peter Anvin wrote:
>
>>> Sorry... this makes no sense.
>>>
>>> For x86-64, there is no direct connection between the physical and
>>> virtual address spaces that the kernel runs in...
>>
>> I am sorry I did not understand this one. I thought that initial
>> relocatable kernel implementaion did not have any direct connection
>> between virtual and physical address. One could load kernel anywhere
>> and kernel virtual address will not change and we will just adjust
>> page tables to map virtual address to right physical address.
>>
>> Now handle_relocation() stuff seems to introduce a close coupling
>> between physical and virtual address. So if kernel shifts by 16MB
>> in physical address space, then it will shift by equal amount
>> in virtual address space. So there seems to be a direct connection
>> between virtual and physical address space in this case.
>
> Yeah, it's exactly as Vivek said.
>
> Before kaslr was introduced, x86_64 kernel can be put anywhere, and
> always _text is 0xffffffff81000000. Meanwhile phys_base contains the
> offset between the compiled addr (namely 0x1000000) and kernel loaded
> addr. After kaslr implementation was added, as long as kernel loaded
> addr is different 0x1000000, it will call handle_relocations(). The
> offset now is added onto each symbols including _text and phys_base
> becomes 0.
>
> It's clearly showing that by checking /proc/kallsyms and value of
> phys_base.
>

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?

	-hpa


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