Re: [PATCH 6/6] x86: kaslr: relocate base offset at boot

["H. Peter Anvin" <hpa@zytor.com>]

On 04/15/2013 02:06 PM, Eric Northup wrote:
> On Sat, Apr 13, 2013 at 8:06 PM, H. Peter Anvin <hpa@zytor.com> wrote:
>> On 04/13/2013 05:37 PM, Yinghai Lu wrote:
>>>
>>> so decompress code position is changed?
>>>
>>> You may push out bss and other data area of run-time kernel of limit
>>> that boot loader
>>> chose according to setup_header.init_size.
>>> aka that make those area overlap with ram hole or other area like
>>> boot command line or initrd....
>>>
>>
>> Is there a strong reason to randomize the physical address on 64 bits
>> (and if so, shouldn't we do it right?)
> 
> The reason to randomize the physical address is because of the kernel
> direct mapping range -- a predictable-to-attackers physical address
> implies a predictable-to-attackers virtual address.
> 
> It had seemed to me like changing the virtual base of the direct
> mapping would be much more involved than physically relocating the
> kernel, but better suggestions would be most welcome :-)
> 

You seem to be missing something here...

There are *two* mappings in 64-bit mode.  Physically, if you're going to
randomize you might as well randomize over the entire range... except
not too far down (on either 32 or 64 bit mode)... in particular, you
don't want to drop below 16 MiB if you can avoid it.

On 64 bits, there is no reason the virtual address has to be randomized
the same way.

	-hpa


-- 
H. Peter Anvin, Intel Open Source Technology Center
I work for Intel.  I don't speak on their behalf.