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

[Kees Cook]

On Mon, Apr 15, 2013 at 2:25 PM, H. Peter Anvin <hpa@xxxxxxxxx> wrote:
> On 04/15/2013 02:06 PM, Eric Northup wrote:
>> On Sat, Apr 13, 2013 at 8:06 PM, H. Peter Anvin <hpa@xxxxxxxxx> 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.

Aren't we bound by the negative 2GB addressing due to -mcmodel=kernel?

-Kees

--
Kees Cook
Chrome OS Security
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