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

From: Eric Northup
Date: Mon Apr 15 2013 - 17:06:40 EST

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 :-)
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