Re: [PATCH 3/3] x86: kernel base offset ASLR

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

On 04/05/2013 01:19 PM, Yinghai Lu wrote:
> On Fri, Apr 5, 2013 at 1:05 PM, H. Peter Anvin <hpa@zytor.com> wrote:
> 
>> That makes zero difference, since the issue at hand is the *virtual*
>> addresses the kernel are running at.  Currently, the 64-bit kernel
>> always runs at 0xffffffff81000000 virtual.  We can't run out of
>> arbitrary bits of the 64-bit address space because of the memory model.
> 
> Not sure if I understand this.
> 
> when bzImage64 is loaded high,  the kernel high address 0xffffffff81000000
> is pointed to phys address above 4G without problem.
> 

That' s the problem.

KASLR is about randomizing the *virtual* address space, not the
*physical* address space.  On 32 bits those are connected (which is a
problem all on its own), on 64 bits not so much.

>>
>> Furthermore, dealing with the boot loaders means dealing with the boot
>> loader maintainers, which can be insanely painful.  Consider that Grub2,
>> 10 years after this was implemented, still can't load more than one
>> initramfs component.
> 
> syslinux and pxelinux could do that?
> 

Yes, they can.