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

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

On 04/04/2013 01:47 PM, Eric Northup wrote:
>>
>> 1. actually compose the kernel of multiple independently relocatable
>>    pieces (maybe chunk it on 2M boundaries or something.)
> 
> Without increasing the entropy bits, does this actually increase the #
> of tries necessary for an attacker to guess correctly?  It
> dramatically increases the number of possible configurations of kernel
> address space, but for any given piece there are only 256 possible
> locations.
> 

The 2M chunk was a red herring; one would of course effectively pack
blocks together, probably packed back to back, in random order.

>> 2. compile the kernel as one of the memory models which can be executed
>>    anywhere in the 64-bit address space.  The cost of this would have
>>    to be quantified, of course.
> 
> I attempted to do this, but was limited by my knowledge of the
> toolchain.  I would welcome help or suggestions!

Start by looking at the ABI document.  I suspect what we need is some
variant of the small PIC model.

	-hpa


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