Re: x86: PIE support and option to extend KASLR randomization

[Thomas Garnier <thgarnie@google.com>]

On Fri, Aug 11, 2017 at 5:41 AM, Ingo Molnar <mingo@kernel.org> wrote:
>
> * Thomas Garnier <thgarnie@google.com> wrote:
>
>> Changes:
>>  - v2:
>>    - Add support for global stack cookie while compiler default to fs without
>>      mcmodel=kernel
>>    - Change patch 7 to correctly jump out of the identity mapping on kexec load
>>      preserve.
>>
>> These patches make the changes necessary to build the kernel as Position
>> Independent Executable (PIE) on x86_64. A PIE kernel can be relocated below
>> the top 2G of the virtual address space. It allows to optionally extend the
>> KASLR randomization range from 1G to 3G.
>
> So this:
>
>  61 files changed, 923 insertions(+), 299 deletions(-)
>
> ... is IMHO an _awful_ lot of churn and extra complexity in pretty fragile pieces
> of code, to gain what appears to be only ~1.5 more bits of randomization!

The range increase is a way to use PIE right away.

>
> Do these changes get us closer to being able to build the kernel as truly position
> independent, i.e. to place it anywhere in the valid x86-64 address space? Or any
> other advantages?

Yes, PIE allows us to put the kernel anywhere in memory. It will allow
us to have a full randomized address space where position and order of
sections are completely random. There is still some work to get there
but being able to build a PIE kernel is a significant step.

>
> Thanks,
>
>         Ingo



-- 
Thomas