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

[Ingo Molnar <mingo@kernel.org>]

* Pavel Machek <pavel@ucw.cz> wrote:

> On Mon 2017-09-25 09:33:42, Ingo Molnar wrote:
> > 
> > * Pavel Machek <pavel@ucw.cz> wrote:
> > 
> > > > For example, there would be collision with regular user-space mappings, right? 
> > > > Can local unprivileged users use mmap(MAP_FIXED) probing to figure out where 
> > > > the kernel lives?
> > > 
> > > Local unpriviledged users can probably get your secret bits using cache probing 
> > > and jump prediction buffers.
> > > 
> > > Yes, you don't want to leak the information using mmap(MAP_FIXED), but CPU will 
> > > leak it for you, anyway.
> > 
> > Depends on the CPU I think, and CPU vendors are busy trying to mitigate this 
> > angle.
> 
> I believe any x86 CPU running Linux will leak it. And with CPU vendors
> putting "artifical inteligence" into branch prediction, no, I don't
> think it is going to get better.
> 
> That does not mean we shoudl not prevent mmap() info leak, but...

That might or might not be so, but there's a world of a difference between
running a relatively long statistical attack figuring out the kernel's
location, versus being able to programmatically probe the kernel's location
by using large MAP_FIXED user-space mmap()s, within a few dozen microseconds
or so and a 100% guaranteed, non-statistical result.

Thanks,

	Ingo