Re: [Xen-devel] x86: PIE support and option to extend KASLR randomization

[Peter Zijlstra <peterz@xxxxxxxxxxxxx>]

On Mon, Aug 21, 2017 at 03:32:22PM +0200, Peter Zijlstra wrote:
> On Wed, Aug 16, 2017 at 05:12:35PM +0200, Ingo Molnar wrote:
> > Unfortunately mcmodel=large looks pretty heavy too AFAICS, at the machine 
> > instruction level.
> > 
> > Function calls look like this:
> > 
> >  -mcmodel=medium:
> > 
> >    757:   e8 98 ff ff ff          callq  6f4 <test_code>
> > 
> >  -mcmodel=large
> > 
> >    77b:   48 b8 10 f7 df ff ff    movabs $0xffffffffffdff710,%rax
> >    782:   ff ff ff 
> >    785:   48 8d 04 03             lea    (%rbx,%rax,1),%rax
> >    789:   ff d0                   callq  *%rax
> > 
> > And we'd do this for _EVERY_ function call in the kernel. That kind of crap 
> > is 
> > totally unacceptable.
> 
> So why does this need to be computed for every single call? How often
> will we move the kernel around at runtime?
> 
> Why can't we process the relocation at load time and then discard the
> relocation tables along with the rest of __init ?

Ah, I see, this is large mode and that needs to use MOVABS to load 64bit
immediates. Still, small RIP relative should be able to live at any
point as long as everything lives inside the same 2G relative range, so
would still allow the goal of increasing the KASLR range.

So I'm not seeing how we need large mode for that. That said, after
reading up on all this, RIP relative will not be too pretty either,
while CALL is naturally RIP relative, data still needs an explicit %rip
offset, still loads better than the large model.

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