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

[Ingo Molnar <mingo@kernel.org>]

* Thomas Garnier <thgarnie@google.com> wrote:

> On Tue, Aug 15, 2017 at 12:56 AM, Ingo Molnar <mingo@kernel.org> wrote:
> >
> > * Thomas Garnier <thgarnie@google.com> wrote:
> >
> >> > 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.
> >
> > So I _really_ dislike the whole PIE approach, because of the huge slowdown:
> >
> > +config RANDOMIZE_BASE_LARGE
> > +       bool "Increase the randomization range of the kernel image"
> > +       depends on X86_64 && RANDOMIZE_BASE
> > +       select X86_PIE
> > +       select X86_MODULE_PLTS if MODULES
> > +       default n
> > +       ---help---
> > +         Build the kernel as a Position Independent Executable (PIE) and
> > +         increase the available randomization range from 1GB to 3GB.
> > +
> > +         This option impacts performance on kernel CPU intensive workloads up
> > +         to 10% due to PIE generated code. Impact on user-mode processes and
> > +         typical usage would be significantly less (0.50% when you build the
> > +         kernel).
> > +
> > +         The kernel and modules will generate slightly more assembly (1 to 2%
> > +         increase on the .text sections). The vmlinux binary will be
> > +         significantly smaller due to less relocations.
> >
> > To put 10% kernel overhead into perspective: enabling this option wipes out about
> > 5-10 years worth of painstaking optimizations we've done to keep the kernel fast
> > ... (!!)
> 
> Note that 10% is the high-bound of a CPU intensive workload.

Note that the 8-10% hackbench or even a 2%-4% range would be 'huge' in terms of 
modern kernel performance. In many cases we are literally applying cycle level 
optimizations that are barely measurable. A 0.1% speedup in linear execution speed 
is already a big success.

> I am going to start doing performance testing on -mcmodel=large to see if it is 
> faster than -fPIE.

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.

> > I think the fundamental flaw is the assumption that we need a PIE executable 
> > to have a freely relocatable kernel on 64-bit CPUs.
> >
> > Have you considered a kernel with -mcmodel=small (or medium) instead of -fpie 
> > -mcmodel=large? We can pick a random 2GB window in the (non-kernel) canonical 
> > x86-64 address space to randomize the location of kernel text. The location of 
> > modules can be further randomized within that 2GB window.
> 
> -model=small/medium assume you are on the low 32-bit. It generates instructions 
> where the virtual addresses have the high 32-bit to be zero.

How are these assumptions hardcoded by GCC? Most of the instructions should be 
relocatable straight away, as most call/jump/branch instructions are RIP-relative.

I.e. is there no GCC code generation mode where code can be placed anywhere in the 
canonical address space, yet call and jump distance is within 31 bits so that the 
generated code is fast?

Thanks,

	Ingo

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