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

[Thomas Garnier <thgarnie@xxxxxxxxxx>]

On Tue, Aug 15, 2017 at 12:56 AM, Ingo Molnar <mingo@xxxxxxxxxx> wrote:
>
> * Thomas Garnier <thgarnie@xxxxxxxxxx> 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.

>
> 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.

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

>
> It should have far less performance impact than the register-losing and
> overhead-inducing -fpie / -mcmodel=large (for modules) execution models.
>
> My quick guess is tha the performance impact might be close to zero in fact.

If mcmodel=small/medium was possible for kernel, I don't think it
would have less performance impact than mcmodel=large. It would still
need to set the high 32-bit to be a static value, only the relocation
would be a different size.

>
> Thanks,
>
>         Ingo



-- 
Thomas

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