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

[Thomas Garnier <thgarnie@xxxxxxxxxx>]

On Tue, Aug 15, 2017 at 7:47 AM, Daniel Micay <danielmicay@xxxxxxxxx> wrote:
> On 15 August 2017 at 10:20, Thomas Garnier <thgarnie@xxxxxxxxxx> wrote:
>> 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.
>
> The cost can be reduced by using -fno-plt these days but some work
> might be required to make that work with the kernel.
>
> Where does that 10% estimate in the kernel config docs come from? I'd
> be surprised if it really cost that much on x86_64. That's a realistic
> cost for i386 with modern GCC (it used to be worse) but I'd expect
> x86_64 to be closer to 2% even for CPU intensive workloads. It should
> be very close to zero with -fno-plt.

I got 8 to 10% on hackbench. Other benchmarks were 4% or lower.

I will do look at more recent compiler and no-plt as well.

-- 
Thomas

_______________________________________________
Xen-devel mailing list
Xen-devel@xxxxxxxxxxxxx
https://lists.xen.org/xen-devel