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

[Daniel Micay <danielmicay@gmail.com>]

On 15 August 2017 at 10:20, 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.

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.