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

On Tue, Aug 15, 2017 at 7:47 AM, Daniel Micay [off-list ref] wrote:

On 15 August 2017 at 10:20, Thomas Garnier [off-list ref] wrote:

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On Tue, Aug 15, 2017 at 12:56 AM, Ingo Molnar [off-list ref] wrote:

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* Thomas Garnier [off-list ref] wrote:

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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