kernel-hardening - Re: x86: PIE support and option to extend KASLR randomization

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Date: Thu, 17 Aug 2017 10:09:20 +0200
From: Ingo Molnar <mingo@...nel.org>
To: Thomas Garnier <thgarnie@...gle.com>
Cc: Herbert Xu <herbert@...dor.apana.org.au>,
	"David S . Miller" <davem@...emloft.net>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>, "H . Peter Anvin" <hpa@...or.com>,
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Subject: Re: x86: PIE support and option to extend KASLR randomization


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

> > > -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 think PIE is capable to use relative instructions well. mcmodel=large assumes 
> symbols can be anywhere.

So if the numbers in your changelog and Kconfig text cannot be trusted, there's 
this description of the size impact which I suspect is less susceptible to 
measurement error:

+         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.

... but describing a 1-2% kernel text size increase as "slightly more assembly" 
shows a gratituous disregard to kernel code generation quality! In reality that's 
a huge size increase that in most cases will almost directly transfer to a 1-2% 
slowdown for kernel intense workloads.

Where does that size increase come from, if PIE is capable of using relative 
instructins well? Does it come from the loss of a generic register and the 
resulting increase in register pressure, stack spills, etc.?

So I'm still unhappy about this all, and about the attitude surrounding it.

Thanks,

	Ingo
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