Re: [PATCH v11 00/11] x86: PIE support to extend KASLR randomization

Thomas Garnier
2020-03-04T19:26:07Z

On Wed, Mar 4, 2020 at 10:45 AM H. Peter Anvin <hpa@zytor.com> wrote:
>
> On 2020-03-04 10:21, Kees Cook wrote:
> > On Wed, Mar 04, 2020 at 10:21:36AM +0100, Peter Zijlstra wrote:
> >> But at what cost; it does unspeakable ugly to the asm. And didn't a
> >> kernel compiled with the extended PIE range produce a measurably slower
> >> kernel due to all the ugly?
> >
> > Was that true? I thought the final results were a wash and that earlier
> > benchmarks weren't accurate for some reason? I can't find the thread
> > now. Thomas, do you have numbers on that?

I have never seen a significant performance impact. Performance and
size is better on more recent versions of gcc as it has better
generation of PIE code (for example generation of switches).

> >
> > BTW, I totally agree that fgkaslr is the way to go in the future. I
> > am mostly arguing for this under the assumption that it doesn't
> > have meaningful performance impact and that it gains the kernel some
> > flexibility in the kinds of things it can do in the future. If the former
> > is not true, then I'd agree, the benefit needs to be more clear.
> >
>
> "Making the assembly really ugly" by itself is a reason not to do it, in my
> Not So Humble Opinion[TM]; but the reason the kernel and small memory models
> exist in the first place is because there is a nonzero performance impact of
> the small-PIC memory model. Having modules in separate regions would further
> add the cost of a GOT references all over the place (PLT is optional, useless
> and deprecated for eager binding) *plus* might introduce at least one new
> vector of attack: overwrite a random GOT slot, and just wait until it gets hit
> by whatever code path it happens to be in; the exact code path doesn't matter.
> From an kASLR perspective this is *very* bad, since you only need to guess the
> general region of a GOT rather than an exact address.

I agree that it would add GOT references and I can explore that more
in terms of performance impact and size. This patchset makes the GOT
readonly too so I don't think the attack vector applies.

>
> The huge memory model, required for arbitrary placement, has a very
> significant performance impact.

I assume you mean mcmodel=large, it doesn't use it. It uses -fPIE and
removes -mcmodel=kernel. It favors relative references whenever
possible.

>
> The assembly code is *very* different across memory models.
>
> -hpa