Re: [PATCH v8 00/14] Function Granular KASLR

Peter Zijlstra
2021-12-03T16:33:16Z

On Fri, Dec 03, 2021 at 03:41:36PM +0100, Alexander Lobakin wrote:
> From: Peter Zijlstra <peterz@infradead.org>
> Date: Fri, 3 Dec 2021 11:38:30 +0100
>
> > On Thu, Dec 02, 2021 at 11:32:00PM +0100, Alexander Lobakin wrote:
> >
> > > feat make -j65 boot vmlinux.o vmlinux bzImage bogoops/s
> > > Relocatable 4m38.478s 24.440s 72014208 58579520 9396192 57640.39
> > > KASLR 4m39.344s 24.204s 72020624 87805776 9740352 57393.80
> > > FG-K 16 fps 6m16.493s 25.429s 83759856 87194160 10885632 57784.76
> > > FG-K 8 fps 6m20.190s 25.094s 83759856 88741328 10985248 56625.84
> > > FG-K 1 fps 7m09.611s 25.922s 83759856 95681128 11352192 56953.99
> >
> > :sadface: so at best it makes my kernel compiles ~50% slower. Who would
> > ever consider doing that? It's like retpolines weren't bad enough; lets
> > heap on the fail?
>
> I was waiting for that :D
>
> I know it's horrible for now, but there are some points to consider:
> - folks who are placing hardening over everything don't mind
> compile times most likely;
> - linkers choking on huge LD scripts is actually a bug in their
> code. They process 40k sections as orphans (without a generated
> LD script) for a split second, so they're likely able to do the
> same with it. Our position here is that after FG-KASLR landing
> we'll report it and probably look into linkers' code to see if
> that can be addressed (Kees et al are on this AFAIU);
> - ClangLTO (at least "Fat", not sure about Thin as I didn't used
> it) thinks on vmlinux.o for ~5 minutes on 8-core Skylake. Still,
> it is here in mainline and is widely (relatively) used.
> I know FG-KASLR stuff is way more exotic, but anyways.
> - And the last one: I wouldn't consider FG-KASLR production ready
> as Kees would like to see it. Apart from compilation time, you
> get random performance {in,de}creases here-and-there all over
> the kernel and modules you can't predict at all.
> I guess it would become better later on when/if we introduce
> profiling-based function placement (there are some discussions
> around that and one related article is referred in the orig
> cover letter), but dunno for now.
> There's one issue in the current code as well -- PTI switching
> code is in .entry.text which doesn't currently get randomized.
> So it can probably be hunted using gadget collectors I guess?

Oooh, so those compile times are not, as one would expect the compile
times for a set .config but with different kernel, but instead for a
varying .config on the same kernel?

IOW, they don't represent the run-time overhead of this thing, but
merely the toolchain overhead of all this.

So what is the actual runtime overhead of all this?