Re: [PATCH v4 00/10] Function Granular KASLR

From: Kees Cook
Date: Thu Aug 06 2020 - 15:27:56 EST

On Thu, Aug 06, 2020 at 05:32:58PM +0200, Ingo Molnar wrote:
> * Kristen Carlson Accardi <kristen@xxxxxxxxxxxxxxx> wrote:
> > [...]
> > Performance Impact
> > ------------------
> > * Run time
> > The performance impact at run-time of function reordering varies by workload.
> > Using kcbench, a kernel compilation benchmark, the performance of a kernel
> > build with finer grained KASLR was about 1% slower than a kernel with standard
> > KASLR. Analysis with perf showed a slightly higher percentage of
> > L1-icache-load-misses. Other workloads were examined as well, with varied
> > results. Some workloads performed significantly worse under FGKASLR, while
> > others stayed the same or were mysteriously better. In general, it will
> > depend on the code flow whether or not finer grained KASLR will impact
> > your workload, and how the underlying code was designed. Because the layout
> > changes per boot, each time a system is rebooted the performance of a workload
> > may change.
> I'd guess that the biggest performance impact comes from tearing apart
> 'groups' of functions that particular workloads are using.
> In that sense it might be worthwile to add a '__kaslr_group' function
> tag to key functions, which would keep certain performance critical
> functions next to each other.

We kind of already do this manually for things like the scheduler, etc,
using macros like ".whatever.text", so we might be able to create a more
generalized approach for those. Right now they require a "section" macro
usage and a linker script __start* and __end* marker, etc:

#define SCHED_TEXT \
__sched_text_start = .; \
*(.sched.text) \
__sched_text_end = .;

Manually collected each whatever_TEXT define and building out each
__whatever_start, etc is annoying. It'd be really cool to have linker
script have wildcard replacements for build a syntax like this, based
on the presences of matching input sections:

.%.text : {
__%_start = .;
__%_end = .;

> I'd also suggest allowing the passing in of a boot-time pseudo-random
> generator seed number, which would allow the creation of a
> pseudo-randomized but repeatable layout across reboots.

This was present in earlier versions of the series.

Kees Cook