Re: [RFC PATCH 03/11] x86/boot: Allow a "silent" kaslr random byte fetch

Kees Cook
2020-02-06T11:48:23Z

On Wed, Feb 05, 2020 at 05:08:55PM -0800, Andy Lutomirski wrote:
>
>
> > On Feb 5, 2020, at 2:39 PM, Kristen Carlson Accardi <kristen@linux.intel.com> wrote:
> >
> > ﻿From: Kees Cook <keescook@chromium.org>
> >
> > Under earlyprintk, each RNG call produces a debug report line. When
> > shuffling hundreds of functions, this is not useful information (each
> > line is identical and tells us nothing new). Instead, allow for a NULL
> > "purpose" to suppress the debug reporting.
>
> Have you counted how many RDRAND calls this causes? RDRAND is
> exceedingly slow on all CPUs I’ve looked at. The whole “RDRAND
> has great bandwidth” marketing BS actually means that it has decent
> bandwidth if all CPUs hammer it at the same time. The latency is abysmal.
> I have asked Intel to improve this, but the latency of that request will
> be quadrillions of cycles :)

In an earlier version of this series, it was called once per function
section (so, about 50,000 times). The (lack of) speed was quite
measurable.

> I would suggest adding a little ChaCha20 DRBG or similar to the KASLR
> environment instead. What crypto primitives are available there?

Agreed. The simple PRNG in the next patch was most just a POC initially,
but Kristen kept it due to its debugging properties (specifying an
external seed). Pulling in ChaCha20 seems like a good approach.

--
Kees Cook