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

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Message-ID: <202002060345.FAF7517CA4@keescook>
Date:   Thu, 6 Feb 2020 03:48:01 -0800
From:   Kees Cook <keescook@...omium.org>
To:     Andy Lutomirski <luto@...capital.net>
Cc:     Kristen Carlson Accardi <kristen@...ux.intel.com>,
        tglx@...utronix.de, mingo@...hat.com, bp@...en8.de, hpa@...or.com,
        arjan@...ux.intel.com, rick.p.edgecombe@...el.com, x86@...nel.org,
        linux-kernel@...r.kernel.org, kernel-hardening@...ts.openwall.com
Subject: Re: [RFC PATCH 03/11] x86/boot: Allow a "silent" kaslr random byte
 fetch

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@...ux.intel.com> wrote:
> > 
> > From: Kees Cook <keescook@...omium.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