linux-kernel - Re: [PATCH 1/2] x86/random: Retry on RDSEED failure

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Date: Tue, 30 Jan 2024 20:16:20 +0100
From: "Jason A. Donenfeld" <Jason@...c4.com>
To: "Reshetova, Elena" <elena.reshetova@...el.com>
Cc: "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>, Thomas Gleixner <tglx@...utronix.de>, 
	Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>, 
	Dave Hansen <dave.hansen@...ux.intel.com>, "H. Peter Anvin" <hpa@...or.com>, 
	"x86@...nel.org" <x86@...nel.org>, "Theodore Ts'o" <tytso@....edu>, 
	Kuppuswamy Sathyanarayanan <sathyanarayanan.kuppuswamy@...ux.intel.com>, 
	"Nakajima, Jun" <jun.nakajima@...el.com>, Tom Lendacky <thomas.lendacky@....com>, 
	"Kalra, Ashish" <ashish.kalra@....com>, Sean Christopherson <seanjc@...gle.com>, 
	"linux-coco@...ts.linux.dev" <linux-coco@...ts.linux.dev>, 
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/2] x86/random: Retry on RDSEED failure

Hi Elena,

On Tue, Jan 30, 2024 at 8:06 PM Reshetova, Elena
<elena.reshetova@...el.com> wrote:
> Yes, sorry, I am just behind answering this thread and it is getting late here.
> This is exactly what I would like to have an open discussion about
> with inputs from everyone.
> We have to remember that it is not only about host 'producing'
> a fully deterministic environment, but also about host being able to
> *observe* the entropy input. So the more precise question to ask is
> how much can a host observe?

Right, observation is just as relevant.

> My personal understanding is that host can
> observe all guest interrupts and their timings, including APIC timer interrupts
> (and IPIs), so what is actually left for the guest as unobservable entropy
> input?

Check out try_to_generate_entropy() and random_get_entropy(), for
example. How observable is RDTSC? Other HPTs?

> > > I imagine the attestation part of CoCo means these VMs need to run on
> > > real Intel silicon and so it can't be single stepped in TCG or
> > > something, right?
>
> Yes, there is an attestation of a confidential VM and some protections in place
> that helps against single-stepping attacks. But I am not sure how this is relevant
> for this, could you please clarify?

I was just thinking that if this didn't require genuine Intel hardware
with prebaked keys in it that you could emulate a CPU and all its
peripherals and ram with defined latencies and such, and run the VM in
a very straightforwardly deterministic environment, because nothing
would be real. But if this does have to hit metal somewhere, then
there's some possibility you at least interact with some hard-to-model
physical hardware.

Jason