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

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Date: Tue, 30 Jan 2024 16:12:22 +0100
From: "Jason A. Donenfeld" <Jason@...c4.com>
To: Daniel P. Berrangé <berrange@...hat.com>
Cc: "Reshetova, Elena" <elena.reshetova@...el.com>,
	"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

On Tue, Jan 30, 2024 at 02:43:19PM +0000, Daniel P. Berrangé wrote:
> On Tue, Jan 30, 2024 at 03:06:14PM +0100, Jason A. Donenfeld wrote:
> > Is that an accurate summary? If it is, then the actual problem is that
> > the hardware provided to solve this problem doesn't actually solve it
> > that well, so we're caught deciding between guest-guest DoS (some
> > other guest on the system uses all RDRAND resources) and cryptographic
> > failure because of a malicious host creating a deterministic
> > environment.
> 
> In a CoCo VM environment, a guest DoS is not a unique threat
> scenario, as it is unrelated to confidentiality. Ensuring
> fair subdivision of resources between competeing guests is
> just a general VM threat. There are many easy ways a host
> admin can stop a guest making computational progress. Simply
> not scheduling the guest vCPU threads is one. CoCo doesn't
> try to solve this problem.
> 
> Preserving confidentiality is the primary aim of CoCo.
> 
> IOW, if the guest boot is stalled because the kernel is spinning
> waiting on RDRAND to return data, that's fine. If the kernel
> panics after "n" RDRAND failures in a row that's fine too. They
> are both just yet another DoS scenario. 
> 
> If the kernel ignores the RDRAND failure and lets it boot with
> degraded RNG state there were susceptible to attacks, that would
> not be OK for CoCo. 

Yea, that's why I said "we're caught deciding..." One case is a DoS that
would affect all VMs, so while one guest preventing new guests from
booting seems like not a CoCo problem, yes, it is still a problem.

At least in theory. And in practice this is easy with RDSEED too. In
practice, could you actually indefinably starve RDRAND between guests?
Is this pretty easy to do with a little tinkering, or is this a
practically impossible DoS vector? I don't actually know.