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

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Message-ID: <20240203143547.GC36616@mit.edu>
Date: Sat, 3 Feb 2024 09:35:47 -0500
From: "Theodore Ts'o" <tytso@....edu>
To: James Bottomley <jejb@...ux.ibm.com>
Cc: "Jason A. Donenfeld" <Jason@...c4.com>,
        "Reshetova, Elena" <elena.reshetova@...el.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>,
        Borislav Petkov <bp@...en8.de>, "H. Peter Anvin" <hpa@...or.com>,
        "x86@...nel.org" <x86@...nel.org>,
        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 Fri, Feb 02, 2024 at 10:28:01PM +0100, James Bottomley wrote:
> 
> My big concern is older cpus where rdrand/rdseed don't produce useful
> entropy.  Exhaustion attacks are going to be largely against VMs not
> physical systems, so I worry about physical systems with older CPUs
> that might have rdrand issues which then trip our Confidential
> Computing checks.

For (non-CC) VM's the answer is virtio-rng.  This solves the
exhaustion problem, since if you can't trust the host, the VM's
security is taost anyway (again, ignoring Confidential Compute).

> The signal for rdseed failing is fairly clear, so if the node has other
> entropy sources, it should continue otherwise it should signal failure.
> Figuring out how a confidential computing environment signals that
> failure is TBD.

That's a design decision, and I believe we've been converging on a
panic during early boot.  Post boot, if we've successfully succeeded
in initializing the guest kernel's RNG, we're secure so long as the
cryptographic primitives haven't been defeated --- and if we have,
such as if Quantuum Computing because practical, we've got bigger
problems anyway.

					- Ted