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

["Theodore Ts'o" <tytso@mit.edu>]

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