virtio_rng should be the source of hardware entropy

[Ian Whitfield]

SRU Justification

[Impact]

GKE made an inquiry about the source of entropy for /dev/hwrng. Their public documentation (https://cloud.google.com/compute/docs/instances/enabling-virtio-rng) specifies that virtio_rng is the default, but they observed that the TPM's RNG is used instead on current GKE images. Besides aligning with their public docs, using virtio_rng means that the host is responsible for providing the most secure hardware entropy source, which is a better default than assuming that the most secure source on that particular machine is the TPM (or RDRAND instructions, etc).

[Fix]

Configure CONFIG_HW_RANDOM_VIRTIO=y for all targeted kernels.

[Test Plan]

Executing
$ cat /sys/devices/virtual/misc/hw_random/rng_current
Should return "virtio_rng.0"

[Regression potential]

There should be a very low chance of regression. Hardware RNG entropy sources in theory are identical in behavior, and the test plan above can determine what the active source of entropy is.

[Other]

PIT: 400861474
SF: 00409265