[PATCH v3 0/2] hwrng, virtio-rng: init-time fixes

From: Amit Shah
Date: Thu Jul 10 2014 - 06:13:13 EST

- Kees Cook pointed out a weird side-effect: devices which have
->init() registered get their randomness added to the system each
time they're switched in, but devices that don't have the init
callback don't contribute to system randomness more than once. The
weirdness is resolved here by using the randomness each time
hwrng_init() is attempted, irrespective of the existence of the
device's ->init() callback.

- this now separates both the patches; the virtio-rng fix is self-contained
- re-work hwrng core to fetch randomness at device init time if
->init() is registered by the device, instead of not calling it at all.
- virtio-rng: introduce a probe_done bool to ensure we don't ask host
for data before successful probe


When booting a recent kernel under KVM with the virtio-rng device
enabled, the boot process was stalling. Bisect pointed to a commit
made during the 3.15 window to fetch randomness from newly-registered
devices in the hwrng core. The details are in the patches.

Turns out there were two bugs: the initial randomness was being
fetched w/o the device being initialized in cases where the init
callback was registered and the device wasn't the first device being
added to the hwrng core (unrelated to the virtio-rng case). The
second bug is virtio can't communicate with the host without the
device probe is successfully completed.

The impact of this change is that a sole virtio-rng device in a system
won't be able to contribute to the initial randomness. If a second
virtio-rng device is hot-plugged, its input will be used. Though that
scenario is extremely unlikely (why have two virtio-rng devices at

This isn't any different from the current behaviour, though - virtio-rng
currently doesn't contribute to initial system randomness, and continues
not to do so. A better way to get virtio-rng to contribute initially
to the system random pool can be worked out later.

Please apply,

Amit Shah (2):
hwrng: fetch randomness only after device init
virtio: rng: ensure reads happen after successful probe

drivers/char/hw_random/core.c | 47 ++++++++++++++++++++++++++++++-------
drivers/char/hw_random/virtio-rng.c | 10 ++++++++
2 files changed, 49 insertions(+), 8 deletions(-)


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