Re: [Qemu-devel] [PATCH 1/1] virtio-rng: device to send host entropy to guest

[Daniel P. Berrange]

On Wed, May 16, 2012 at 08:24:22AM -0500, Anthony Liguori wrote:
> On 05/16/2012 06:30 AM, Amit Shah wrote:
> >The Linux kernel already has a virtio-rng driver, this is the device
> >implementation.
> >
> >When Linux needs more entropy, it puts a buffer in the vq.  We then put
> >entropy into that buffer, and push it back to the guest.
> >
> >Feeding randomness from host's /dev/urandom into the guest is
> >sufficient, so this is a simple implementation that opens /dev/urandom
> >and reads from it whenever required.
> >
> >Invocation is simple:
> >
> >   qemu ... -device virtio-rng-pci
> >
> >In the guest, we see
> >
> >   $ cat /sys/devices/virtual/misc/hw_random/rng_available
> >   virtio
> >
> >   $ cat /sys/devices/virtual/misc/hw_random/rng_current
> >   virtio
> >
> >There are ways to extend the device to be more generic and collect
> >entropy from other sources, but this is simple enough and works for now.
> >
> >Signed-off-by: Amit Shah<address@hidden>
> 
> It's not this simple unfortunately.
> 
> If you did this with libvirt, one guest could exhaust the available
> entropy for the remaining guests.  This could be used as a mechanism
> for one guest to attack another (reducing the available entropy for
> key generation).
> 
> You need to rate limit the amount of entropy that a guest can obtain
> to allow management tools to mitigate this attack.

Ultimately I think you need to have a push mechanism, where an external
process feeds entropy to QEMU, rather than a pull mechanism where QEMU
grabs entropy itself.

I tend to think that virtio-rng should have a chardev backend associated
with it. The driver should just read from this chardev to get its entropy.
Either libvirtd, or better yet a separate virt-entropyd daemonm would
connect to each guest & feed the entropy into each guest according to
some desired metrics.

Regards,
Daniel
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