On (Wed) 16 May 2012 [08:24:22], 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<amit.s...@redhat.com>
>
> 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.
Hm, rate-limiting is a good point. However, we're using /dev/urandom
here, which is nonblocking, and will keep on providing data as long as
we keep reading.
Amit
