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

[Anthony Liguori]

On 05/16/2012 08:53 AM, Daniel P. Berrange wrote:
> On Wed, May 16, 2012 at 08:48:20AM -0500, Anthony Liguori wrote:
> > On 05/16/2012 08:45 AM, Daniel P. Berrange wrote:
> > > 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.
> >
> > A previous patch didn't open urandom directly but instead talked to
> > an entropy daemon.  This approach would allow libvirt to hand out
> > entropy as it saw fit without requiring a new driver.
>
> The nice thing about just using a plain chardev backend for virtiorng
> is that it would let you have the flexibility to integrate with any kind
> of entropy daemon, or even just run without a daemon&  rely on some other
> process to periodically open the chardev&  feed in data.

Ack.

But there is an entropy daemon that does use a protocol.  The protocol may be 
"just read raw random data" but we should at least check to make sure that is 
the protocol.

Regards,

Anthony Liguori

> > > 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