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<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. > > > >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. > >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 -- |: http://berrange.com -o- http://www.flickr.com/photos/dberrange/ :| |: http://libvirt.org -o- http://virt-manager.org :| |: http://autobuild.org -o- http://search.cpan.org/~danberr/ :| |: http://entangle-photo.org -o- http://live.gnome.org/gtk-vnc :|
