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

From: Daniel P. Berrange
Subject: Re: [Qemu-devel] [PATCH 1/1] virtio-rng: device to send host entropy to guest
Date: Wed, 16 May 2012 14:53:28 +0100
User-agent: Mutt/1.5.21 (2010-09-15)

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.

> >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.

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