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

[Anthony Liguori <anthony@codemonkey.ws>]

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<amit.shah@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.

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