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

[Anthony Liguori <anthony@codemonkey.ws>]

On 05/16/2012 12:21 PM, Amit Shah wrote:
> 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.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.
>
> 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.

But you're still exhausting the entropy pool (which is a global resource). 
That's the problem.

Regards,

Anthony Liguori

>
> 		Amit