[libvirt-users] virRandomBits - not very random

[Michal Privoznik]

On 05/25/2018 02:58 PM, Eric Blake wrote:
> Reviving an ancient thread:
> 
> On 11/04/2014 02:18 AM, Daniel P. Berrange wrote:
>> On Mon, Nov 03, 2014 at 11:09:12AM -0500, Brian Rak wrote:
>>> I just ran into an issue where I had about 30 guests get duplicate mac
>>> addresses assigned.  These were scattered across 30 different machines.
>>>
>>> Some debugging revealed that:
>>>
>>> 1) All the host machines were restarted within a couple seconds of each
>>> other
>>> 2) All the host machines had fairly similar libvirtd pids (within
>>> ~100 PIDs
>>> of each other)
>>> 3) Libvirt seeds the RNG using 'time(NULL) ^ getpid()'
>>>
>>> This perfectly explains why I saw so many duplicate mac addresses.
>>>
>>> Why is the RNG seed such a predictable value?  Surely there has to be a
>>> better source of a random seed then the timestamp and the pid?
>>>
>>> The PID seems to me to be a very bad source of any randomness.  I
>>> just ran a
>>> test across 60 of our hosts.  43 of them shared their PID with at
>>> least one
>>> other machine.
>>
>> We should probably seed it with data from /dev/urandom, and/or the new
>> Linux getrandom() syscall (or BSD equivalent).

I'm not quite sure that right after reboot there's going to be enough
entropy. Every service that's starting wants some random bits. But it's
probably better than what we have now.

Michal