[libvirt-users] virRandomBits - not very random

[Eric Blake]

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).

Did anyone ever open a BZ to track this?  As far as I can tell, we still 
have a very predictable (meaning bad) seeding algorithm that permits 
large clusters to create collisions when their random number sequences 
sync up.

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3266
Virtualization:  qemu.org | libvirt.org