[libvirt] should we use new Linux syscall getrandom(2)?

[Eric Blake]

On 12/09/2014 09:49 AM, Daniel P. Berrange wrote:

>>>>> The question is how should we make use of it ?  Should we use it as the
>>>>> seed for initstate_r, or just use it for virRandomBits directly ?
>>>>
>>>> Well, consider that libvirt might be run in a VM with snapshot. IIUC
>>>> nowadays when the VM is started from the snapshot virRandomBits() produces
>>>> the same sequence. If we want to prevent that we must use the new syscall
>>>> every time the virRandomBits() is called. I'm afraid using the syscall just
>>>> to set the seed won't be sufficient.

Forking two guests from the same point in time is NOT my concern with
this proposal (when forking, you should already be coordinating with the
forks to clear their entropy pool; virt-sysprep can do this).  Rather,
this is about making libvirtd startup use a better seed, when it can.

>>>
>>> If you are restoring a VM from a snapshot, the entropy pool for /dev/random
>>> will have been preseved too, so you'll still have the same problem. This is
>>> just one of 100's of examples of why you should never try to use snapshots
>>> as the basis for forking multiple independant VM instances.
>>
>> Correct. But after some time (and some packets, keypresses and mouse
>> movements too) both the syscall and the /dev/random will produce
>> unpredictable sequence. But that's not the case for our RNG. What I'm saying
>> is, if we feel that our RNG is not good enough we should use a better one.

It's not our RNG that is bad, just our seeding.

>> Improving seed setting is just deferring problem. Although I can live with
>> Eric's approach too.
> 
> I don't think we are saying the RNG is not good enough. We're saying the
> way we initialize it when libvirtd starts is not good enough. Basically
> two libvirtd processes started on identical OS instances should not get
> the same random numbers. Giving a good seed achieves that.

Furthermore, my biggest reason for using getrandom(2) _only_ for the
seed is that this is the documentation recommended by the syscall itself:

http://lists.openwall.net/linux-kernel/2014/07/17/235


	The system call getrandom() fills the buffer pointed to by buf
	with up to buflen random bytes which can be used to seed user
	space random number generators (i.e., DRBG's) or for other
	cryptographic processes.  It should not be used Monte Carlo
	simulations or for other probabilistic sampling applications.

We do NOT want to be draining /dev/[u]random on every call, but merely
seeding our PRNG sufficiently that the rest of our random bits are
decently "random" because the seed was unpredictable.

-- 
Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org

-------------- next part --------------
A non-text attachment was scrubbed...
Name: signature.asc
Type: application/pgp-signature
Size: 539 bytes
Desc: OpenPGP digital signature
URL: <http://listman.redhat.com/archives/libvir-list/attachments/20141209/cb475543/attachment-0001.sig>