[systemd-devel] Delaying (SSH) key generation until the urandom pool is initialized

[Florian Weimer]

On 05/05/2014 04:35 PM, Lennart Poettering wrote:
> On Wed, 30.04.14 17:06, Florian Weimer (fweimer at redhat.com) wrote:
>
>>
>> On 04/30/2014 02:28 PM, Daniel P. Berrange wrote:
>>
>>>> Interesting suggestion.  I just used virt-manager to create the VM.
>>>> I don't see any trace for "rng" or "random" in the domain XML file.
>>>> If it is supported, I think it should be enabled by default.
>>>
>>> I'm told that it isn't turned on by default, but you can add it to
>>> a VM post-install. Since it feeds VMs from the host's /dev/random
>>> or /dev/hwrng, there was a question mark as to whether it was right
>>> to enable it by default or not, and if so what kind of rate limiting
>>> might be wanted by default.
>>
>> Ah, so it builds down to our distrust of hardware RNGs?  How
>> annoying. We should be able to trust Fedora-on-Fedora (or
>> Debian-on-Debian or whatever) scenarios.  But I get that in the
>> general case, it's impossible to know what's on the other side of
>> the virtio_rng side, so reservations remain.
>
> Hmm? Well, a virtualized OS has to trust the hypervisor, there's no way
> around that.

I'm referring to this:

  * This function will use the architecture-specific hardware random
  * number generator if it is available.  The arch-specific hw RNG will
  * almost certainly be faster than what we can do in software, but it
  * is impossible to verify that it is implemented securely (as
  * opposed, to, say, the AES encryption of a sequence number using a
  * key known by the NSA).  So it's useful if we need the speed, but
  * only if we're willing to trust the hardware manufacturer not to
  * have put in a back door.

I think this is the reason why the pool isn't considered initialized 
even if its contents has been randomized with RDRAND or similar 
instructions.

I wouldn't be surprised if these minds have a similar concern about 
randomness coming from a hypervisor.

-- 
Florian Weimer / Red Hat Product Security Team