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

Daniel P. Berrange berrange at redhat.com
Tue Dec 9 16:49:21 UTC 2014


On Tue, Dec 09, 2014 at 05:46:54PM +0100, Michal Privoznik wrote:
> On 09.12.2014 17:36, Daniel P. Berrange wrote:
> >On Tue, Dec 09, 2014 at 05:29:51PM +0100, Michal Privoznik wrote:
> >>On 09.12.2014 16:07, Daniel P. Berrange wrote:
> >>>On Tue, Dec 09, 2014 at 08:03:13AM -0700, Eric Blake wrote:
> >>>>Now that Linux has a syscall for getting secure random bytes, should we
> >>>>use that when available in our src/util/virrandom.c implementation?
> >>>
> >>>Yes, we should. I remember reading a few weeks back that someone found
> >>>our current random seed is rather predictable when the libvirt host is
> >>>booted from a cut-down image running systemd. Since there is no longer
> >>>1000000000 lines of shell in the init process the initial PIDs are very
> >>>stable across each boot attempt.
> >>>
> >>>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.
> >
> >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.
> 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.

Regards,
Daniel
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