[libvirt] [libvirt-users] virRandomBits - not very random

[Daniel P. Berrangé]

On Wed, May 30, 2018 at 10:21:54PM +0200, Martin Kletzander wrote:
> On Tue, May 29, 2018 at 10:06:25AM -0400, John Ferlan wrote:
> > 
> > 
> > On 05/29/2018 09:44 AM, Michal Privoznik wrote:
> > > On 05/29/2018 03:38 PM, Martin Kletzander wrote:
> > > > On Fri, May 25, 2018 at 09:37:44AM -0500, Eric Blake wrote:
> > > > > On 05/25/2018 09:17 AM, Michal Privoznik wrote:
> > > > > 
> > > > > > > > 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.
> > > > > 
> > > > > Here's where we left things last time it came up:
> > > > > 
> > > > > https://www.redhat.com/archives/libvir-list/2014-December/msg00573.html
> > > > > 
> > > > > If gnutls has an interface that will give us random bits
> > > > > (gnutls_key_generate() in 3.0, perhaps), we should use THAT for all of
> > > > > our random bits (and forget about a seed), except when we are mocking
> > > > > things in our testsuite, and need a deterministic PRNG from a
> > > > > deterministic seed.
> > > > > 
> > > > > If not (including if we are not linked with gnutls), then we should
> > > > > prefer the new Linux syscall but fall back to /dev/urandom for JUST
> > > > > enough bits for a seed; once we're seeded, stick with using our existing
> > > > > PRNG for all future bits (after all, we aren't trying to generate
> > > > > cryptographically secure keys using virRandomBits - and the places where
> > > > > we DO need crypto-strong randomness such as setting up TLS migration is
> > > > > where we are relying on gnutls to provide it rather than virRandomBits).
> > > > > 
> > > > > So at this point, it's just a matter of someone writing the patches.
> > > > > 
> > > > 
> > > > Actually, do we need to have a fallback at all?  Can't we just drop all the
> > > > gross parts of the code the conditionally compile based on GNUTLS
> > > > support?  Why
> > > > don't we have gnutls required?
> > > 
> > > That's exactly what I'm suggesting in my patches [1]. gnutls is widely
> > > available (including Linux, Windows, *BSD, Mac Os X). However, before
> > > doing that we need to fix virRandomBits() to actually call gnutls_rnd().
> > > 
> > > 1: https://www.redhat.com/archives/libvir-list/2018-May/msg02077.html
> > > 
> > 
> > I have this faint recollection of one of the CI platform builds failing
> > because something in the gnutls* family didn't exist there when I was
> > making the changes to add the domain master secret code....  After a bit
> > of digging, it seems it was a perhaps a CENTOS6 environment:
> > 
> > https://www.redhat.com/archives/libvir-list/2016-April/msg00287.html
> > 
> > and since IIUC that's not an issue any more....
> > 
> 
> Oh, cool to know.  Michal also found the patch [1] where Dan switched the gnutls
> from being mandatory to making it optional and there is no explanation for that
> change in the commit message:
> 
> [1] f587c27768ee13f5bed6a9262106307b7a124403

Not all usage scenarios in libvirt have required GNUTLS - only the remote
driver, when using stateful virt drivers. If you're just biulding libvirt
for usage with ESX/HyperV/etc, there's no reason you'd want GNUTLS historically.

Also note when building the setuid libvirt pieces we must never use GNUTLS
because its library constructors do very bad things leading to CVEs.


Regards,
Daniel
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