Re: [chrony-users] Chrony vs. Linux RNG

[Holger Hoffstätte <holger@xxxxxxxxxxxxxxxxxxxxxx>]

On 04/23/18 11:04, Miroslav Lichvar wrote:
> On Sun, Apr 22, 2018 at 07:15:12PM +0200, Holger Hoffstätte wrote:
> > I test stable/LTS kernels to help Greg KH and just updated to 4.16.4-rc1.
> > This contains a few patches that are supposed to help with CVEs around
> > randomness, and which cause an interesting catch-22 that affects chrony,
> > hence this mail.
>
> Thanks for the heads up.
>
> I tried booting a VM with 4.17-rc2, which should include the patches

Yeah, I could have mentioned that..

> you are referring to, but didn't see any delays problems.
>
> On what distro do you test it? Does it save and restore the random
> seed on boot (e.g. the systemd-random-seed)?

Gentoo using OpenRC, chronyd 3.3. It uses start-stop-daemon and it
was definitely chronyd hanging the boot sequence; for tests I disabled
chronyd from the default runlevel and all was back to smooth sailing.
Since s-s-d relies on chronyd going into the background, the temporary
fix was to add the --background flag to s-s-d so that OpenRC returns
immediately.

I just saw that it does indeed have a "urandom" service in the boot
runlevel, reading/writing from/to /var/lib/misc/random-seed.
But that happens way before chrony's default runlevel.

glibc is 2.26, so it should be using getrandom() and not use the
urandom fallbacks. Unfortunately it's really hard to trace/debug
this since the bug only manifests itself during the early stages, and
as soon as I do anything on a freshly booted system I create entropy,
initialising the crng and thus making everything work.

> I guess it could use a non-blocking read for the urandom device (or
> getrandom() syscall) and fall back to random(), but I'm not sure if it
> would be a good idea from the security point of view.

I found in util.c that it *should* be using getrandom() already?
Maybe the HAVE_GETRANDOM detection didn't work, but even then the
urandom fallback should not be blocking either. I'll double-check the
package script's autoconf log.

thanks,
Holger

--
To unsubscribe email chrony-users-request@xxxxxxxxxxxxxxxxxxxx 
with "unsubscribe" in the subject.
For help email chrony-users-request@xxxxxxxxxxxxxxxxxxxx 
with "help" in the subject.
Trouble?  Email listmaster@xxxxxxxxxxxxxxxxxxxx.