On Wed, Apr 17, 2019 at 10:55:58AM -0400, Steve Grubb wrote:
On Wednesday, April 17, 2019 4:38:18 AM EDT Lennart Poettering
> On Di, 16.04.19 09:06, Adam Williamson (adamwill(a)fedoraproject.org) wrote:
> > > I think all of these are good ideas. "No udev-settle" seems like
> > > nice
> > > highlevel goal to shoot for.
> > >
> > >
> > >
> > > Another one I might add: "No stuck stop jobs" - it annoys me
> > > single
> > > time when I reboot and something like rngd or conmon holds up my
> > > reboot
> > > for several minutes for no reason at all.
> > I've seen the rngd stop thing, hadn't had time to investigate it yet
> > more urgent fires keep showing up :/
> What's the story anyway for rngd? Why would userspace be better at
> providing entropy to the kernel than the kernel itself? Why do we
> enable it on desktops at all, such systems should not be
> entropy-starved. Do we need this at all now that the kernel can use
> RDRAND itself?
The kernel uses RDRAND/SEED but it does not increment the entropy estimate
based on it. Another interesting thing is that TPM chips also have entropy
available, but the kernel does not use it. So, if you have a hardware based
entropy source such as TPM, you need rngd to move the entropy to the kernel.
And it also can mine CPU jitter to create some entropy on its own. And it
also supports the NIST beacon if you want that kind of entropy. Rngd greatly
helps system recover from low entropy situations.
> rngd runs as regular system service, hence what's the point of that
> altogether? I mean, it runs so late during boot, at a point where the
> entropy pool is full anyway,
I'd really like to see it start much earlier. Any way to make that happen?
> and we need the kernel's RNG much much earlier already (already because
> systemd assigns a uuid to each service invocation that derives from kernel
> RNG, and it does that super early). So, why run a service that is supposed
> to fill up the entropy pool at a point where we don't need it anymore, and
> if the kernel can do what it does most likely already on its own?
The kernel cannot recover quickly when stressed for continued entropy
depletion. For example, we are required to be able to supply all guest VM's
with entropy from the host. They draw down the entropy pools which need
replenishment. The kernel is constantly starved for entropy.
The recommendation is for virtio-rng to be backed by /dev/urandom
these days, so you won't deplete the host /dev/random anymore.