Re: [RFC PATCH] hwrng: sysfs entry rng_seed_kernel, was: "Re: [PATCH v2 1/2] hwrng: fetch randomness only after device init"

[Jason Cooper <jason@lakedaemon.net>]

On Fri, Jul 11, 2014 at 06:56:26PM +0530, Amit Shah wrote:
> On (Wed) 09 Jul 2014 [12:07:25], Jason Cooper wrote:
> > Amit, Kees,
> 
> (snip)
> 
> > I'm cooling to the idea of the init function for virtio-rng, and it
> > might be best just to admit that there's no way to seed the entropy pool
> > from the virtio-rng at probe time.  After all, once userspace is up, the
> > system should take advantage of /dev/hwrng for the generation of
> > long-term keys.  Either via rngd feeding /dev/random, or directly.
> > 
> > As for the follow-on patch you asked about, I think that's fine.  More
> > entropy can't hurt.
> > 
> > The below patch might be worth considering so that the user of a system
> > with only virtio-rng can kick the entropy pool as they see fit.  It's
> > probably not too kosher as is, but if the idea is liked, I could clean
> > it up and submit.
> > 
> > The advantage is that users don't need to have rngd installed and
> > running on the system in order to jump-start the entropy pool.
> 
> ... so a udev rule that looks for the new sysfs file, and asks the
> kernel to do its thing?

Or, as simple as:

[ -e /sys/.../rng_seed_kernel ] && echo "0" >/sys/.../rng_seed_kernel

in the initrd.  It needs to run *before* any init scripts which may create
keys.

> And maybe even a patch to rngd that looks for this file and does a
> similar thing?

I'm not opposed to that, but it doesn't fit the problem I'm trying to
solve.  Basically, average systems, not trying to be Ft Knox-secure, but
needing to generate long-term keys at first boot.  These systems won't
have an hwrng installed, but should use one if available.  eg
virtio-rng, or any of the on-die SoC hwrngs.

> There's also the option to use a delayed workqueue item, that will
> succeed if probe has finished.  This method doesn't have userspace
> dependencies.

Hmm, I like that idea better.  No ABI change to maintain, no userspace
changes...  You obviously know virtio-rng better than I do, care to take
a crack at it?

thx,

Jason.