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

From: Kees Cook
Date: Mon Jul 14 2014 - 17:50:55 EST

On Fri, Jul 11, 2014 at 8:44 AM, Jason Cooper <jason@xxxxxxxxxxxxxx> wrote:
> 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?

I like this idea too. It's much nicer to just have the kernel Do The
Right Thing automatically. :)


Kees Cook
Chrome OS Security
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