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

[Kees Cook]

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

-- 
Kees Cook
Chrome OS Security
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