[PATCH v2 1/2] lib: uuid: use RNG device if present

[Torsten Duwe]

On Sun, 20 Dec 2020 11:17:50 -0700
Simon Glass <sjg at chromium.org> wrote:

> Hi Torsten,
> 
> On Sun, 20 Dec 2020 at 10:00, Torsten Duwe <duwe at lst.de> wrote:
> >
> > On Fri, 18 Dec 2020 19:29:12 -0700
> > Simon Glass <sjg at chromium.org> wrote:
> >
> > > > -       int i;
> > > > -
> > > > -       srand(get_ticks() + rand());
> > > > +       int i, ret;
> > > > +       struct udevice *devp;
> > > > +       u8 randv = 0;
> > > > +
> > > > +#if defined(CONFIG_DM_RNG)
> > >
> > > This seems a little backwards to me. The caller should request a
> > > RNG device, getting either a hardware one or a software one, and
> > > then call the uclass method to get the uuid.
> >
> > Strictly speaking, there's no such thing as a "software RNG". The
> > term "DRBG" was coined for accurateness, "deterministic random bit
> > generator". The oxymoron "deterministic random" pretty much nails
> > it. Alternatively, it can be called "pseudo" RNG.
> > rand() and srand() exactly implement such a mechanism already, with
> > low coding overhead. U-Boot runs fine with them most of the time,
> > but there are rare cases where real entropy would be needed. This
> > is what these two patches are about. In case there's more, I
> > already speculated about a centralised entity in my response to the
> > v1 cover letter, but for now these two changes should do.
> 
> I am used to the term pseudo-random, but it doesn't much matter what
> kind of random number is used. It is still covered by the RNG class.

Well, in these 2 cases, it _does_ matter. And besides, as I wrote
above, pseudo randomness is produced by the rand() function, and RNG
devices provide _real_ entropy to a system.

So, while every other entity in U-Boot is happy with a DRBG, a UUID and
a BOOTP delay need this real entropy, hence the different code, for a
start.

> You are currently burying device access in a utility function. That
> really isn't the right way to do it. See my comment above. There is no
> way to control which RNG device is used and no visibility that this is
> happening at all, outside this function.

The code looks a bit odd to me, too, as I mentioned. I imagined
something not so full blown as in the Linux kernel, but still some
central mechanism to get entropy from, for those who really need it (in
the current situation, just these 2 cases). This alternative would
result in a real /dev/random in U-Boot, which would yield a cleaner
structure, but would require more code to be produced and more code
needing change. That given, I'd agree to these 2 hacks, especially
because there are security implications. 

What's your opinion, how would you like to create really unique UUIDs?
How should BOOTP clients wait randomly (esp. in a large group)?
Shall we create some U-Boot version of /dev/random and haveged?

I'm really open to suggestions.

	Torsten