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

[Simon Glass]

Hi guys,

On Wed, 23 Dec 2020 at 03:24, Torsten Duwe <duwe at lst.de> wrote:
>
> 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.
>

I really don't mind too much about that side of it. But I do worry
when I see code that buries hard-coded DM access in a lib/ function.

If boot wants a random device it should get one, at the top level, not
in the bowels of U-Boot. It can pass that device down to other
functions as needed.

To my reading, the current definition of the RNG uclass looks similar
to a /dev/random

Let me know if I am missing something.

Regards,
Simon