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

[Matthias Brugger]

On Wed, Jan 20, 2021 at 12:50:56PM -0700, Simon Glass wrote:
> Hi Matthias,
> 
> On Wed, 20 Jan 2021 at 04:17, Matthias Brugger <mbrugger at suse.com> wrote:
> >
> >
> >
> > On 29/12/2020 04:32, Simon Glass wrote:
> > > 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.
> > >
> >
> > v3 of this series got merged, which should not stop us from making the code better.
> >
> >
> > > 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.
> > >
> >
> > So you mean we should get the RNG udevice at the caller of gen_rand_uuid_str and
> > then pass it down to gen_rand_uuid? And if no device has been found, we will
> > call srand(get_ticks() + rand())?
> 
> Yes. I suppose you could call it with NULL if there is no device, but
> I think it would be much better to have a software RNG device that can
> be enabled on platforms that don't have hardware support. Then your
> get_ticks() + rand() can be in that.

Well the thing is that up to now the SW RNG implementation uses
different ways in uuid and bootp to create the seeds.

> 
> Also the command stuff in uuid.c should move to cmd/
> 

Yes, realized that as well :)

> >
> > I'm not quite sure how we would need to implement this in the case of
> > srand_mac() though.
> 
> You can put it in struct eth_uclass_priv and provide a function in
> eth-uclass.c to return it? Then call that from bootp.c
> 
> At least then the caller is in control.
> 

I'll have a look on that.

Regards,
Matthias

> >
> > Regards,
> > Matthias
> >
> > > To my reading, the current definition of the RNG uclass looks similar
> > > to a /dev/random
> > >
> > > Let me know if I am missing something.
> > >
> >