[U-Boot] [RFC] SPL -> U-Boot Chain of Trust

[Simon Glass]

Hi Andreas,

On 14 April 2016 at 17:07, Andreas Dannenberg <dannenberg at ti.com> wrote:
>
> Hi Simon, thanks for the feedback. Additional comments inlined...
>
> On Mon, Apr 04, 2016 at 06:04:15PM -0600, Simon Glass wrote:
> > Hi Andreas,
> >
> > On 28 March 2016 at 14:19, Andreas Dannenberg <dannenberg at ti.com> wrote:
> > > On Mon, Mar 28, 2016 at 03:32:40PM -0400, Tom Rini wrote:
> > >> I'm interested in getting secure device support going, but it seems
> > >> like we should need more than that, ie something to keep the chain of
> > >> trust going.
> > >
> > > Tom et al.,
> > > I just saw your reply to Vitaly's email and I'm actually just looking
> > > into something along the lines you brought up but I didn't want to
> > > hijack that discussion so here's a new thread.
> > >
> > > As for the chain of trust for ARMv7, my understanding is that when
> > > using a combination of SPL and U-Boot there will always be a vendor-
> > > specific initial boot (ROM) code that authenticates SPL, and then there
> > > will need to be some code inserted into SPL that authenticates U-Boot
> > > after it's loaded (for example by using some secure ROM API call and
> > > such).
> > >
> > > So I was looking into if there is already some generic framework for
> > > this in U-Boot but didn't see anything obvious. One "easy" way would be
> > > to add a simple call to an authentication routine to board_init_r
> > > (u-boot/common/spl/spl.c) but let's say we add such a call for TI or
> > > other vendor's stuff I suppose this would not scale very well.
> > >
> > > But what about adding one generic call to a default authentication
> > > function declared as __weak for spl_image that doesn't do anything, but
> > > can be overwritten in vendor-specific files to provide means of
> > > authenticating spl_image. Would this be a good approach?
> > >
> > >
> > >
> > > Beyond that I was reviewing some of the awesome work from the Chromium
> > > team and I think on ARMv7 after we get MLO to authenticate U-Boot
> > > everything beyond that is already looking very solid and thorough (with
> > > FIT, DTB/Kernel and initramfs authentication).
> >
> > It should be possible to use this from SPL, if you can enable FIT in
> > SPL. The current implementation does not support verification, and is
> > deliberately cut down. See common/spl/spl_fit.c.
>
> Oh, I just noticed this file after doing a pull, that's really one step
> ahead of the U-Boot versions I've worked with so far. Upon further
> digging I found that the general SPL FIT approach is actually something
> we are trying to enable for our own customers moving forward. So adding/
> enabling FIT auth in SPL would really help connecting the dots and
> closing the current authentication gap not just for us but actually for
> all U-Boot users.

Agreed.

>
> Will look at this more closely and see how much overhead this would
> involve since for SPL memory can be of an issue, as using SPL
> authenticated FIT will probably mean pulling in the U-Boot crypto stuff
> as in such case we would be using U-Boot tooling to generate the signed
> FIT image (as opposed to a vendor-specific signing tool generating an
> image compatible with a simple SoC ROM API auth call). But looking at
> the already memory-optimized U-Boot RSA verification code in
> rsa-verify.c and rsa-checksum.c I would hope the impact would not be too
> bad. I'd guess maybe 10-20KB total with SHA256, RSA, and the needed code
> changes to spl_fit.c.

The overhead for U-Boot itself is covered in the 'Verified Boot on
Chrome OS and How to do it yourself' talk here:

http://elinux.org/ELC_Europe_2013_Presentations

Verified boot itself is about 6KB on Thumb 2, on top of the FIT overhead.

>
> > but you could perhaps
> > provide an option to use the full U-Boot implementation instead.
>
> ...which would mean that the entire U-Boot would need to be loaded
> initially as one piece which wouldn't work on some of our SoCs due to
> memory constraints (hence the SPL approach).

No I don't mean that. I mean use the full U-Boot FIT implementation,
i.e. just the same code. It would still run in FIT. Much of the bloat
is messages which can be dropped. I did an experiment on this a while
ago and already have it on my list to see if I can find any remnants
of it.

>
> --
> Andreas Dannenberg
> Texas Instruments Inc
>

Regards,
Simon