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

[Andreas Dannenberg]

On Wed, Apr 27, 2016 at 08:34:50AM -0600, Simon Glass wrote:
> 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.

Hi Simon,
I had actually seen/reviewed this presentation earlier and thought it was
very helpful to get started so thanks for pointing this out again.

> > > 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.

Ok understood, that would seem to make it easier/cleaner from an
implementation POV. Still, I've concerns specifically with one of our
SoCs of which its high-security device variant apparently only has
something to the order of 45KB for SPL use, which folks had to make a
lot of effort to strip down/squeeze in a custom SPL, even going as far
as trimming down strings (and that's using a proprietary authentication
scheme for U-Boot that almost doesn't take any space at all). I need to
find some time to dig deeper and play with the bits and bytes but my
current thinking is to try to pursue the SPL-based FIT approach you
suggested for the general use case, but have one exception for that
particular one SoC using a vendor-specific authentication approach since
there might not be another way due to the memory constraints.

> 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.

Yeah please let me/us know if there is something you have that could be
recycled.

Regards,

--
Andreas Dannenberg
Texas Instruments Inc