On Wed, Apr 27, 2016 at 08:34:50AM -0600, Simon Glass wrote: > Hi Andreas, > > On 14 April 2016 at 17:07, Andreas Dannenberg <dannenb...@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 <dannenb...@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 _______________________________________________ U-Boot mailing list U-Boot@lists.denx.de http://lists.denx.de/mailman/listinfo/u-boot
