[U-Boot] Complete verified uboot example

[Simon Glass]

Hi Ron,

On 21 February 2017 at 11:08, Ron Brash <ron.brash at gmail.com> wrote:
> Hello all,
>
> I am adding verified kernel support on a board we are using and I am
> struggling to fully understand all of the concepts and steps required to
> pull everything together (on ARM, using ZImages and booting with a working
> DTB on 4.4.3x).  I also looked at the test script inside of examples, but
> it left me with more questions than understanding.
>
> Please correct me where appropriate in my understanding, but if I am
> confused, likely others are too and I hope this helps everyone involved
> overall.

See also the beaglebone_vboot.txt file which has an end-to-end example
for that board. That is a much better example than the test.

>
> Steps:
> ---------------------------------------------------------------
>
> First, u-boot needs to have the appropriate features enabled and to be
> built using them.  At a minimum, I suspect:
>
> CONFIG_RSA=y
> CONFIG_FIT=y
> CONFIG_FIT_SIGNATURE=y
> CONFIG_OF_CONTROL=y

Yes.

>
> Next, we need to derive the appropriate cryptographic primitives/keys.
>
> #Generate a private signing key (RSA2048):
> openssl genrsa -F4 -out \
> "${key_dir}"/"${key_name}".key 2048
>
> # Generate a public key:
> openssl req -batch -new -x509 \
> -key "${key_dir}"/"${key_name}".key \
> -out "${key_dir}"/"${key_name}".crt
>
> Then we derive the ITS or image source file - a file that hints/describes
> the elements that will be verified and/or inside of the FIT image?  Lets
> call this $FIT_ITS
>
> / dts - v1 /;
> / {
> description = "Configuration to load a Xen Kernel";
> #address-cells = <1>;
> images {
> linux_kernel @ 1 {
> description = "Linux zImage";
> data = /incbin / ("pathToImage/zImage");
> type = "kernel";
> arch = "arm";
> os = "linux";
> compression = "none";
> load = <0xaf600000 >;
> entry = <0xaf600000 >;
> hash @ 1 {
> algo = "sha1";
> };
> };
> fdt @ 1 {
> description = "FDT blob";
> data = /incbin / ("PathToDTBUsedByBootingKernel/ex.dtb");
> type = "flat_dt";
> arch = "arm";
> compression = "none";
> load = <0xaec00000 >;
> hash @ 1 {
> algo = "sha1";
> };
> };
> };
> configurations {
> default = "config at 1";
> config @ 1 {
> description = "Plain Linux";
> kernel = "linux_kernel at 1";
> fdt = "fdt at 1";
> loadables = "linux_kernel at 1";

You need a signature at 1 node in here, otherwise mkimage will not sign anything.

> };
> };
> };
>
> Question: Does a signature section go into this as well? underneath the
> hash node for each value?
>
> signature at 1 {
>      algo = "sha1,rsa2048";
>      value = <...kernel signature 1...>
>  };

That goes in the config at 1 node, as above. See sign-configs.txt for an example.

>
> Then using the device-tree-compiler (dtc), I create a DTB for u-boot.  This
> is the control FDT and this defines what keys are used etc..
>
> #Assemble control FDT for U-Boot with space for public key:
> $DTC -p 0x1000 u-boot.dts -O dtb -o u-boot.dtb

Well if you are using CONFIG_OF_CONTROL, U-Boot will create this file
automatically. Also mkimage will add space if needed automatically. So
I don't think you need this step.

>
> Question: What is required inside of the u-boot.dts for u-boot?  Is it
> simply the same .dts used by the booting kernel, but with a section
> proclaiming the keys?

You don't even need that. It will create it for you.

>
> Question:  Where will the compiled u-boot.dtb eventually go?  Is this put
> into a FIT image, or flashed onto the board alongside the u-boot bootloader
> itself?

It needs to be protected from being written. Normally it is added to
the end of U-Boot:

cat u-boot-nodtb.bin u-boot.dtb >u-boot.bin

>
> Next, given that the above steps are completed, I need to create a FIT
> image with space for the signature.
>
> # Generate fitImage with space for signature:
> $MKIMG -D "-I dts -O dtb -p 2000" \
> -f f$FIT_ITS $FIT_IMG

Again, mkimage will add space if needed.

>
> Question: Is the FIT_IMAGE the actual zimage or is it an output image that
> contains all of the values contained within the ITS?

The latter.

>
> Next this FIT_IMAGE (assuming that this is the final FIT image that
> contains the FDT and zImage) needs to be signed and the public key added to
> it; given that that the key information is in the uboot.
>
> # Sign fitImage and add public key into u-boot.dtb:
> $MKIMG -D "-I dts -O dtb -p 2000" -F \
> -k "${key dir}" -K u-boot.dtb -r $FIT_IMG

Yes.

>
> Then, we sign the subsequent fitImage again - correct?
>
> # Signing subsequent fitImage:
> $MKIMG -D "-I dts -O dtb -p 2000" \
> -k "${key dir}" -f $FIT_ITS -r $FIT_IMG

No, that step isn't needed.

>
> Now that all of the above is done - we need to:
> 1. Write our uboot to the flash
> 2. Write our FIT_IMAGE to flash
>
> Question: Do we write anything else to persistent storage? The ITS? etc..

No.

>
> Question: Do we just boot using anything else or just bootm
> 0xLocationOfTheFitImageInRAM

Just bootm.

>
> Greatly appreciate any assistance to all of these questions and I'm sure
> this threat will be of interest to anyone else too.

Reading your questions I can't helping thinking that you may have
missed the documentation.

See the files in doc/uImage.FIT and it will probably help a lot.

There are quite a few more things in this thread now that I have found
it, so I will take a look.

Regards,
Simon