[U-Boot] Complete verified uboot example

[Lukasz Majewski]

On Tue, 21 Feb 2017 13:08:19 -0500
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.

Some time ago I've gave a try to verified boot on BBB (beagle bone
black).

Please refer to:
/home/lukma/work/embedded/u-boot-denx/doc/uImage.FIT/beaglebone_vboot.txt

It should shed some more light to your problem and provide reference.

> 
> 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
> 
> 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";
> };
> };
> };
> 
> 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...>
>  };
> 
> 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
> 
> 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?

u-boot dts is not 100% compatible with Linux kernel, but does the
same job.

Yes, you put your public key to it IIRC.

> 
> 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?

The u-boot dtb will be packed into FIT image, which comprise u-boot and
several dtb's. This image is not the one, which kernel uses to boot the
kernel.

(There is a switch CONFIG_SPL_LOAD_FIT in spl.c, so please look for a
reference).

> 
> 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
> 
> Question: Is the FIT_IMAGE the actual zimage or is it an output image
> that contains all of the values contained within the ITS?
> 
> 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
> 
> 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
> 
> 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..
> 
> Question: Do we just boot using anything else or just bootm
> 0xLocationOfTheFitImageInRAM
> 
> Greatly appreciate any assistance to all of these questions and I'm
> sure this threat will be of interest to anyone else too.
> 
> Thanks!
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Best regards,

Lukasz Majewski

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