+===============================================================+
+ i.MX6, i.MX7 U-Boot HABv4 Secure Boot guide for SPL targets +
+===============================================================+
1. HABv4 secure boot process
-----------------------------
This document is an addendum of mx6_mx7_secure_boot.txt guide describing a
step-by-step procedure on how to sign and securely boot an U-Boot image for
SPL targets.
Details about HAB can be found in the application note AN4581[1] and in the
introduction_habv4.txt document.
1.1 Building a SPL target supporting secure boot
-------------------------------------------------
The U-Boot provides Second Program Loader (SPL) support which generates two
final images, SPL and U-Boot proper. The HABv4 can be used to authenticate
both binaries.
Out of reset the ROM code authenticates the SPL which is responsible for
initializing essential features such as DDR, UART, PMIC and clock
enablement. Once the DDR is available, the SPL code loads the U-Boot proper
image to its specific execution address and call the HAB APIs to extend the
root of trust.
The U-Boot provides support to secure boot configuration and also provide
access to the HAB APIs exposed by the ROM vector table, the support is
enabled by selecting the CONFIG_IMX_HAB option.
When built with this configuration the U-Boot correctly pads the final SPL
image by aligning to the next 0xC00 address, so the CSF signature data
generated by CST can be concatenated to the image.
The U-Boot also append an Image Vector Table (IVT) in the final U-Boot proper
binary (u-boot-ivt.img) so it can be used by HAB API in a post ROM stage.
The diagram below illustrate a signed SPL image layout:
------- +-----------------------------+ <-- *start
^ | Image Vector Table |
| +-----------------------------+ <-- *boot_data
| | Boot Data |
| +-----------------------------+
Signed | | Padding |
Data | +-----------------------------+ <-- *entry
| | |
| | SPL |
| | |
| +-----------------------------+
v | Padding |
------- +-----------------------------+ <-- *csf
| |
| Command Sequence File (CSF) |
| |
+-----------------------------+
| Padding (optional) |
+-----------------------------+
The diagram below illustrate a signed u-boot-ivt.img image layout:
------- +-----------------------------+ <-- *load_address
^ | |
| | |
| | u-boot.img |
Signed | | |
Data | | |
| +-----------------------------+
| | Padding Next Boundary |
| +-----------------------------+ <-- *ivt
v | Image Vector Table |
------- +-----------------------------+ <-- *csf
| |
| Command Sequence File (CSF) |
| |
+-----------------------------+
| Padding (optional) |
+-----------------------------+
1.2 Enabling the secure boot support
-------------------------------------
The first step is to generate an U-Boot image supporting the HAB features
mentioned above, this can be achieved by adding CONFIG_IMX_HAB to the
build configuration:
- Defconfig:
CONFIG_IMX_HAB=y
- Kconfig:
ARM architecture -> Support i.MX HAB features
1.3 Creating the CSF description file
--------------------------------------
The CSF contains all the commands that the HAB executes during the secure
boot. These commands instruct the HAB code on which memory areas of the image
to authenticate, which keys to install, use and etc.
CSF examples are available under doc/imx/habv4/csf_examples/ directory.
Build logs containing the "Authenticate Data" parameters are available after
the U-Boot build, the example below is a log for mx6sabresd_defconfig target:
- SPL build log:
$ cat SPL.log
Image Type: Freescale IMX Boot Image
Image Ver: 2 (i.MX53/6/7 compatible)
Mode: DCD
Data Size: 69632 Bytes = 68.00 KiB = 0.07 MiB
Load Address: 00907420
Entry Point: 00908000
HAB Blocks: 0x00907400 0x00000000 0x0000ec00
- u-boot-ivt.img build log:
$ cat u-boot-ivt.img.log
Image Name: U-Boot 2019.01-00003-g78ee492eb3
Created: Mon Jan 14 17:58:10 2019
Image Type: ARM U-Boot Firmware with HABv4 IVT (uncompressed)
Data Size: 458688 Bytes = 447.94 KiB = 0.44 MiB
Load Address: 17800000
Entry Point: 00000000
HAB Blocks: 0x177fffc0 0x0000 0x0006e020
As explained in section above the SPL is first authenticated by the ROM code
and the root of trust is extended to the U-Boot image, hence two CSF files are
necessary to completely sign a bootloader image.
In "Authenticate Data" CSF command users can copy and past the output
addresses, the csf_uboot.txt can be used as example:
- In csf_SPL.txt:
Block = 0x00907400 0x00000000 0x0000ec00 "SPL"
- In csf_uboot-ivt.txt:
Block = 0x177fffc0 0x0000 0x0006e020 "u-boot-ivt.img"
1.4 Signing the images
-----------------------
The CST tool is used for signing the U-Boot binary and generating a CSF binary,
users should input the CSF description file created in the step above and
receive a CSF binary, which contains the CSF commands, SRK table, signatures
and certificates.
- Create SPL CSF binary file:
$ ./cst -i csf_SPL.txt -o csf_SPL.bin
- Append CSF signature to the end of SPL image:
$ cat SPL csf_SPL.bin > SPL-signed
- Create U-Boot proper CSF binary file:
$ ./cst -i csf_uboot-ivt.txt -o csf_uboot-ivt.bin
- Append CSF signature to the end of U-Boot proper image:
$ cat u-boot-ivt.img csf_uboot-ivt.bin > u-boot-signed.img
The bootloader is signed and can be flashed into the boot media.
1.5 Closing the device
-----------------------
The procedure for closing the device is similar as in Non-SPL targets, for a
complete procedure please refer to section "1.5 Programming SRK Hash" in
mx6_mx7_secure_boot.txt document available under doc/imx/habv4/guides/
directory.
References:
[1] AN4581: "Secure Boot on i.MX 50, i.MX 53, i.MX 6 and i.MX 7 Series using
HABv4" - Rev 2.