1. High Assurance Boot (HAB) for i.MX CPUs
------------------------------------------
To enable the authenticated or encrypted boot mode of U-Boot, it is
required to set the proper configuration for the target board. This
is done by adding the following configuration in the defconfig file:
CONFIG_SECURE_BOOT=y
In addition, the U-Boot image to be programmed into the
boot media needs to be properly constructed, i.e. it must contain a
proper Command Sequence File (CSF).
The CSF itself is generated by the i.MX High Assurance Boot Reference
Code Signing Tool.
https://www.nxp.com/webapp/sps/download/license.jsp?colCode=IMX_CST_TOOL
More information about the CSF and HAB can be found in the AN4581.
https://www.nxp.com/docs/en/application-note/AN4581.pdf
We don't want to explain how to create a PKI tree or SRK table as
this is well explained in the Application Note.
2. Secure Boot on non-SPL targets
---------------------------------
On non-SPL targets a singe U-Boot binary is generated, mkimage will
output additional information about "HAB Blocks" which can be used
in the CST to authenticate the U-Boot image (entries in the CSF file).
Image Type: Freescale IMX Boot Image
Image Ver: 2 (i.MX53/6 compatible)
Data Size: 327680 Bytes = 320.00 kB = 0.31 MB
Load Address: 177ff420
Entry Point: 17800000
HAB Blocks: 0x177ff400 0x00000000 0x0004dc00
^^^^^^^^^^ ^^^^^^^^^^ ^^^^^^^^^^
| | |
| | ----- (1)
| |
| ---------------- (2)
|
--------------------------- (3)
(1) Size of area in file u-boot-dtb.imx to sign
This area should include the IVT, the Boot Data the DCD
and U-Boot itself.
(2) Start of area in u-boot-dtb.imx to sign
(3) Start of area in RAM to authenticate
CONFIG_SECURE_BOOT currently enables only an additional command
'hab_status' in U-Boot to retrieve the HAB status and events. This
can be useful while developing and testing HAB.
Commands to generate a signed U-Boot using i.MX HAB CST tool:
# Compile CSF and create signature
cst --o csf-u-boot.bin --i command_sequence_uboot.csf
# Append compiled CSF to Binary
cat u-boot-dtb.imx csf-u-boot.bin > u-boot-signed.imx
3. Secure Boot on SPL targets
-----------------------------
This version of U-Boot is able to build a signable version of the SPL
as well as a signable version of the U-Boot image. The signature can
be verified through High Assurance Boot (HAB).
After building, you need to create a command sequence file and use
i.MX HAB Code Signing Tool to sign both binaries. After creation,
the mkimage tool outputs the required information about the HAB Blocks
parameter for the CSF. During the build, the information is preserved
in log files named as the binaries. (SPL.log and u-boot-ivt.log).
Example Output of the SPL (imximage) creation:
Image Type: Freescale IMX Boot Image
Image Ver: 2 (i.MX53/6/7 compatible)
Mode: DCD
Data Size: 61440 Bytes = 60.00 kB = 0.06 MB
Load Address: 00907420
Entry Point: 00908000
HAB Blocks: 0x00907400 0x00000000 0x0000cc00
Example Output of the u-boot-ivt.img (firmware_ivt) creation:
Image Name: U-Boot 2016.11-rc1-31589-g2a4411
Created: Sat Nov 5 21:53:28 2016
Image Type: ARM U-Boot Firmware with HABv4 IVT (uncompressed)
Data Size: 352192 Bytes = 343.94 kB = 0.34 MB
Load Address: 17800000
Entry Point: 00000000
HAB Blocks: 0x177fffc0 0x0000 0x00054020
# Compile CSF and create signature
cst --o csf-u-boot.bin --i command_sequence_uboot.csf
cst --o csf-SPL.bin --i command_sequence_spl.csf
# Append compiled CSF to Binary
cat SPL csf-SPL.bin > SPL-signed
cat u-boot-ivt.img csf-u-boot.bin > u-boot-signed.img
These two signed binaries can be used on an i.MX in closed
configuration when the according SRK Table Hash has been flashed.
4. Setup U-Boot Image for Encrypted Boot
----------------------------------------
An authenticated U-Boot image is used as starting point for
Encrypted Boot. The image is encrypted by i.MX Code Signing
Tool (CST). The CST replaces only the image data of
u-boot-dtb.imx with the encrypted data. The Initial Vector Table,
DCD, and Boot data, remains in plaintext.
The image data is encrypted with a Encryption Key (DEK).
Therefore, this key is needed to decrypt the data during the
booting process. The DEK is protected by wrapping it in a Blob,
which needs to be appended to the U-Boot image and specified in
the CSF file.
The DEK blob is generated by an authenticated U-Boot image with
the dek_blob cmd enabled. The image used for DEK blob generation
needs to have the following configurations enabled in Kconfig:
CONFIG_SECURE_BOOT=y
CONFIG_CMD_DEKBLOB=y
Note: The encrypted boot feature is only supported by HABv4 or
greater.
The dek_blob command then can be used to generate the DEK blob of
a DEK previously loaded in memory. The command is used as follows:
dek_blob <DEK address> <Output Address> <Key Size in Bits>
example: dek_blob 0x10800000 0x10801000 192
The resulting DEK blob then is used to construct the encrypted
U-Boot image. Note that the blob needs to be transferred back
to the host.Then the following commands are used to construct
the final image.
cat u-boot-dtb.imx csf-u-boot.bin > u-boot-signed.imx
objcopy -I binary -O binary --pad-to <blob_dst> --gap-fill=0x00 \
u-boot-signed.imx u-boot-signed-pad.bin
cat u-boot-signed-pad.imx DEK_blob.bin > u-boot-encrypted.imx
NOTE: u-boot-signed.bin needs to be padded to the value
equivalent to the address in which the DEK blob is specified
in the CSF.