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High Assurance Boot (HAB) for i.MX6 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 Initial Vector Table contains a pointer to the CSF. Please see
doc/README.imximage for how to prepare u-boot.imx.
The CSF itself is being generated by Freescale HAB tools.
mkimage will output additional information about "HAB Blocks"
which can be used in the Freescale tooling to authenticate U-Boot
(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: 177ff400 00000000 0004dc00
^^^^^^^^ ^^^^^^^^ ^^^^^^^^
| | |
| | -------- (1)
| |
| ------------------- (2)
|
--------------------------- (3)
(1) Size of area in file u-boot.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.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 Freescale HAB tools:
cst --o U-Boot_CSF.bin < U-Boot.CSF
objcopy -I binary -O binary --pad-to 0x2000 --gap-fill=0x00 \
U-Boot_CSF.bin U-Boot_CSF_pad.bin
cat u-boot.imx U-Boot_CSF_pad.bin > u-boot-signed.imx
NOTE: U-Boot_CSF.bin needs to be padded to the value specified in
the imximage.cfg file.
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 Freescale's Code
Signing Tool (CST). The CST replaces only the image data of
u-boot.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.
objcopy -I binary -O binary --pad-to 0x2000 --gap-fill=0x00 \
U-Boot_CSF.bin U-Boot_CSF_pad.bin
cat u-boot.imx U-Boot_CSF_pad.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.
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