High Assurance Boot (HAB) for i.MX6 CPUs
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To enable the authenticated or encrypted boot mode of U-Boot, it is
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required to set the proper configuration for the target board. This
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is done by adding the following configuration in the defconfig file:
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CONFIG_SECURE_BOOT=y
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In addition, the U-Boot image to be programmed into the
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boot media needs to be properly constructed, i.e. it must contain a
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proper Command Sequence File (CSF).
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The Initial Vector Table contains a pointer to the CSF. Please see
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doc/README.imximage for how to prepare u-boot.imx.
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The CSF itself is being generated by Freescale HAB tools.
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mkimage will output additional information about "HAB Blocks"
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which can be used in the Freescale tooling to authenticate U-Boot
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(entries in the CSF file).
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Image Type: Freescale IMX Boot Image
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Image Ver: 2 (i.MX53/6 compatible)
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Data Size: 327680 Bytes = 320.00 kB = 0.31 MB
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Load Address: 177ff420
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Entry Point: 17800000
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HAB Blocks: 177ff400 00000000 0004dc00
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^^^^^^^^ ^^^^^^^^ ^^^^^^^^
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| | |
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| | -------- (1)
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| ------------------- (2)
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--------------------------- (3)
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(1) Size of area in file u-boot.imx to sign
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This area should include the IVT, the Boot Data the DCD
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and U-Boot itself.
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(2) Start of area in u-boot.imx to sign
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(3) Start of area in RAM to authenticate
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CONFIG_SECURE_BOOT currently enables only an additional command
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'hab_status' in U-Boot to retrieve the HAB status and events. This
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can be useful while developing and testing HAB.
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Commands to generate a signed U-Boot using Freescale HAB tools:
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cst --o U-Boot_CSF.bin < U-Boot.CSF
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objcopy -I binary -O binary --pad-to 0x2000 --gap-fill=0x00 \
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U-Boot_CSF.bin U-Boot_CSF_pad.bin
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cat u-boot.imx U-Boot_CSF_pad.bin > u-boot-signed.imx
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NOTE: U-Boot_CSF.bin needs to be padded to the value specified in
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the imximage.cfg file.
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Setup U-Boot Image for Encrypted Boot
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-------------------------------------
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An authenticated U-Boot image is used as starting point for
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Encrypted Boot. The image is encrypted by Freescale's Code
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Signing Tool (CST). The CST replaces only the image data of
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u-boot.imx with the encrypted data. The Initial Vector Table,
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DCD, and Boot data, remains in plaintext.
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The image data is encrypted with a Encryption Key (DEK).
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Therefore, this key is needed to decrypt the data during the
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booting process. The DEK is protected by wrapping it in a Blob,
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which needs to be appended to the U-Boot image and specified in
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the CSF file.
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The DEK blob is generated by an authenticated U-Boot image with
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the dek_blob cmd enabled. The image used for DEK blob generation
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needs to have the following configurations enabled:
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CONFIG_SECURE_BOOT
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CONFIG_SYS_FSL_SEC_COMPAT 4 /* HAB version */
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CONFIG_FSL_CAAM
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CONFIG_CMD_DEKBLOB
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CONFIG_SYS_FSL_SEC_LE
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Note: The encrypted boot feature is only supported by HABv4 or
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greater.
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The dek_blob command then can be used to generate the DEK blob of
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a DEK previously loaded in memory. The command is used as follows:
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dek_blob <DEK address> <Output Address> <Key Size in Bits>
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example: dek_blob 0x10800000 0x10801000 192
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The resulting DEK blob then is used to construct the encrypted
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U-Boot image. Note that the blob needs to be transferred back
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to the host.Then the following commands are used to construct
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the final image.
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objcopy -I binary -O binary --pad-to 0x2000 --gap-fill=0x00 \
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U-Boot_CSF.bin U-Boot_CSF_pad.bin
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cat u-boot.imx U-Boot_CSF_pad.bin > u-boot-signed.imx
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objcopy -I binary -O binary --pad-to <blob_dst> --gap-fill=0x00 \
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u-boot-signed.imx u-boot-signed-pad.bin
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cat u-boot-signed-pad.imx DEK_blob.bin > u-boot-encrypted.imx
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NOTE: u-boot-signed.bin needs to be padded to the value
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equivalent to the address in which the DEK blob is specified
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in the CSF.
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