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- 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.
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