1 1. High Assurance Boot (HAB) for i.MX CPUs 2 ------------------------------------------ 3 4 To enable the authenticated or encrypted boot mode of U-Boot, it is 5 required to set the proper configuration for the target board. This 6 is done by adding the following configuration in the defconfig file: 7 8 CONFIG_SECURE_BOOT=y 9 10 In addition, the U-Boot image to be programmed into the 11 boot media needs to be properly constructed, i.e. it must contain a 12 proper Command Sequence File (CSF). 13 14 The CSF itself is generated by the i.MX High Assurance Boot Reference 15 Code Signing Tool. 16 https://www.nxp.com/webapp/sps/download/license.jsp?colCode=IMX_CST_TOOL 17 18 More information about the CSF and HAB can be found in the AN4581. 19 https://www.nxp.com/docs/en/application-note/AN4581.pdf 20 21 We don't want to explain how to create a PKI tree or SRK table as 22 this is well explained in the Application Note. 23 24 2. Secure Boot on non-SPL targets 25 --------------------------------- 26 27 On non-SPL targets a singe U-Boot binary is generated, mkimage will 28 output additional information about "HAB Blocks" which can be used 29 in the CST to authenticate the U-Boot image (entries in the CSF file). 30 31 Image Type: Freescale IMX Boot Image 32 Image Ver: 2 (i.MX53/6 compatible) 33 Data Size: 327680 Bytes = 320.00 kB = 0.31 MB 34 Load Address: 177ff420 35 Entry Point: 17800000 36 HAB Blocks: 0x177ff400 0x00000000 0x0004dc00 37 ^^^^^^^^^^ ^^^^^^^^^^ ^^^^^^^^^^ 38 | | | 39 | | ----- (1) 40 | | 41 | ---------------- (2) 42 | 43 --------------------------- (3) 44 45 (1) Size of area in file u-boot-dtb.imx to sign 46 This area should include the IVT, the Boot Data the DCD 47 and U-Boot itself. 48 (2) Start of area in u-boot-dtb.imx to sign 49 (3) Start of area in RAM to authenticate 50 51 CONFIG_SECURE_BOOT currently enables only an additional command 52 'hab_status' in U-Boot to retrieve the HAB status and events. This 53 can be useful while developing and testing HAB. 54 55 Commands to generate a signed U-Boot using i.MX HAB CST tool: 56 # Compile CSF and create signature 57 cst --o csf-u-boot.bin --i command_sequence_uboot.csf 58 # Append compiled CSF to Binary 59 cat u-boot-dtb.imx csf-u-boot.bin > u-boot-signed.imx 60 61 3. Secure Boot on SPL targets 62 ----------------------------- 63 64 This version of U-Boot is able to build a signable version of the SPL 65 as well as a signable version of the U-Boot image. The signature can 66 be verified through High Assurance Boot (HAB). 67 68 After building, you need to create a command sequence file and use 69 i.MX HAB Code Signing Tool to sign both binaries. After creation, 70 the mkimage tool outputs the required information about the HAB Blocks 71 parameter for the CSF. During the build, the information is preserved 72 in log files named as the binaries. (SPL.log and u-boot-ivt.log). 73 74 Example Output of the SPL (imximage) creation: 75 Image Type: Freescale IMX Boot Image 76 Image Ver: 2 (i.MX53/6/7 compatible) 77 Mode: DCD 78 Data Size: 61440 Bytes = 60.00 kB = 0.06 MB 79 Load Address: 00907420 80 Entry Point: 00908000 81 HAB Blocks: 0x00907400 0x00000000 0x0000cc00 82 83 Example Output of the u-boot-ivt.img (firmware_ivt) creation: 84 Image Name: U-Boot 2016.11-rc1-31589-g2a4411 85 Created: Sat Nov 5 21:53:28 2016 86 Image Type: ARM U-Boot Firmware with HABv4 IVT (uncompressed) 87 Data Size: 352192 Bytes = 343.94 kB = 0.34 MB 88 Load Address: 17800000 89 Entry Point: 00000000 90 HAB Blocks: 0x177fffc0 0x0000 0x00054020 91 92 # Compile CSF and create signature 93 cst --o csf-u-boot.bin --i command_sequence_uboot.csf 94 cst --o csf-SPL.bin --i command_sequence_spl.csf 95 # Append compiled CSF to Binary 96 cat SPL csf-SPL.bin > SPL-signed 97 cat u-boot-ivt.img csf-u-boot.bin > u-boot-signed.img 98 99 These two signed binaries can be used on an i.MX in closed 100 configuration when the according SRK Table Hash has been flashed. 101 102 4. Setup U-Boot Image for Encrypted Boot 103 ---------------------------------------- 104 An authenticated U-Boot image is used as starting point for 105 Encrypted Boot. The image is encrypted by i.MX Code Signing 106 Tool (CST). The CST replaces only the image data of 107 u-boot-dtb.imx with the encrypted data. The Initial Vector Table, 108 DCD, and Boot data, remains in plaintext. 109 110 The image data is encrypted with a Encryption Key (DEK). 111 Therefore, this key is needed to decrypt the data during the 112 booting process. The DEK is protected by wrapping it in a Blob, 113 which needs to be appended to the U-Boot image and specified in 114 the CSF file. 115 116 The DEK blob is generated by an authenticated U-Boot image with 117 the dek_blob cmd enabled. The image used for DEK blob generation 118 needs to have the following configurations enabled in Kconfig: 119 120 CONFIG_SECURE_BOOT=y 121 CONFIG_CMD_DEKBLOB=y 122 123 Note: The encrypted boot feature is only supported by HABv4 or 124 greater. 125 126 The dek_blob command then can be used to generate the DEK blob of 127 a DEK previously loaded in memory. The command is used as follows: 128 129 dek_blob <DEK address> <Output Address> <Key Size in Bits> 130 example: dek_blob 0x10800000 0x10801000 192 131 132 The resulting DEK blob then is used to construct the encrypted 133 U-Boot image. Note that the blob needs to be transferred back 134 to the host.Then the following commands are used to construct 135 the final image. 136 137 cat u-boot-dtb.imx csf-u-boot.bin > u-boot-signed.imx 138 objcopy -I binary -O binary --pad-to <blob_dst> --gap-fill=0x00 \ 139 u-boot-signed.imx u-boot-signed-pad.bin 140 cat u-boot-signed-pad.imx DEK_blob.bin > u-boot-encrypted.imx 141 142 NOTE: u-boot-signed.bin needs to be padded to the value 143 equivalent to the address in which the DEK blob is specified 144 in the CSF. 145
