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doc: imx: habv4: Add Secure Boot guide for i.MX6 and i.MX7 SPL targets
The current U-Boot implementation includes SPL targets for some NXP development boards: - mx6sabreauto_defconfig - mx6sabresd_defconfig - mx6ul_14x14_evk_defconfig - mx6ul_9x9_evk_defconfig Add additional steps needed to completly secure the bootloader image. Signed-off-by: Breno Lima <breno.lima@nxp.com>
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Breno Matheus Lima
Stefano Babic
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doc/imx/habv4/guides/mx6_mx7_spl_secure_boot.txt
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doc/imx/habv4/guides/mx6_mx7_spl_secure_boot.txt
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+===============================================================+
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+ i.MX6, i.MX7 U-Boot HABv4 Secure Boot guide for SPL targets +
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+===============================================================+
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1. HABv4 secure boot process
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-----------------------------
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This document is an addendum of mx6_mx7_secure_boot.txt guide describing a
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step-by-step procedure on how to sign and securely boot an U-Boot image for
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SPL targets.
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Details about HAB can be found in the application note AN4581[1] and in the
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introduction_habv4.txt document.
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1.1 Building a SPL target supporting secure boot
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-------------------------------------------------
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The U-Boot provides Second Program Loader (SPL) support which generates two
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final images, SPL and U-Boot proper. The HABv4 can be used to authenticate
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both binaries.
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Out of reset the ROM code authenticates the SPL which is responsible for
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initializing essential features such as DDR, UART, PMIC and clock
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enablement. Once the DDR is available, the SPL code loads the U-Boot proper
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image to its specific execution address and call the HAB APIs to extend the
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root of trust.
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The U-Boot provides support to secure boot configuration and also provide
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access to the HAB APIs exposed by the ROM vector table, the support is
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enabled by selecting the CONFIG_SECURE_BOOT option.
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When built with this configuration the U-Boot correctly pads the final SPL
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image by aligning to the next 0xC00 address, so the CSF signature data
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generated by CST can be concatenated to the image.
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The U-Boot also append an Image Vector Table (IVT) in the final U-Boot proper
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binary (u-boot-ivt.img) so it can be used by HAB API in a post ROM stage.
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The diagram below illustrate a signed SPL image layout:
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------- +-----------------------------+ <-- *start
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^ | Image Vector Table |
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| +-----------------------------+ <-- *boot_data
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| | Boot Data |
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| +-----------------------------+
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Signed | | Padding |
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Data | +-----------------------------+ <-- *entry
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| | |
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| | SPL |
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| | |
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| +-----------------------------+
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v | Padding |
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------- +-----------------------------+ <-- *csf
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| |
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| Command Sequence File (CSF) |
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| |
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+-----------------------------+
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| Padding (optional) |
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+-----------------------------+
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The diagram below illustrate a signed u-boot-ivt.img image layout:
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------- +-----------------------------+ <-- *load_address
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^ | |
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| | |
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| | u-boot.img |
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Signed | | |
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Data | | |
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| +-----------------------------+
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| | Padding Next Boundary |
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| +-----------------------------+ <-- *ivt
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v | Image Vector Table |
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------- +-----------------------------+ <-- *csf
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| |
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| Command Sequence File (CSF) |
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| |
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+-----------------------------+
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| Padding (optional) |
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+-----------------------------+
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1.2 Enabling the secure boot support
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-------------------------------------
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The first step is to generate an U-Boot image supporting the HAB features
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mentioned above, this can be achieved by adding CONFIG_SECURE_BOOT to the
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build configuration:
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- Defconfig:
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CONFIG_SECURE_BOOT=y
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- Kconfig:
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ARM architecture -> Support i.MX HAB features
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1.3 Creating the CSF description file
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--------------------------------------
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The CSF contains all the commands that the HAB executes during the secure
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boot. These commands instruct the HAB code on which memory areas of the image
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to authenticate, which keys to install, use and etc.
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CSF examples are available under doc/imx/habv4/csf_examples/ directory.
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Build logs containing the "Authenticate Data" parameters are available after
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the U-Boot build, the example below is a log for mx6sabresd_defconfig target:
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- SPL build log:
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$ cat SPL.log
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Image Type: Freescale IMX Boot Image
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Image Ver: 2 (i.MX53/6/7 compatible)
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Mode: DCD
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Data Size: 69632 Bytes = 68.00 KiB = 0.07 MiB
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Load Address: 00907420
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Entry Point: 00908000
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HAB Blocks: 0x00907400 0x00000000 0x0000ec00
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- u-boot-ivt.img build log:
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$ cat u-boot-ivt.img.log
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Image Name: U-Boot 2019.01-00003-g78ee492eb3
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Created: Mon Jan 14 17:58:10 2019
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Image Type: ARM U-Boot Firmware with HABv4 IVT (uncompressed)
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Data Size: 458688 Bytes = 447.94 KiB = 0.44 MiB
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Load Address: 17800000
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Entry Point: 00000000
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HAB Blocks: 0x177fffc0 0x0000 0x0006e020
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As explained in section above the SPL is first authenticated by the ROM code
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and the root of trust is extended to the U-Boot image, hence two CSF files are
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necessary to completely sign a bootloader image.
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In "Authenticate Data" CSF command users can copy and past the output
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addresses, the csf_uboot.txt can be used as example:
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- In csf_SPL.txt:
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Block = 0x00907400 0x00000000 0x0000ec00 "SPL"
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- In csf_uboot-ivt.txt:
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Block = 0x177fffc0 0x0000 0x0006e020 "u-boot-ivt.img"
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1.4 Signing the images
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-----------------------
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The CST tool is used for singing the U-Boot binary and generating a CSF binary,
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users should input the CSF description file created in the step above and
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receive a CSF binary, which contains the CSF commands, SRK table, signatures
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and certificates.
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- Create SPL CSF binary file:
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$ ./cst -i csf_SPL.txt -o csf_SPL.bin
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- Append CSF signature to the end of SPL image:
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$ cat SPL csf_SPL.bin > SPL-signed
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- Create U-Boot proper CSF binary file:
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$ ./cst -i csf_uboot-ivt.txt -o csf_uboot-ivt.bin
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- Append CSF signature to the end of U-Boot proper image:
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$ cat u-boot-ivt.img csf_uboot-ivt.bin > u-boot-signed.img
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The bootloader is signed and can be flashed into the boot media.
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1.5 Closing the device
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-----------------------
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The procedure for closing the device is similar as in Non-SPL targets, for a
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complete procedure please refer to section "1.5 Programming SRK Hash" in
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mx6_mx7_secure_boot.txt document available under doc/imx/habv4/guides/
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directory.
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References:
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[1] AN4581: "Secure Boot on i.MX 50, i.MX 53, i.MX 6 and i.MX 7 Series using
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HABv4" - Rev 2.
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