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Secure Boot Support in L4T R35.2.1 and later
Jetson secure boot support in L4T R35.2.1 implements a different chain of trust from what was present in the L4T R32 releases:
- The Trusty secure OS has been replaced by OP-TEE, which allows for dynamic loading of trusted applications (TAs) from the non-secure world. TAs must be signed, and the public key used for checking the signature is compiled into the OP-TEE OS.
- The cboot bootloader has been replaced by UEFI, which uses its own set of keys for validating signatures on binaries that it loads (Linux kernel, EFI applications, and EFI capsules).
NOTE NVIDIA made some changes to the UEFI bootloader in L4T R35.5.0 that require that an "authentication key" be programmed into the Encrypted Key Block on secured devices. If you are updating your secured device from an earlier R35.x release to R35.5.0, you must update the EKB on the device with the added key. See this developer forum thread for more information.
Start by reading the Secure Boot section of the Jetson Linux Developer's Guide.
The sections below cover specifics of how secure boot and signing are implemented for OE/Yocto builds with meta-tegra.
Follow the instructions in the NVIDIA documentation for generating keys and burning secure boot fuses for your Jetson device. Be warned that burning the fuses is a one-time operation, so be extremely careful. You could render your Jetson permanently unbootable if something goes wrong during the fuse burning process.
If you have the bootloader signing and encryption key files available, you can add the following setting to your local.conf to create signed boot images and BUP packages:
TEGRA_SIGNING_ARGS = "-u /path/to/pkc-signing-key.pem -v /path/to/sbk.key --user_key /path/to/user.key"
These arguments parallel the ones used with the L4T flash.sh script for signing:
- The
-uoption takes the path name of the RSA private key for PKC signing. - The
-voption takes the path name of the SBK key used for encrypting the binaries loaded at boot time. - The
--user_keyoption takes the path name of the encryption key you create for use with the NVIDIA sample OP-TEE TAs.
Note that with R35.2.1, the --user_key encryption key is used only for the XUSB firmware. Starting with R35.3.1, the user encryption key is not used for any of the boot firmware.
Build-time bootloader signing will be performed on the boot-related files in the tegraflash package for flashing, as well as the entries in any bootloader update payloads (BUPs).
You can elect to perform bootloader signing outside of the build process by adding the -u, -v, and --user_key options when running the doflash.sh or initrd-flash script during flashing of your tegraflash package. For BUP generation, add those options when running the generate_bup_payload.sh script to have the bootloader components signed.
To enable UEFI secure boot support, start by generating the PK, KEK, and DB keys and related configuration files, as described in the UEFI Secure Boot section of the Jetson Linux documentation.
It should be noted that UEFI boot is not compatible with the legacy secure boot supported on Tegra devices.
During the build, signing of the EFI launcher app, the kernel, and device tree files is performed automatically when the following settings are present in your build configuration:
TEGRA_UEFI_DB_KEY = "/path/to/db.key"
TEGRA_UEFI_DB_CERT = "/path/to/db.crt"
Both settings must be present, and must point to one of the DB keys you generated (you do not need the PK or KEK keys).
Post-build UEFI signing is not currently supported.
To enable UEFI secure boot, the PK, KEK, and DB keys you generated must be "enrolled" at boot time. On Jetson platforms, this done by adding the needed key enrollment variable settings to the bootloader's device tree via the UefiDefaultSecurityKeys.dts file you generated when creating the keys and configuration files. For meta-tegra builds, you can supply this file by adding a bbappend for the tegra-uefi-keys-dtb.bb recipe in one of your own metadata layers:
$ mkdir -p meta-my-layer/recipes-bsp/uefi
$ cat > meta-my-layer/recipes-bsp/uefi/tegra-uefi-keys-dtb.bbappend <<EOF
FILESEXTRAPATHS:prepend := "${THISDIR}/files:"
EOF
$ mkdir meta-my-layer/recipes-bsp/uefi/files
$ cp /path/to/generated/UefiDefaultSecurityKeys.dts meta-my-layer/recipes-bsp/uefi/files/
$ cp /path/to/generated/UefiUpdateSecurityKeys.dts meta-my-layer/recipes-bsp/uefi/files/
The Jetson Linux documentation describes the process for enrolling UEFI keys and enabling UEFI secure boot at runtime. You will need to add some packages to your image build to make the necessary commands available. As of this writing, runtime enrollment has not been tested.
OP-TEE provides a mechanism for loading TAs from the "Rich Execution Environment" (REE, another term for the normal, non-secure OS), which must be signed with a key that is known the OP-TEE OS. Read the OP-TEE documentation on TAs for more information.
By default, a development/test key from the upstream OP-TEE source is compiled in; this configuration should not be used in any production device, since the key is publicly available. You should generate a suitable RSA keypair as described in the OP-TEE documentation. For build-time signing, add a bbappend for the optee-os recipe in one of your layers. For build-time signing, your bbappend should resemble the following:
FILESEXTRAPATHS:prepend := "${THISDIR}/files:"
SRC_URI += "file://optee-signing-key.pem"
EXTRA_OEMAKE += "TA_SIGN_KEY=${WORKDIR}/optee-signing-key.pem"
Post-build signing of TAs is more difficult, since external TAs are generally packaged and installed into the root filesystem as part of the build. For that approach, though, you would include the public key file in the optee-os bbappend, and set TA_PUBLIC_KEY instead of TA_SIGN_KEY. The OP-TEE makefiles will sign TAs with the a dummy private key, but the public key you specify will be compiled into the secure OS. You will have to figure out how to re-sign the TAs with your actual private key before they get used.
To make use of the encryption/decryption functions NVIDIA provides by default with their OP-TEE implementation, you will need to supply an "Encrypted Keyblob" (EKB) that corresponds to the KEK/K2 fuses you have burned on your Jetson device. Instructions for generating an EKB are in this section of the Jetson Linux documentation. See the note at the top of this page for information about changes in L4T R35.5.0 that require the re-generation of the EKB.
The tegra-bootfiles recipe installs the default EKB from the L4T kit. Add a bbappend for that recipe to replace the default with the custom EKB for your device.