Secure Boot

Secure Boot refers to hardware and software that does not allow an attacker to obtain sensitive data or boot altered firmware. This can be accomplished on modern embedded System on Chip devices by creating a Chain of Trust.

Chain of Trust

A Chain of Trust is established by validating each component of software to ensure that only trusted software can be used.

For a typical embedded Linux board the chain of trust may look like:

• Boot firmware (ie SPL) validated by embedded SoC BOOT ROM against signature hashes fused into one-time-programmable memory
• U-Boot validated by the Boot firmware (ie SPL)
• Kernel / FDT / initramfs images validated by U-Boot
• Filesystem encryption (if needed) unlocked by the kernel+initramfs

Secure the Boot firmware

Securing your product boot involves using SoC specific methods to verify the initial boot code that is fetched from the storage device and executed.

The initial boot code and how the various SoC manages secure boot varies per processor:

• Venice: IMX8M HABv4: initial boot code is U-Boot SPL
• Newport: CN803x Trusted Boot; initial boot code is Marvell BDK
• Ventana: IMX6 HABv4; initial boot code is U-Boot SPL

Secure U-Boot

For a secure U-Boot you want to disable the ability to stop autoboot and get to a U-Boot console. Additionally you do not want to use env variables that can be used by an attacker to affect the boot sequence.

To do this you need to understand where U-Boot env comes from:

• static default env embedded in uboot itself is from include/env_default.h default_environment and is built up depending on a lot of various configs. You can see this by doing a 'make u-boot-initial-env' which will produce a 'u-boot-initial-env' file
• programmatic env vars set at runtime such as eth*addr, fdtcontroladdr, stdin, stdout, stderr (and more depending on your config)
• FLASH storage backed env such as MMC

First and foremost you need to disable the ability to stop U-Boot's autoboot for security. This is done by setting CONFIG_BOOTDELAY=-2 which will disallow stopping from user input as well as disable any delay. Additionally your bootcmd should either be in an endless loop or end with a 'reset' so that any failure of the bootcmd does not leave you in an insecure U-Boot console environment. You probably want to do this step last however after debugging and development cycles as you may need to break into your bootloader during design and development.

Secondly you need to ensure there are no env vars that can be set outside of U-Boot that can alter the boot path. There are two common cases you may fall into:

1. Your Linux environment never needs to access or alter U-Boot env (ie via fw_setenv/fw_printenv). In this case you can:
   • define the following in your U-Boot config:

     CONFIG_ENV_IS_NOWHERE=y
     # CONFIG_ENV_IS_IN_EEPROM is not set
     # CONFIG_ENV_IS_IN_FAT is not set
     # CONFIG_ENV_IS_IN_EXT4 is not set
     # CONFIG_ENV_IS_IN_FLASH is not set
     # CONFIG_ENV_IS_IN_MMC is not set
     # CONFIG_ENV_IS_IN_NAND is not set
     # CONFIG_ENV_IS_IN_NVRAM is not set
     # CONFIG_ENV_IS_IN_ONENAND is not set
     # CONFIG_ENV_IS_IN_REMOTE is not set
     CONFIG_USE_BOOTCOMMAND=y
     CONFIG_BOOTCOMMAND="<your simple boot-command which uses 'root' variable>; reset"
     # CONFIG_LEGACY_IMAGE_FORMAT is not set
     CONFIG_FIT_SIGNATURE=y
     

     • Note the 'reset' at the end of the bootcmd. This is to ensure that a failure to boot does not drop you into an insecure U-boot console
2. Your Linux environment needs to alter a variable (ie SWUpdate may need to alter a variable indicating which partition your rootfs is on if you are using a A/B ping-pong update method). In this case to allow U-boot to import the variable 'root' treated as a decimal you can define:
   • define the following in your U-Boot config:

     CONFIG_ENV_IS_NOWHERE=y
     CONFIG_ENV_IS_MMC=y
     CONFIG_ENV_WRITEABLE_LIST=y
     CONFIG_USE_BOOTCOMMAND=y
     CONFIG_BOOTCOMMAND="<your simple boot-command which uses 'root' variable>; reset"
     # CONFIG_LEGACY_IMAGE_FORMAT is not set
     CONFIG_FIT_SIGNATURE=y
     

     • Note the 'reset' at the end of the bootcmd. This is to ensure that a failure to boot does not drop you into an insecure U-boot console
   • patch the appropriate board config file in include/configs (ie include/configs/imx8mm_venice.h for Venice, include/configs/octeontx_common.h for Newport, or include/configs/gw_ventana.h for Ventana) to set:

     CONFIG_ENV_FLAGS_LIST_DEFAULT="root:dw"
     CONFIG_ENV_FLAGS_LIST_STATIC="root:dw"
     

     • ENV_FLAGS_LIST_DEFAULT is the default environment .flags variable that sets up permissions and validations
     • ENV_FLAGS_LIST_STATIC is used when CONFIG_ENV_WRITEABLE_LIST=y and is an unchangeable list of vars with permissions and validations
     • make sure your writable variables do not appear in the static default env as that will always override what gets imported from a FLASH based env
   • use U-Boot 'mkenvimage' on your development host to create a binary FLASH env that sets necessary defaults for any writeable vars you declare

For additional details on securing U-Boot see the following excellent presentation by F-Secure:

• ​https://labs.f-secure.com/assets/BlogFiles/2020-05-u-booting-securely-wp-final.pdf

Securing the Kernel, FDT, ramdisk via FIT images

The simplest way to secure the Kernel, the FDT, and the (optional) ramdisk image used to boot a Linux based OS is to use a U-Boot FIT image to contain signed versions of these.

A FIT (Flattened Image Tree) file is a container designed to hold one or more binary objects. It also can contain meta-data about each object such as addresses, hash values, and signing key information which can be used to validate the binary objects.

For more info on FIT images see:

• ​http://www.denx.de/wiki/pub/U-Boot/Documentation/multi_image_booting_scenarios.pdf
• ​https://elixir.bootlin.com/u-boot/latest/source/doc/uImage.FIT

FIT images are created using the U-Boot mkimage application and use a template file (.its extension typically) which describes the image. The output is a binary image (.itb extension typically) which can be booted in U-Boot with the bootm command. The syntax of the its template is well documented in U-Boot source:

• ​its file format
• ​signed images
• ​verified boot

There are two parts to using signed FIT images:

• the FDT used by U-Boot itself needs to have a signature node that describes the public key details needed to verify the FIT image components
• the FIT image needs to contain signatures covering the images inside as well as the configuration

Both steps above are achieved by using the mkimage application from U-Boot which is built for your host OS in the tools directory. Note that it is important to use the mkimage app built for your U-Boot to ensure it has FIT signatures enabled and is compatible. In other words, do not use a mkimage binary that came from a package in your development host distro.

Procedure for using a FIT image:

1. Create a key. It is recommended to use a sha256,rsa2048 key:

   openssl genpkey -algorithm RSA -out fit.key -pkeyopt rsa_keygen_bits:2048 -pkeyopt rsa_keygen_pubexp:65537
   

   • see ​verified-boot.txt for more info on key options

2. add a signature node to an existing compiled FDT (myboard.dtb in this example) with a key named $KEY_NAME in the $KEY_DIR directory:

   cat << EOF > dummy.its
   /dts-v1/;
   
   / {
           description = "Dummy file for signing board .dtb files";
           #address-cells = <1>;
   
           /* dummy image just to keep mkimage tool happy */
           images {
                   kernel@1 {
                           description = "dummy kernel image";
                           data = /incbin/("dummy.bin");
                           type = "kernel";
                           arch = "arm64";
                           os = "linux";
                           compression = "none";
                           load = <0x0>;
                           entry = <0x0>;
                           hash@1 {
                                   algo = "sha256";
                           };
                   };
           };
           configurations {
                   default = "config@1";
                   config@1 {
                           description = "Linux configuration";
                           kernel = "kernel@1";
                           signature@1 {
                                   algo = "sha256,rsa2048";
                                   key-name-hint = "dummy";
                           };
                   };
           };
   };
   EOF
   touch dummy.bin
   # replace dummy placeholders for key-name
   sed -i "s;key-name-hint = \"dummy\";key-name-hint = \"$KEY_NAME\";g" dummy.its
   # create cert for key
   openssl req -batch -new -x509 -key $KEY_DIR/$KEY_NAME.key -out $KEY_DIR/$KEY_NAME.crt
   # add signature node to myboard.dtb
   mkimage -r -k $KEY_DIR -K myboard.dtb -f dummy.its dummy.itb
   

   • Note that the kernel image defined above and the created dummy.itb is just a dummy config to keep mkimage happy - the important thing above is the signature node in the configurations
   • Note that if you are using U-Boot SPL with a FIT image (such as Venice) you will need to perform a 'make flash.bin' again following the update of any dtb's to re-create the FIT image.

3. create a signed FIT image containing $KERNEL, $FDT, $INITRAMFS with a key named $KEY_NAME in the $KEY_DIR directory:

   cat << EOF > fit.its
   /dts-v1/;
   
   / {
           description = "Image for Linux Kernel";
           #address-cells = <1>;
   
           images {
                   kernel {
                           description = "Linux Kernel";
                           data = /incbin/("kernel");
                           type = "kernel";
                           arch = "arm64";
                           os = "linux";
                           compression = "gzip";
                           load = <0x20080000>;
                           entry = <0x20080000>;
                           hash@1 {
                                   algo = "sha256";
                           };
                   };
                   ramdisk {
                           description = "ramdisk";
                           data = /incbin/("ramdisk");
                           type = "ramdisk";
                           arch = "arm64";
                           os = "linux";
                           compression = "none";
                           hash@1 {
                                   algo = "sha256";
                           };
                   };
                   fdt {
                           description = "fdt";
                           data = /incbin/("fdt");
                           type = "flat_dt";
                           arch = "arm64";
                           compression = "none";
                           hash@1 {
                                   algo = "sha256";
                           };
                   };
           };
           configurations {
                   default = "config@1";
                   config@1 {
                           description = "Linux configuration";
                           kernel = "kernel";
                           ramdisk = "ramdisk";
                           fdt = "fdt";
                           signature@1 {
                                   algo = "sha256,rsa2048";
                                   key-name-hint = "key";
                           };
                   };
           };
   };
   EOF
   # replace dummy placeholders for kernel, ramdisk, fdt binaries
   sed -i "s;data = /incbin/(\"kernel\");data = /incbin/(\"$KERNEL\");g" fit.its
   sed -i "s;data = /incbin/(\"ramdisk\");data = /incbin/(\"$RAMDISK\");g" fit.its
   sed -i "s;data = /incbin/(\"fdt\");data = /incbin/(\"$FDT\");g" fit.its
   # replace dummy placeholders for key-name
   sed -i "s;key-name-hint = \"key\";key-name-hint = \"$KEY_NAME\";g" fit.its
   mkimage -k $KEY_DIR -f fit.its fit.itb
   

   • If not using a ramdisk or fdt you can remove those sections above including the references in config and sign-images
   • Note the kernel compression is set to gzip so you will want to make sure your $KERNEL points to a compressed kernel image (Image.gz)
   • Note there is no compression type for a ramdisk as the kernel supports self-decompression of ramdisks. Use whatever compression type you want as long as you have enabled that compression type in your kernel.
   • Note that you need a valid load/entry address above which varies depending on your processor (0x40080000 for Venice, 0x20080000 for Newport, 0x10008000 for Ventana)

4. boot a FIT image using the U-boot bootm command:
   • from network

     tftp $loadaddr fit.itb && bootm $loadaddr
     

   • from mmc device 0, partition 1, /boot/fit.itb:

     load mmc 0:1 $loadaddr /boot/fit.itb && bootm $loadaddr
     

   • Make sure the address you are loading the FIT image to does not cause an overlap in memory with where the kernel load/entry point is. The bootm command will copy or uncompress the kernel to the load/entry point and copy the initramfs and the fdt to a location following that. You may need to alter loadaddr to somewhere in memory other than the default.