doc/uImage.FIT/beaglebone_vboot.txt - platform/external/u-boot - Gitiles

 Verified Boot on the Beaglebone Black
 =====================================

 Introduction
 ------------

 Before reading this, please read verified-boot.txt and signature.txt. These
 instructions are for mainline U-Boot from v2014.07 onwards.

 There is quite a bit of documentation in this directory describing how
 verified boot works in U-Boot. There is also a test which runs through the
 entire process of signing an image and running U-Boot (sandbox) to check it.
 However, it might be useful to also have an example on a real board.

 Beaglebone Black is a fairly common board so seems to be a reasonable choice
 for an example of how to enable verified boot using U-Boot.

 First a note that may to help avoid confusion. U-Boot and Linux both use
 device tree. They may use the same device tree source, but it is seldom useful
 for them to use the exact same binary from the same place. More typically,
 U-Boot has its device tree packaged wtih it, and the kernel's device tree is
 packaged with the kernel. In particular this is important with verified boot,
 since U-Boot's device tree must be immutable. If it can be changed then the
 public keys can be changed and verified boot is useless. An attacker can
 simply generate a new key and put his public key into U-Boot so that
 everything verifies. On the other hand the kernel's device tree typically
 changes when the kernel changes, so it is useful to package an updated device
 tree with the kernel binary. U-Boot supports the latter with its flexible FIT
 format (Flat Image Tree).


 Overview
 --------

 The steps are roughly as follows:

 1. Build U-Boot for the board, with the verified boot options enabled.

 2. Obtain a suitable Linux kernel

 3. Create a Image Tree Source file (ITS) file describing how you want the
 kernel to be packaged, compressed and signed.

 4. Create a key pair

 5. Sign the kernel

 6. Put the public key into U-Boot's image

 7. Put U-Boot and the kernel onto the board

 8. Try it


 Step 1: Build U-Boot
 --------------------

 a. Set up the environment variable to point to your toolchain. You will need
 this for U-Boot and also for the kernel if you build it. For example if you
 installed a Linaro version manually it might be something like:

    export CROSS_COMPILE=/opt/linaro/gcc-linaro-arm-linux-gnueabihf-4.8-2013.08_linux/bin/arm-linux-gnueabihf-

 or if you just installed gcc-arm-linux-gnueabi then it might be

    export CROSS_COMPILE=arm-linux-gnueabi-

 b. Configure and build U-Boot with verified boot enabled:

    export ARCH=arm
    export UBOOT=/path/to/u-boot
    cd $UBOOT
    # You can add -j10 if you have 10 CPUs to make it faster
    make O=b/am335x_boneblack_vboot am335x_boneblack_vboot_config all
    export UOUT=$UBOOT/b/am335x_boneblack_vboot

 c. You will now have a U-Boot image:

    file b/am335x_boneblack_vboot/u-boot-dtb.img
 b/am335x_boneblack_vboot/u-boot-dtb.img: u-boot legacy uImage, U-Boot 2014.07-rc2-00065-g2f69f8, Firmware/ARM, Firmware Image (Not compressed), 395375 bytes, Sat May 31 16:19:04 2014, Load Address: 0x80800000, Entry Point: 0x00000000, Header CRC: 0x0ABD6ACA, Data CRC: 0x36DEF7E4


 Step 2: Build Linux
 --------------------

 a. Find the kernel image ('Image') and device tree (.dtb) file you plan to
 use. In our case it is am335x-boneblack.dtb and it is built with the kernel.
 At the time of writing an SD Boot image can be obtained from here:

    http://www.elinux.org/Beagleboard:Updating_The_Software#Image_For_Booting_From_microSD

 You can write this to an SD card and then mount it to extract the kernel and
 device tree files.

 You can also build a kernel. Instructions for this are are here:

    http://elinux.org/Building_BBB_Kernel

 or you can use your favourite search engine. Following these instructions
 produces a kernel Image and device tree files. For the record the steps were:

    export KERNEL=/path/to/kernel
    cd $KERNEL
    git clone git://github.com/beagleboard/kernel.git .
    git checkout v3.14
    ./patch.sh
    cp configs/beaglebone kernel/arch/arm/configs/beaglebone_defconfig
    cd kernel
    make beaglebone_defconfig
    make uImage dtbs   # -j10 if you have 10 CPUs
    export OKERNEL=$KERNEL/kernel/arch/arm/boot

 c. You now have the 'Image' and 'am335x-boneblack.dtb' files needed to boot.


 Step 3: Create the ITS
 ----------------------

 Set up a directory for your work.

    export WORK=/path/to/dir
    cd $WORK

 Put this into a file in that directory called sign.its:

 /dts-v1/;

 / {
 	description = "Beaglebone black";
 	#address-cells = <1>;

 	images {
 		kernel {
 			data = /incbin/("Image.lzo");
 			type = "kernel";
 			arch = "arm";
 			os = "linux";
 			compression = "lzo";
 			load = <0x80008000>;
 			entry = <0x80008000>;
 			hash-1 {
 				algo = "sha1";
 			};
 		};
 		fdt-1 {
 			description = "beaglebone-black";
 			data = /incbin/("am335x-boneblack.dtb");
 			type = "flat_dt";
 			arch = "arm";
 			compression = "none";
 			hash-1 {
 				algo = "sha1";
 			};
 		};
 	};
 	configurations {
 		default = "conf-1";
 		conf-1 {
 			kernel = "kernel";
 			fdt = "fdt-1";
 			signature-1 {
 				algo = "sha1,rsa2048";
 				key-name-hint = "dev";
 				sign-images = "fdt", "kernel";
 			};
 		};
 	};
 };


 The explanation for this is all in the documentation you have already read.
 But briefly it packages a kernel and device tree, and provides a single
 configuration to be signed with a key named 'dev'. The kernel is compressed
 with LZO to make it smaller.


 Step 4: Create a key pair
 -------------------------

 See signature.txt for details on this step.

    cd $WORK
    mkdir keys
    openssl genrsa -F4 -out keys/dev.key 2048
    openssl req -batch -new -x509 -key keys/dev.key -out keys/dev.crt

 Note: keys/dev.key contains your private key and is very secret. If anyone
 gets access to that file they can sign kernels with it. Keep it secure.


 Step 5: Sign the kernel
 -----------------------

 We need to use mkimage (which was built when you built U-Boot) to package the
 Linux kernel into a FIT (Flat Image Tree, a flexible file format that U-Boot
 can load) using the ITS file you just created.

 At the same time we must put the public key into U-Boot device tree, with the
 'required' property, which tells U-Boot that this key must be verified for the
 image to be valid. You will make this key available to U-Boot for booting in
 step 6.

    ln -s $OKERNEL/dts/am335x-boneblack.dtb
    ln -s $OKERNEL/Image
    ln -s $UOUT/u-boot-dtb.img
    cp $UOUT/arch/arm/dts/am335x-boneblack.dtb am335x-boneblack-pubkey.dtb
    lzop Image
    $UOUT/tools/mkimage -f sign.its -K am335x-boneblack-pubkey.dtb -k keys -r image.fit

 You should see something like this:

 FIT description: Beaglebone black
 Created:         Sun Jun  1 12:50:30 2014
  Image 0 (kernel)
   Description:  unavailable
   Created:      Sun Jun  1 12:50:30 2014
   Type:         Kernel Image
   Compression:  lzo compressed
   Data Size:    7790938 Bytes = 7608.34 kB = 7.43 MB
   Architecture: ARM
   OS:           Linux
   Load Address: 0x80008000
   Entry Point:  0x80008000
   Hash algo:    sha1
   Hash value:   c94364646427e10f423837e559898ef02c97b988
  Image 1 (fdt-1)
   Description:  beaglebone-black
   Created:      Sun Jun  1 12:50:30 2014
   Type:         Flat Device Tree
   Compression:  uncompressed
   Data Size:    31547 Bytes = 30.81 kB = 0.03 MB
   Architecture: ARM
   Hash algo:    sha1
   Hash value:   cb09202f889d824f23b8e4404b781be5ad38a68d
  Default Configuration: 'conf-1'
  Configuration 0 (conf-1)
   Description:  unavailable
   Kernel:       kernel
   FDT:          fdt-1


 Now am335x-boneblack-pubkey.dtb contains the public key and image.fit contains
 the signed kernel. Jump to step 6 if you like, or continue reading to increase
 your understanding.

 You can also run fit_check_sign to check it:

    $UOUT/tools/fit_check_sign -f image.fit -k am335x-boneblack-pubkey.dtb

 which results in:

 Verifying Hash Integrity ... sha1,rsa2048:dev+
 ## Loading kernel from FIT Image at 7fc6ee469000 ...
    Using 'conf-1' configuration
    Verifying Hash Integrity ...
 sha1,rsa2048:dev+
 OK

    Trying 'kernel' kernel subimage
      Description:  unavailable
      Created:      Sun Jun  1 12:50:30 2014
      Type:         Kernel Image
      Compression:  lzo compressed
      Data Size:    7790938 Bytes = 7608.34 kB = 7.43 MB
      Architecture: ARM
      OS:           Linux
      Load Address: 0x80008000
      Entry Point:  0x80008000
      Hash algo:    sha1
      Hash value:   c94364646427e10f423837e559898ef02c97b988
    Verifying Hash Integrity ...
 sha1+
 OK

 Unimplemented compression type 4
 ## Loading fdt from FIT Image at 7fc6ee469000 ...
    Using 'conf-1' configuration
    Trying 'fdt-1' fdt subimage
      Description:  beaglebone-black
      Created:      Sun Jun  1 12:50:30 2014
      Type:         Flat Device Tree
      Compression:  uncompressed
      Data Size:    31547 Bytes = 30.81 kB = 0.03 MB
      Architecture: ARM
      Hash algo:    sha1
      Hash value:   cb09202f889d824f23b8e4404b781be5ad38a68d
    Verifying Hash Integrity ...
 sha1+
 OK

    Loading Flat Device Tree ... OK

 ## Loading ramdisk from FIT Image at 7fc6ee469000 ...
    Using 'conf-1' configuration
 Could not find subimage node

 Signature check OK


 At the top, you see "sha1,rsa2048:dev+". This means that it checked an RSA key
 of size 2048 bits using SHA1 as the hash algorithm. The key name checked was
 'dev' and the '+' means that it verified. If it showed '-' that would be bad.

 Once the configuration is verified it is then possible to rely on the hashes
 in each image referenced by that configuration. So fit_check_sign goes on to
 load each of the images. We have a kernel and an FDT but no ramkdisk. In each
 case fit_check_sign checks the hash and prints sha1+ meaning that the SHA1
 hash verified. This means that none of the images has been tampered with.

 There is a test in test/vboot which uses U-Boot's sandbox build to verify that
 the above flow works.

 But it is fun to do this by hand, so you can load image.fit into a hex editor
 like ghex, and change a byte in the kernel:

    $UOUT/tools/fit_info -f image.fit -n /images/kernel -p data
 NAME: kernel
 LEN: 7790938
 OFF: 168

 This tells us that the kernel starts at byte offset 168 (decimal) in image.fit
 and extends for about 7MB. Try changing a byte at 0x2000 (say) and run
 fit_check_sign again. You should see something like:

 Verifying Hash Integrity ... sha1,rsa2048:dev+
 ## Loading kernel from FIT Image at 7f5a39571000 ...
    Using 'conf-1' configuration
    Verifying Hash Integrity ...
 sha1,rsa2048:dev+
 OK

    Trying 'kernel' kernel subimage
      Description:  unavailable
      Created:      Sun Jun  1 13:09:21 2014
      Type:         Kernel Image
      Compression:  lzo compressed
      Data Size:    7790938 Bytes = 7608.34 kB = 7.43 MB
      Architecture: ARM
      OS:           Linux
      Load Address: 0x80008000
      Entry Point:  0x80008000
      Hash algo:    sha1
      Hash value:   c94364646427e10f423837e559898ef02c97b988
    Verifying Hash Integrity ...
 sha1 error
 Bad hash value for 'hash-1' hash node in 'kernel' image node
 Bad Data Hash

 ## Loading fdt from FIT Image at 7f5a39571000 ...
    Using 'conf-1' configuration
    Trying 'fdt-1' fdt subimage
      Description:  beaglebone-black
      Created:      Sun Jun  1 13:09:21 2014
      Type:         Flat Device Tree
      Compression:  uncompressed
      Data Size:    31547 Bytes = 30.81 kB = 0.03 MB
      Architecture: ARM
      Hash algo:    sha1
      Hash value:   cb09202f889d824f23b8e4404b781be5ad38a68d
    Verifying Hash Integrity ...
 sha1+
 OK

    Loading Flat Device Tree ... OK

 ## Loading ramdisk from FIT Image at 7f5a39571000 ...
    Using 'conf-1' configuration
 Could not find subimage node

 Signature check Bad (error 1)


 It has detected the change in the kernel.

 You can also be sneaky and try to switch images, using the libfdt utilities
 that come with dtc (package name is device-tree-compiler but you will need a
 recent version like 1.4:

    dtc -v
 Version: DTC 1.4.0

 First we can check which nodes are actually hashed by the configuration:

    fdtget -l image.fit /
 images
 configurations

    fdtget -l image.fit /configurations
 conf-1
 fdtget -l image.fit /configurations/conf-1
 signature-1

    fdtget -p image.fit /configurations/conf-1/signature-1
 hashed-strings
 hashed-nodes
 timestamp
 signer-version
 signer-name
 value
 algo
 key-name-hint
 sign-images

    fdtget image.fit /configurations/conf-1/signature-1 hashed-nodes
 / /configurations/conf-1 /images/fdt-1 /images/fdt-1/hash /images/kernel /images/kernel/hash-1

 This gives us a bit of a look into the signature that mkimage added. Note you
 can also use fdtdump to list the entire device tree.

 Say we want to change the kernel that this configuration uses
 (/images/kernel). We could just put a new kernel in the image, but we will
 need to change the hash to match. Let's simulate that by changing a byte of
 the hash:

     fdtget -tx image.fit /images/kernel/hash-1 value
 c9436464 6427e10f 423837e5 59898ef0 2c97b988
     fdtput -tx image.fit /images/kernel/hash-1 value c9436464 6427e10f 423837e5 59898ef0 2c97b981

 Now check it again:

    $UOUT/tools/fit_check_sign -f image.fit -k am335x-boneblack-pubkey.dtb
 Verifying Hash Integrity ... sha1,rsa2048:devrsa_verify_with_keynode: RSA failed to verify: -13
 rsa_verify_with_keynode: RSA failed to verify: -13
 -
 Failed to verify required signature 'key-dev'
 Signature check Bad (error 1)

 This time we don't even get as far as checking the images, since the
 configuration signature doesn't match. We can't change any hashes without the
 signature check noticing. The configuration is essentially locked. U-Boot has
 a public key for which it requires a match, and will not permit the use of any
 configuration that does not match that public key. The only way the
 configuration will match is if it was signed by the matching private key.

 It would also be possible to add a new signature node that does match your new
 configuration. But that won't work since you are not allowed to change the
 configuration in any way. Try it with a fresh (valid) image if you like by
 running the mkimage link again. Then:

    fdtput -p image.fit /configurations/conf-1/signature-1 value fred
    $UOUT/tools/fit_check_sign -f image.fit -k am335x-boneblack-pubkey.dtb
 Verifying Hash Integrity ... -
 sha1,rsa2048:devrsa_verify_with_keynode: RSA failed to verify: -13
 rsa_verify_with_keynode: RSA failed to verify: -13
 -
 Failed to verify required signature 'key-dev'
 Signature check Bad (error 1)


 Of course it would be possible to add an entirely new configuration and boot
 with that, but it still needs to be signed, so it won't help.


 6. Put the public key into U-Boot's image
 -----------------------------------------

 Having confirmed that the signature is doing its job, let's try it out in
 U-Boot on the board. U-Boot needs access to the public key corresponding to
 the private key that you signed with so that it can verify any kernels that
 you sign.

    cd $UBOOT
    make O=b/am335x_boneblack_vboot EXT_DTB=${WORK}/am335x-boneblack-pubkey.dtb

 Here we are overrriding the normal device tree file with our one, which
 contains the public key.

 Now you have a special U-Boot image with the public key. It can verify can
 kernel that you sign with the private key as in step 5.

 If you like you can take a look at the public key information that mkimage
 added to U-Boot's device tree:

    fdtget -p am335x-boneblack-pubkey.dtb /signature/key-dev
 required
 algo
 rsa,r-squared
 rsa,modulus
 rsa,n0-inverse
 rsa,num-bits
 key-name-hint

 This has information about the key and some pre-processed values which U-Boot
 can use to verify against it. These values are obtained from the public key
 certificate by mkimage, but require quite a bit of code to generate. To save
 code space in U-Boot, the information is extracted and written in raw form for
 U-Boot to easily use. The same mechanism is used in Google's Chrome OS.

 Notice the 'required' property. This marks the key as required - U-Boot will
 not boot any image that does not verify against this key.


 7. Put U-Boot and the kernel onto the board
 -------------------------------------------

 The method here varies depending on how you are booting. For this example we
 are booting from an micro-SD card with two partitions, one for U-Boot and one
 for Linux. Put it into your machine and write U-Boot and the kernel to it.
 Here the card is /dev/sde:

    cd $WORK
    export UDEV=/dev/sde1   # Change thes two lines to the correct device
    export KDEV=/dev/sde2
    sudo mount $UDEV /mnt/tmp && sudo cp $UOUT/u-boot-dtb.img /mnt/tmp/u-boot.img  && sleep 1 && sudo umount $UDEV
    sudo mount $KDEV /mnt/tmp && sudo cp $WORK/image.fit /mnt/tmp/boot/image.fit && sleep 1 && sudo umount $KDEV


 8. Try it
 ---------

 Boot the board using the commands below:

    setenv bootargs console=ttyO0,115200n8 quiet root=/dev/mmcblk0p2 ro rootfstype=ext4 rootwait
    ext2load mmc 0:2 82000000 /boot/image.fit
    bootm 82000000

 You should then see something like this:

 U-Boot# setenv bootargs console=ttyO0,115200n8 quiet root=/dev/mmcblk0p2 ro rootfstype=ext4 rootwait
 U-Boot# ext2load mmc 0:2 82000000 /boot/image.fit
 7824930 bytes read in 589 ms (12.7 MiB/s)
 U-Boot# bootm 82000000
 ## Loading kernel from FIT Image at 82000000 ...
    Using 'conf-1' configuration
    Verifying Hash Integrity ... sha1,rsa2048:dev+ OK
    Trying 'kernel' kernel subimage
      Description:  unavailable
      Created:      2014-06-01  19:32:54 UTC
      Type:         Kernel Image
      Compression:  lzo compressed
      Data Start:   0x820000a8
      Data Size:    7790938 Bytes = 7.4 MiB
      Architecture: ARM
      OS:           Linux
      Load Address: 0x80008000
      Entry Point:  0x80008000
      Hash algo:    sha1
      Hash value:   c94364646427e10f423837e559898ef02c97b988
    Verifying Hash Integrity ... sha1+ OK
 ## Loading fdt from FIT Image at 82000000 ...
    Using 'conf-1' configuration
    Trying 'fdt-1' fdt subimage
      Description:  beaglebone-black
      Created:      2014-06-01  19:32:54 UTC
      Type:         Flat Device Tree
      Compression:  uncompressed
      Data Start:   0x8276e2ec
      Data Size:    31547 Bytes = 30.8 KiB
      Architecture: ARM
      Hash algo:    sha1
      Hash value:   cb09202f889d824f23b8e4404b781be5ad38a68d
    Verifying Hash Integrity ... sha1+ OK
    Booting using the fdt blob at 0x8276e2ec
    Uncompressing Kernel Image ... OK
    Loading Device Tree to 8fff5000, end 8ffffb3a ... OK

 Starting kernel ...

 [    0.582377] omap_init_mbox: hwmod doesn't have valid attrs
 [    2.589651] musb-hdrc musb-hdrc.0.auto: Failed to request rx1.
 [    2.595830] musb-hdrc musb-hdrc.0.auto: musb_init_controller failed with status -517
 [    2.606470] musb-hdrc musb-hdrc.1.auto: Failed to request rx1.
 [    2.612723] musb-hdrc musb-hdrc.1.auto: musb_init_controller failed with status -517
 [    2.940808] drivers/rtc/hctosys.c: unable to open rtc device (rtc0)
 [    7.248889] libphy: PHY 4a101000.mdio:01 not found
 [    7.253995] net eth0: phy 4a101000.mdio:01 not found on slave 1
 systemd-fsck[83]: Angstrom: clean, 50607/218160 files, 306348/872448 blocks

 .---O---.
 |       |                  .-.           o o
 |   |   |-----.-----.-----.| |   .----..-----.-----.
 |       |     | __  |  ---'| '--.|  .-'|     |     |
 |   |   |  |  |     |---  ||  --'|  |  |  '  | | | |
 '---'---'--'--'--.  |-----''----''--'  '-----'-'-'-'
                 -'  |
                 '---'

 The Angstrom Distribution beaglebone ttyO0

 Angstrom v2012.12 - Kernel 3.14.1+

 beaglebone login:

 At this point your kernel has been verified and you can be sure that it is one
 that you signed. As an exercise, try changing image.fit as in step 5 and see
 what happens.


 Further Improvements
 --------------------

 Several of the steps here can be easily automated. In particular it would be
 capital if signing and packaging a kernel were easy, perhaps a simple make
 target in the kernel.

 Some mention of how to use multiple .dtb files in a FIT might be useful.

 U-Boot's verified boot mechanism has not had a robust and independent security
 review. Such a review should look at the implementation and its resistance to
 attacks.

 Perhaps the verified boot feature could could be integrated into the Amstrom
 distribution.


 Simon Glass
 sjg@chromium.org
 2-June-14
	Verified Boot on the Beaglebone Black
	=====================================

	Introduction
	------------

	Before reading this, please read verified-boot.txt and signature.txt. These
	instructions are for mainline U-Boot from v2014.07 onwards.

	There is quite a bit of documentation in this directory describing how
	verified boot works in U-Boot. There is also a test which runs through the
	entire process of signing an image and running U-Boot (sandbox) to check it.
	However, it might be useful to also have an example on a real board.

	Beaglebone Black is a fairly common board so seems to be a reasonable choice
	for an example of how to enable verified boot using U-Boot.

	First a note that may to help avoid confusion. U-Boot and Linux both use
	device tree. They may use the same device tree source, but it is seldom useful
	for them to use the exact same binary from the same place. More typically,
	U-Boot has its device tree packaged wtih it, and the kernel's device tree is
	packaged with the kernel. In particular this is important with verified boot,
	since U-Boot's device tree must be immutable. If it can be changed then the
	public keys can be changed and verified boot is useless. An attacker can
	simply generate a new key and put his public key into U-Boot so that
	everything verifies. On the other hand the kernel's device tree typically
	changes when the kernel changes, so it is useful to package an updated device
	tree with the kernel binary. U-Boot supports the latter with its flexible FIT
	format (Flat Image Tree).


	Overview
	--------

	The steps are roughly as follows:

	1. Build U-Boot for the board, with the verified boot options enabled.

	2. Obtain a suitable Linux kernel

	3. Create a Image Tree Source file (ITS) file describing how you want the
	kernel to be packaged, compressed and signed.

	4. Create a key pair

	5. Sign the kernel

	6. Put the public key into U-Boot's image

	7. Put U-Boot and the kernel onto the board

	8. Try it


	Step 1: Build U-Boot
	--------------------

	a. Set up the environment variable to point to your toolchain. You will need
	this for U-Boot and also for the kernel if you build it. For example if you
	installed a Linaro version manually it might be something like:

	export CROSS_COMPILE=/opt/linaro/gcc-linaro-arm-linux-gnueabihf-4.8-2013.08_linux/bin/arm-linux-gnueabihf-

	or if you just installed gcc-arm-linux-gnueabi then it might be

	export CROSS_COMPILE=arm-linux-gnueabi-

	b. Configure and build U-Boot with verified boot enabled:

	export ARCH=arm
	export UBOOT=/path/to/u-boot
	cd $UBOOT
	# You can add -j10 if you have 10 CPUs to make it faster
	make O=b/am335x_boneblack_vboot am335x_boneblack_vboot_config all
	export UOUT=$UBOOT/b/am335x_boneblack_vboot

	c. You will now have a U-Boot image:

	file b/am335x_boneblack_vboot/u-boot-dtb.img
	b/am335x_boneblack_vboot/u-boot-dtb.img: u-boot legacy uImage, U-Boot 2014.07-rc2-00065-g2f69f8, Firmware/ARM, Firmware Image (Not compressed), 395375 bytes, Sat May 31 16:19:04 2014, Load Address: 0x80800000, Entry Point: 0x00000000, Header CRC: 0x0ABD6ACA, Data CRC: 0x36DEF7E4


	Step 2: Build Linux
	--------------------

	a. Find the kernel image ('Image') and device tree (.dtb) file you plan to
	use. In our case it is am335x-boneblack.dtb and it is built with the kernel.
	At the time of writing an SD Boot image can be obtained from here:

	http://www.elinux.org/Beagleboard:Updating_The_Software#Image_For_Booting_From_microSD

	You can write this to an SD card and then mount it to extract the kernel and
	device tree files.

	You can also build a kernel. Instructions for this are are here:

	http://elinux.org/Building_BBB_Kernel

	or you can use your favourite search engine. Following these instructions
	produces a kernel Image and device tree files. For the record the steps were:

	export KERNEL=/path/to/kernel
	cd $KERNEL
	git clone git://github.com/beagleboard/kernel.git .
	git checkout v3.14
	./patch.sh
	cp configs/beaglebone kernel/arch/arm/configs/beaglebone_defconfig
	cd kernel
	make beaglebone_defconfig
	make uImage dtbs # -j10 if you have 10 CPUs
	export OKERNEL=$KERNEL/kernel/arch/arm/boot

	c. You now have the 'Image' and 'am335x-boneblack.dtb' files needed to boot.


	Step 3: Create the ITS
	----------------------

	Set up a directory for your work.

	export WORK=/path/to/dir
	cd $WORK

	Put this into a file in that directory called sign.its:

	/dts-v1/;

	/ {
	description = "Beaglebone black";
	#address-cells = <1>;

	images {
	kernel {
	data = /incbin/("Image.lzo");
	type = "kernel";
	arch = "arm";
	os = "linux";
	compression = "lzo";
	load = <0x80008000>;
	entry = <0x80008000>;
	hash-1 {
	algo = "sha1";
	};
	};
	fdt-1 {
	description = "beaglebone-black";
	data = /incbin/("am335x-boneblack.dtb");
	type = "flat_dt";
	arch = "arm";
	compression = "none";
	hash-1 {
	algo = "sha1";
	};
	};
	};
	configurations {
	default = "conf-1";
	conf-1 {
	kernel = "kernel";
	fdt = "fdt-1";
	signature-1 {
	algo = "sha1,rsa2048";
	key-name-hint = "dev";
	sign-images = "fdt", "kernel";
	};
	};
	};
	};


	The explanation for this is all in the documentation you have already read.
	But briefly it packages a kernel and device tree, and provides a single
	configuration to be signed with a key named 'dev'. The kernel is compressed
	with LZO to make it smaller.


	Step 4: Create a key pair
	-------------------------

	See signature.txt for details on this step.

	cd $WORK
	mkdir keys
	openssl genrsa -F4 -out keys/dev.key 2048
	openssl req -batch -new -x509 -key keys/dev.key -out keys/dev.crt

	Note: keys/dev.key contains your private key and is very secret. If anyone
	gets access to that file they can sign kernels with it. Keep it secure.


	Step 5: Sign the kernel
	-----------------------

	We need to use mkimage (which was built when you built U-Boot) to package the
	Linux kernel into a FIT (Flat Image Tree, a flexible file format that U-Boot
	can load) using the ITS file you just created.

	At the same time we must put the public key into U-Boot device tree, with the
	'required' property, which tells U-Boot that this key must be verified for the
	image to be valid. You will make this key available to U-Boot for booting in
	step 6.

	ln -s $OKERNEL/dts/am335x-boneblack.dtb
	ln -s $OKERNEL/Image
	ln -s $UOUT/u-boot-dtb.img
	cp $UOUT/arch/arm/dts/am335x-boneblack.dtb am335x-boneblack-pubkey.dtb
	lzop Image
	$UOUT/tools/mkimage -f sign.its -K am335x-boneblack-pubkey.dtb -k keys -r image.fit

	You should see something like this:

	FIT description: Beaglebone black
	Created: Sun Jun 1 12:50:30 2014
	Image 0 (kernel)
	Description: unavailable
	Created: Sun Jun 1 12:50:30 2014
	Type: Kernel Image
	Compression: lzo compressed
	Data Size: 7790938 Bytes = 7608.34 kB = 7.43 MB
	Architecture: ARM
	OS: Linux
	Load Address: 0x80008000
	Entry Point: 0x80008000
	Hash algo: sha1
	Hash value: c94364646427e10f423837e559898ef02c97b988
	Image 1 (fdt-1)
	Description: beaglebone-black
	Created: Sun Jun 1 12:50:30 2014
	Type: Flat Device Tree
	Compression: uncompressed
	Data Size: 31547 Bytes = 30.81 kB = 0.03 MB
	Architecture: ARM
	Hash algo: sha1
	Hash value: cb09202f889d824f23b8e4404b781be5ad38a68d
	Default Configuration: 'conf-1'
	Configuration 0 (conf-1)
	Description: unavailable
	Kernel: kernel
	FDT: fdt-1


	Now am335x-boneblack-pubkey.dtb contains the public key and image.fit contains
	the signed kernel. Jump to step 6 if you like, or continue reading to increase
	your understanding.

	You can also run fit_check_sign to check it:

	$UOUT/tools/fit_check_sign -f image.fit -k am335x-boneblack-pubkey.dtb

	which results in:

	Verifying Hash Integrity ... sha1,rsa2048:dev+
	## Loading kernel from FIT Image at 7fc6ee469000 ...
	Using 'conf-1' configuration
	Verifying Hash Integrity ...
	sha1,rsa2048:dev+
	OK

	Trying 'kernel' kernel subimage
	Description: unavailable
	Created: Sun Jun 1 12:50:30 2014
	Type: Kernel Image
	Compression: lzo compressed
	Data Size: 7790938 Bytes = 7608.34 kB = 7.43 MB
	Architecture: ARM
	OS: Linux
	Load Address: 0x80008000
	Entry Point: 0x80008000
	Hash algo: sha1
	Hash value: c94364646427e10f423837e559898ef02c97b988
	Verifying Hash Integrity ...
	sha1+
	OK

	Unimplemented compression type 4
	## Loading fdt from FIT Image at 7fc6ee469000 ...
	Using 'conf-1' configuration
	Trying 'fdt-1' fdt subimage
	Description: beaglebone-black
	Created: Sun Jun 1 12:50:30 2014
	Type: Flat Device Tree
	Compression: uncompressed
	Data Size: 31547 Bytes = 30.81 kB = 0.03 MB
	Architecture: ARM
	Hash algo: sha1
	Hash value: cb09202f889d824f23b8e4404b781be5ad38a68d
	Verifying Hash Integrity ...
	sha1+
	OK

	Loading Flat Device Tree ... OK

	## Loading ramdisk from FIT Image at 7fc6ee469000 ...
	Using 'conf-1' configuration
	Could not find subimage node

	Signature check OK


	At the top, you see "sha1,rsa2048:dev+". This means that it checked an RSA key
	of size 2048 bits using SHA1 as the hash algorithm. The key name checked was
	'dev' and the '+' means that it verified. If it showed '-' that would be bad.

	Once the configuration is verified it is then possible to rely on the hashes
	in each image referenced by that configuration. So fit_check_sign goes on to
	load each of the images. We have a kernel and an FDT but no ramkdisk. In each
	case fit_check_sign checks the hash and prints sha1+ meaning that the SHA1
	hash verified. This means that none of the images has been tampered with.

	There is a test in test/vboot which uses U-Boot's sandbox build to verify that
	the above flow works.

	But it is fun to do this by hand, so you can load image.fit into a hex editor
	like ghex, and change a byte in the kernel:

	$UOUT/tools/fit_info -f image.fit -n /images/kernel -p data
	NAME: kernel
	LEN: 7790938
	OFF: 168

	This tells us that the kernel starts at byte offset 168 (decimal) in image.fit
	and extends for about 7MB. Try changing a byte at 0x2000 (say) and run
	fit_check_sign again. You should see something like:

	Verifying Hash Integrity ... sha1,rsa2048:dev+
	## Loading kernel from FIT Image at 7f5a39571000 ...
	Using 'conf-1' configuration
	Verifying Hash Integrity ...
	sha1,rsa2048:dev+
	OK

	Trying 'kernel' kernel subimage
	Description: unavailable
	Created: Sun Jun 1 13:09:21 2014
	Type: Kernel Image
	Compression: lzo compressed
	Data Size: 7790938 Bytes = 7608.34 kB = 7.43 MB
	Architecture: ARM
	OS: Linux
	Load Address: 0x80008000
	Entry Point: 0x80008000
	Hash algo: sha1
	Hash value: c94364646427e10f423837e559898ef02c97b988
	Verifying Hash Integrity ...
	sha1 error
	Bad hash value for 'hash-1' hash node in 'kernel' image node
	Bad Data Hash

	## Loading fdt from FIT Image at 7f5a39571000 ...
	Using 'conf-1' configuration
	Trying 'fdt-1' fdt subimage
	Description: beaglebone-black
	Created: Sun Jun 1 13:09:21 2014
	Type: Flat Device Tree
	Compression: uncompressed
	Data Size: 31547 Bytes = 30.81 kB = 0.03 MB
	Architecture: ARM
	Hash algo: sha1
	Hash value: cb09202f889d824f23b8e4404b781be5ad38a68d
	Verifying Hash Integrity ...
	sha1+
	OK

	Loading Flat Device Tree ... OK

	## Loading ramdisk from FIT Image at 7f5a39571000 ...
	Using 'conf-1' configuration
	Could not find subimage node

	Signature check Bad (error 1)


	It has detected the change in the kernel.

	You can also be sneaky and try to switch images, using the libfdt utilities
	that come with dtc (package name is device-tree-compiler but you will need a
	recent version like 1.4:

	dtc -v
	Version: DTC 1.4.0

	First we can check which nodes are actually hashed by the configuration:

	fdtget -l image.fit /
	images
	configurations

	fdtget -l image.fit /configurations
	conf-1
	fdtget -l image.fit /configurations/conf-1
	signature-1

	fdtget -p image.fit /configurations/conf-1/signature-1
	hashed-strings
	hashed-nodes
	timestamp
	signer-version
	signer-name
	value
	algo
	key-name-hint
	sign-images

	fdtget image.fit /configurations/conf-1/signature-1 hashed-nodes
	/ /configurations/conf-1 /images/fdt-1 /images/fdt-1/hash /images/kernel /images/kernel/hash-1

	This gives us a bit of a look into the signature that mkimage added. Note you
	can also use fdtdump to list the entire device tree.

	Say we want to change the kernel that this configuration uses
	(/images/kernel). We could just put a new kernel in the image, but we will
	need to change the hash to match. Let's simulate that by changing a byte of
	the hash:

	fdtget -tx image.fit /images/kernel/hash-1 value
	c9436464 6427e10f 423837e5 59898ef0 2c97b988
	fdtput -tx image.fit /images/kernel/hash-1 value c9436464 6427e10f 423837e5 59898ef0 2c97b981

	Now check it again:

	$UOUT/tools/fit_check_sign -f image.fit -k am335x-boneblack-pubkey.dtb
	Verifying Hash Integrity ... sha1,rsa2048:devrsa_verify_with_keynode: RSA failed to verify: -13
	rsa_verify_with_keynode: RSA failed to verify: -13
	-
	Failed to verify required signature 'key-dev'
	Signature check Bad (error 1)

	This time we don't even get as far as checking the images, since the
	configuration signature doesn't match. We can't change any hashes without the
	signature check noticing. The configuration is essentially locked. U-Boot has
	a public key for which it requires a match, and will not permit the use of any
	configuration that does not match that public key. The only way the
	configuration will match is if it was signed by the matching private key.

	It would also be possible to add a new signature node that does match your new
	configuration. But that won't work since you are not allowed to change the
	configuration in any way. Try it with a fresh (valid) image if you like by
	running the mkimage link again. Then:

	fdtput -p image.fit /configurations/conf-1/signature-1 value fred
	$UOUT/tools/fit_check_sign -f image.fit -k am335x-boneblack-pubkey.dtb
	Verifying Hash Integrity ... -
	sha1,rsa2048:devrsa_verify_with_keynode: RSA failed to verify: -13
	rsa_verify_with_keynode: RSA failed to verify: -13
	-
	Failed to verify required signature 'key-dev'
	Signature check Bad (error 1)


	Of course it would be possible to add an entirely new configuration and boot
	with that, but it still needs to be signed, so it won't help.


	6. Put the public key into U-Boot's image
	-----------------------------------------

	Having confirmed that the signature is doing its job, let's try it out in
	U-Boot on the board. U-Boot needs access to the public key corresponding to
	the private key that you signed with so that it can verify any kernels that
	you sign.

	cd $UBOOT
	make O=b/am335x_boneblack_vboot EXT_DTB=${WORK}/am335x-boneblack-pubkey.dtb

	Here we are overrriding the normal device tree file with our one, which
	contains the public key.

	Now you have a special U-Boot image with the public key. It can verify can
	kernel that you sign with the private key as in step 5.

	If you like you can take a look at the public key information that mkimage
	added to U-Boot's device tree:

	fdtget -p am335x-boneblack-pubkey.dtb /signature/key-dev
	required
	algo
	rsa,r-squared
	rsa,modulus
	rsa,n0-inverse
	rsa,num-bits
	key-name-hint

	This has information about the key and some pre-processed values which U-Boot
	can use to verify against it. These values are obtained from the public key
	certificate by mkimage, but require quite a bit of code to generate. To save
	code space in U-Boot, the information is extracted and written in raw form for
	U-Boot to easily use. The same mechanism is used in Google's Chrome OS.

	Notice the 'required' property. This marks the key as required - U-Boot will
	not boot any image that does not verify against this key.


	7. Put U-Boot and the kernel onto the board
	-------------------------------------------

	The method here varies depending on how you are booting. For this example we
	are booting from an micro-SD card with two partitions, one for U-Boot and one
	for Linux. Put it into your machine and write U-Boot and the kernel to it.
	Here the card is /dev/sde:

	cd $WORK
	export UDEV=/dev/sde1 # Change thes two lines to the correct device
	export KDEV=/dev/sde2
	sudo mount $UDEV /mnt/tmp && sudo cp $UOUT/u-boot-dtb.img /mnt/tmp/u-boot.img && sleep 1 && sudo umount $UDEV
	sudo mount $KDEV /mnt/tmp && sudo cp $WORK/image.fit /mnt/tmp/boot/image.fit && sleep 1 && sudo umount $KDEV


	8. Try it
	---------

	Boot the board using the commands below:

	setenv bootargs console=ttyO0,115200n8 quiet root=/dev/mmcblk0p2 ro rootfstype=ext4 rootwait
	ext2load mmc 0:2 82000000 /boot/image.fit
	bootm 82000000

	You should then see something like this:

	U-Boot# setenv bootargs console=ttyO0,115200n8 quiet root=/dev/mmcblk0p2 ro rootfstype=ext4 rootwait
	U-Boot# ext2load mmc 0:2 82000000 /boot/image.fit
	7824930 bytes read in 589 ms (12.7 MiB/s)
	U-Boot# bootm 82000000
	## Loading kernel from FIT Image at 82000000 ...
	Using 'conf-1' configuration
	Verifying Hash Integrity ... sha1,rsa2048:dev+ OK
	Trying 'kernel' kernel subimage
	Description: unavailable
	Created: 2014-06-01 19:32:54 UTC
	Type: Kernel Image
	Compression: lzo compressed
	Data Start: 0x820000a8
	Data Size: 7790938 Bytes = 7.4 MiB
	Architecture: ARM
	OS: Linux
	Load Address: 0x80008000
	Entry Point: 0x80008000
	Hash algo: sha1
	Hash value: c94364646427e10f423837e559898ef02c97b988
	Verifying Hash Integrity ... sha1+ OK
	## Loading fdt from FIT Image at 82000000 ...
	Using 'conf-1' configuration
	Trying 'fdt-1' fdt subimage
	Description: beaglebone-black
	Created: 2014-06-01 19:32:54 UTC
	Type: Flat Device Tree
	Compression: uncompressed
	Data Start: 0x8276e2ec
	Data Size: 31547 Bytes = 30.8 KiB
	Architecture: ARM
	Hash algo: sha1
	Hash value: cb09202f889d824f23b8e4404b781be5ad38a68d
	Verifying Hash Integrity ... sha1+ OK
	Booting using the fdt blob at 0x8276e2ec
	Uncompressing Kernel Image ... OK
	Loading Device Tree to 8fff5000, end 8ffffb3a ... OK

	Starting kernel ...

	[ 0.582377] omap_init_mbox: hwmod doesn't have valid attrs
	[ 2.589651] musb-hdrc musb-hdrc.0.auto: Failed to request rx1.
	[ 2.595830] musb-hdrc musb-hdrc.0.auto: musb_init_controller failed with status -517
	[ 2.606470] musb-hdrc musb-hdrc.1.auto: Failed to request rx1.
	[ 2.612723] musb-hdrc musb-hdrc.1.auto: musb_init_controller failed with status -517
	[ 2.940808] drivers/rtc/hctosys.c: unable to open rtc device (rtc0)
	[ 7.248889] libphy: PHY 4a101000.mdio:01 not found
	[ 7.253995] net eth0: phy 4a101000.mdio:01 not found on slave 1
	systemd-fsck[83]: Angstrom: clean, 50607/218160 files, 306348/872448 blocks

	.---O---.
	\| \| .-. o o
	\| \| \|-----.-----.-----.\| \| .----..-----.-----.
	\| \| \| __ \| ---'\| '--.\| .-'\| \| \|
	\| \| \| \| \| \|--- \|\| --'\| \| \| ' \| \| \| \|
	'---'---'--'--'--. \|-----''----''--' '-----'-'-'-'
	-' \|
	'---'

	The Angstrom Distribution beaglebone ttyO0

	Angstrom v2012.12 - Kernel 3.14.1+

	beaglebone login:

	At this point your kernel has been verified and you can be sure that it is one
	that you signed. As an exercise, try changing image.fit as in step 5 and see
	what happens.


	Further Improvements
	--------------------

	Several of the steps here can be easily automated. In particular it would be
	capital if signing and packaging a kernel were easy, perhaps a simple make
	target in the kernel.

	Some mention of how to use multiple .dtb files in a FIT might be useful.

	U-Boot's verified boot mechanism has not had a robust and independent security
	review. Such a review should look at the implementation and its resistance to
	attacks.

	Perhaps the verified boot feature could could be integrated into the Amstrom
	distribution.


	Simon Glass
	sjg@chromium.org
	2-June-14