Verified Boot on the Beaglebone Black
Introduction
Before reading this, please read U-Boot Verified Boot and U-Boot FIT Signature Verification. 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 with 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:
Build U-Boot for the board, with the verified boot options enabled.
Obtain a suitable Linux kernel
Create a Image Tree Source file (ITS) file describing how you want the kernel to be packaged, compressed and signed.
Create a key pair
Sign the kernel
Put the public key into U-Boot’s image
Put U-Boot and the kernel onto the board
Try it
Step 1: Build U-Boot
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-
Configure and build U-Boot with verified boot enabled:
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
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
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
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 = "sha256";
};
};
fdt-1 {
description = "beaglebone-black";
data = /incbin/("am335x-boneblack.dtb");
type = "flat_dt";
arch = "arm";
compression = "none";
hash-1 {
algo = "sha256";
};
};
};
configurations {
default = "conf-1";
conf-1 {
kernel = "kernel";
fdt = "fdt-1";
signature-1 {
algo = "sha256,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 U-Boot FIT Signature Verification 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: sha256
Hash value: 51b2adf9c1016ed46f424d85dcc6c34c46a20b9bee7227e06a6b6320ca5d35c1
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: sha256
Hash value: 807d5842a04132261ba092373bd40c78991bc7ce173d1175cd976ec37858e7cd
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 ... sha256,rsa2048:dev+
## Loading kernel from FIT Image at 7fc6ee469000 ...
Using 'conf-1' configuration
Verifying Hash Integrity ...
sha256,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: sha256
Hash value: 51b2adf9c1016ed46f424d85dcc6c34c46a20b9bee7227e06a6b6320ca5d35c1
Verifying Hash Integrity ...
sha256+
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: sha256
Hash value: 807d5842a04132261ba092373bd40c78991bc7ce173d1175cd976ec37858e7cd
Verifying Hash Integrity ...
sha256+
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 “sha256,rsa2048:dev+”. This means that it checked an RSA key of size 2048 bits using SHA256 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 sha256+ meaning that the SHA256 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 ... sha256,rsa2048:dev+
## Loading kernel from FIT Image at 7f5a39571000 ...
Using 'conf-1' configuration
Verifying Hash Integrity ...
sha256,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: sha256
Hash value: 51b2adf9c1016ed46f424d85dcc6c34c46a20b9bee7227e06a6b6320ca5d35c1
Verifying Hash Integrity ...
sha256 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: sha256
Hash value: 807d5842a04132261ba092373bd40c78991bc7ce173d1175cd976ec37858e7cd
Verifying Hash Integrity ...
sha256+
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
51b2adf9 c1016ed4 6f424d85 dcc6c34c 46a20b9b ee7227e0 6a6b6320 ca5d35c1
fdtput -tx image.fit /images/kernel/hash-1 value 51b2adf9 c1016ed4 6f424d85 dcc6c34c 46a20b9b ee7227e0 6a6b6320 ca5d35c8
Now check it again:
$UOUT/tools/fit_check_sign -f image.fit -k am335x-boneblack-pubkey.dtb
Verifying Hash Integrity ... sha256,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 ... -
sha256,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 overriding 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 ... sha256,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: sha256
Hash value: 51b2adf9c1016ed46f424d85dcc6c34c46a20b9bee7227e06a6b6320ca5d35c1
Verifying Hash Integrity ... sha256+ 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: sha256
Hash value: 807d5842a04132261ba092373bd40c78991bc7ce173d1175cd976ec37858e7cd
Verifying Hash Integrity ... sha256+ 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 be integrated into the Amstrom distribution.