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 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 ... 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