[U-Boot] [PATCH 11/11] Add documentation for verified boot on Beaglebone Black
Simon Glass
sjg at chromium.org
Tue Jun 3 06:39:51 CEST 2014
As an example of an end-to-end process for using verified boot in U-Boot,
add a detailed description of the steps to be used for a Beaglebone
Black.
Signed-off-by: Simon Glass <sjg at chromium.org>
---
doc/uImage.FIT/beaglebone_vboot.txt | 608 ++++++++++++++++++++++++++++++++++++
1 file changed, 608 insertions(+)
create mode 100644 doc/uImage.FIT/beaglebone_vboot.txt
diff --git a/doc/uImage.FIT/beaglebone_vboot.txt b/doc/uImage.FIT/beaglebone_vboot.txt
new file mode 100644
index 0000000..6a6d58d
--- /dev/null
+++ b/doc/uImage.FIT/beaglebone_vboot.txt
@@ -0,0 +1,608 @@
+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 at 1 {
+ data = /incbin/("Image.lzo");
+ type = "kernel";
+ arch = "arm";
+ os = "linux";
+ compression = "lzo";
+ load = <0x80008000>;
+ entry = <0x80008000>;
+ hash at 1 {
+ algo = "sha1";
+ };
+ };
+ fdt at 1 {
+ description = "beaglebone-black";
+ data = /incbin/("am335x-boneblack.dtb");
+ type = "flat_dt";
+ arch = "arm";
+ compression = "none";
+ hash at 1 {
+ algo = "sha1";
+ };
+ };
+ };
+ configurations {
+ default = "conf at 1";
+ conf at 1 {
+ kernel = "kernel at 1";
+ fdt = "fdt at 1";
+ signature at 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 at 1)
+ 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 at 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 at 1'
+ Configuration 0 (conf at 1)
+ Description: unavailable
+ Kernel: kernel at 1
+ FDT: fdt at 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 at 1' configuration
+ Verifying Hash Integrity ...
+sha1,rsa2048:dev+
+OK
+
+ Trying 'kernel at 1' 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 at 1' configuration
+ Trying 'fdt at 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 at 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 at 1 -p data
+NAME: kernel at 1
+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 at 1' configuration
+ Verifying Hash Integrity ...
+sha1,rsa2048:dev+
+OK
+
+ Trying 'kernel at 1' 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 at 1' hash node in 'kernel at 1' image node
+Bad Data Hash
+
+## Loading fdt from FIT Image at 7f5a39571000 ...
+ Using 'conf at 1' configuration
+ Trying 'fdt at 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 at 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 at 1
+fdtget -l image.fit /configurations/conf at 1
+signature at 1
+
+ fdtget -p image.fit /configurations/conf at 1/signature at 1
+hashed-strings
+hashed-nodes
+timestamp
+signer-version
+signer-name
+value
+algo
+key-name-hint
+sign-images
+
+ fdtget image.fit /configurations/conf at 1/signature at 1 hashed-nodes
+/ /configurations/conf at 1 /images/fdt at 1 /images/fdt at 1/hash at 1 /images/kernel at 1 /images/kernel at 1/hash at 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 at 1). 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 at 1/hash at 1 value
+c9436464 6427e10f 423837e5 59898ef0 2c97b988
+ fdtput -tx image.fit /images/kernel at 1/hash at 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 at 1/signature at 2 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 $UBOOT
+ make O=b/am335x_boneblack_vboot DEV_TREE_BIN=${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.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 at 1' configuration
+ Verifying Hash Integrity ... sha1,rsa2048:dev+ OK
+ Trying 'kernel at 1' 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 at 1' configuration
+ Trying 'fdt at 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 at chromium.org
+2-June-14
--
1.9.1.423.g4596e3a
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