mirror of
https://github.com/AsahiLinux/u-boot
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3790b18444
Move this properties into the main spec. Signed-off-by: Simon Glass <sjg@chromium.org>
696 lines
24 KiB
ReStructuredText
696 lines
24 KiB
ReStructuredText
.. SPDX-License-Identifier: GPL-2.0+
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U-Boot FIT Signature Verification
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=================================
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Introduction
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------------
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FIT supports hashing of images so that these hashes can be checked on
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loading. This protects against corruption of the image. However it does not
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prevent the substitution of one image for another.
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The signature feature allows the hash to be signed with a private key such
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that it can be verified using a public key later. Provided that the private
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key is kept secret and the public key is stored in a non-volatile place,
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any image can be verified in this way.
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See verified-boot.txt for more general information on verified boot.
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Concepts
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--------
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Some familiarity with public key cryptography is assumed in this section.
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The procedure for signing is as follows:
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- hash an image in the FIT
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- sign the hash with a private key to produce a signature
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- store the resulting signature in the FIT
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The procedure for verification is:
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- read the FIT
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- obtain the public key
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- extract the signature from the FIT
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- hash the image from the FIT
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- verify (with the public key) that the extracted signature matches the
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hash
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The signing is generally performed by mkimage, as part of making a firmware
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image for the device. The verification is normally done in U-Boot on the
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device.
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Algorithms
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----------
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In principle any suitable algorithm can be used to sign and verify a hash.
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U-Boot supports a few hashing and verification algorithms. See below for
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details.
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While it is acceptable to bring in large cryptographic libraries such as
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openssl on the host side (e.g. mkimage), it is not desirable for U-Boot.
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For the run-time verification side, it is important to keep code and data
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size as small as possible.
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For this reason the RSA image verification uses pre-processed public keys
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which can be used with a very small amount of code - just some extraction
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of data from the FDT and exponentiation mod n. Code size impact is a little
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under 5KB on Tegra Seaboard, for example.
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It is relatively straightforward to add new algorithms if required. If
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another RSA variant is needed, then it can be added with the
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U_BOOT_CRYPTO_ALGO() macro. If another algorithm is needed (such as DSA) then
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it can be placed in a directory alongside lib/rsa/, and its functions added
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using U_BOOT_CRYPTO_ALGO().
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Creating an RSA key pair and certificate
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----------------------------------------
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To create a new public/private key pair, size 2048 bits::
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$ openssl genpkey -algorithm RSA -out keys/dev.key \
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-pkeyopt rsa_keygen_bits:2048 -pkeyopt rsa_keygen_pubexp:65537
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To create a certificate for this containing the public key::
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$ openssl req -batch -new -x509 -key keys/dev.key -out keys/dev.crt
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If you like you can look at the public key also::
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$ openssl rsa -in keys/dev.key -pubout
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Public Key Storage
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------------------
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In order to verify an image that has been signed with a public key we need to
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have a trusted public key. This cannot be stored in the signed image, since
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it would be easy to alter. For this implementation we choose to store the
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public key in U-Boot's control FDT (using CONFIG_OF_CONTROL).
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Public keys should be stored as sub-nodes in a /signature node. Required
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properties are:
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algo
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Algorithm name (e.g. "sha1,rsa2048" or "sha256,ecdsa256")
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Optional properties are:
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key-name-hint
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Name of key used for signing. This is only a hint since it
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is possible for the name to be changed. Verification can proceed by checking
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all available signing keys until one matches.
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required
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If present this indicates that the key must be verified for the
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image / configuration to be considered valid. Only required keys are
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normally verified by the FIT image booting algorithm. Valid values are
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"image" to force verification of all images, and "conf" to force verification
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of the selected configuration (which then relies on hashes in the images to
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verify those).
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Each signing algorithm has its own additional properties.
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For RSA the following are mandatory:
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rsa,num-bits
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Number of key bits (e.g. 2048)
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rsa,modulus
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Modulus (N) as a big-endian multi-word integer
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rsa,exponent
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Public exponent (E) as a 64 bit unsigned integer
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rsa,r-squared
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(2^num-bits)^2 as a big-endian multi-word integer
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rsa,n0-inverse
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-1 / modulus[0] mod 2^32
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For ECDSA the following are mandatory:
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ecdsa,curve
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Name of ECDSA curve (e.g. "prime256v1")
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ecdsa,x-point
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Public key X coordinate as a big-endian multi-word integer
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ecdsa,y-point
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Public key Y coordinate as a big-endian multi-word integer
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These parameters can be added to a binary device tree using parameter -K of the
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mkimage command::
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tools/mkimage -f fit.its -K control.dtb -k keys -r image.fit
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Here is an example of a generated device tree node::
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signature {
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key-dev {
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required = "conf";
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algo = "sha256,rsa2048";
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rsa,r-squared = <0xb76d1acf 0xa1763ca5 0xeb2f126
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0x742edc80 0xd3f42177 0x9741d9d9
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0x35bb476e 0xff41c718 0xd3801430
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0xf22537cb 0xa7e79960 0xae32a043
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0x7da1427a 0x341d6492 0x3c2762f5
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0xaac04726 0x5b262d96 0xf984e86d
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0xb99443c7 0x17080c33 0x940f6892
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0xd57a95d1 0x6ea7b691 0xc5038fa8
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0x6bb48a6e 0x73f1b1ea 0x37160841
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0xe05715ce 0xa7c45bbd 0x690d82d5
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0x99c2454c 0x6ff117b3 0xd830683b
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0x3f81c9cf 0x1ca38a91 0x0c3392e4
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0xd817c625 0x7b8e9a24 0x175b89ea
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0xad79f3dc 0x4d50d7b4 0x9d4e90f8
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0xad9e2939 0xc165d6a4 0x0ada7e1b
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0xfb1bf495 0xfc3131c2 0xb8c6e604
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0xc2761124 0xf63de4a6 0x0e9565f9
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0xc8e53761 0x7e7a37a5 0xe99dcdae
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0x9aff7e1e 0xbd44b13d 0x6b0e6aa4
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0x038907e4 0x8e0d6850 0xef51bc20
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0xf73c94af 0x88bea7b1 0xcbbb1b30
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0xd024b7f3>;
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rsa,modulus = <0xc0711d6cb 0x9e86db7f 0x45986dbe
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0x023f1e8c9 0xe1a4c4d0 0x8a0dfdc9
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0x023ba0c48 0x06815f6a 0x5caa0654
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0x07078c4b7 0x3d154853 0x40729023
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0x0b007c8fe 0x5a3647e5 0x23b41e20
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0x024720591 0x66915305 0x0e0b29b0
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0x0de2ad30d 0x8589430f 0xb1590325
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0x0fb9f5d5e 0x9eba752a 0xd88e6de9
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0x056b3dcc6 0x9a6b8e61 0x6784f61f
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0x000f39c21 0x5eec6b33 0xd78e4f78
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0x0921a305f 0xaa2cc27e 0x1ca917af
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0x06e1134f4 0xd48cac77 0x4e914d07
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0x0f707aa5a 0x0d141f41 0x84677f1d
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0x0ad47a049 0x028aedb6 0xd5536fcf
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0x03fef1e4f 0x133a03d2 0xfd7a750a
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0x0f9159732 0xd207812e 0x6a807375
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0x06434230d 0xc8e22dad 0x9f29b3d6
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0x07c44ac2b 0xfa2aad88 0xe2429504
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0x041febd41 0x85d0d142 0x7b194d65
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0x06e5d55ea 0x41116961 0xf3181dde
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0x068bf5fbc 0x3dd82047 0x00ee647e
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0x0d7a44ab3>;
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rsa,exponent = <0x00 0x10001>;
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rsa,n0-inverse = <0xb3928b85>;
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rsa,num-bits = <0x800>;
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key-name-hint = "dev";
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};
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};
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Signed Configurations
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---------------------
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While signing images is useful, it does not provide complete protection
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against several types of attack. For example, it is possible to create a
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FIT with the same signed images, but with the configuration changed such
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that a different one is selected (mix and match attack). It is also possible
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to substitute a signed image from an older FIT version into a newer FIT
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(roll-back attack).
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As an example, consider this FIT::
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/ {
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images {
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kernel-1 {
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data = <data for kernel1>
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signature-1 {
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algo = "sha1,rsa2048";
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value = <...kernel signature 1...>
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};
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};
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kernel-2 {
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data = <data for kernel2>
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signature-1 {
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algo = "sha1,rsa2048";
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value = <...kernel signature 2...>
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};
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};
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fdt-1 {
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data = <data for fdt1>;
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signature-1 {
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algo = "sha1,rsa2048";
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value = <...fdt signature 1...>
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};
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};
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fdt-2 {
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data = <data for fdt2>;
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signature-1 {
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algo = "sha1,rsa2048";
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value = <...fdt signature 2...>
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};
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};
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};
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configurations {
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default = "conf-1";
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conf-1 {
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kernel = "kernel-1";
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fdt = "fdt-1";
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};
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conf-2 {
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kernel = "kernel-2";
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fdt = "fdt-2";
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};
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};
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};
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Since both kernels are signed it is easy for an attacker to add a new
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configuration 3 with kernel 1 and fdt 2::
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configurations {
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default = "conf-1";
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conf-1 {
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kernel = "kernel-1";
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fdt = "fdt-1";
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};
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conf-2 {
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kernel = "kernel-2";
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fdt = "fdt-2";
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};
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conf-3 {
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kernel = "kernel-1";
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fdt = "fdt-2";
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};
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};
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With signed images, nothing protects against this. Whether it gains an
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advantage for the attacker is debatable, but it is not secure.
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To solve this problem, we support signed configurations. In this case it
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is the configurations that are signed, not the image. Each image has its
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own hash, and we include the hash in the configuration signature.
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So the above example is adjusted to look like this::
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/ {
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images {
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kernel-1 {
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data = <data for kernel1>
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hash-1 {
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algo = "sha1";
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value = <...kernel hash 1...>
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};
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};
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kernel-2 {
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data = <data for kernel2>
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hash-1 {
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algo = "sha1";
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value = <...kernel hash 2...>
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};
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};
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fdt-1 {
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data = <data for fdt1>;
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hash-1 {
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algo = "sha1";
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value = <...fdt hash 1...>
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};
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};
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fdt-2 {
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data = <data for fdt2>;
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hash-1 {
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algo = "sha1";
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value = <...fdt hash 2...>
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};
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};
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};
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configurations {
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default = "conf-1";
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conf-1 {
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kernel = "kernel-1";
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fdt = "fdt-1";
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signature-1 {
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algo = "sha1,rsa2048";
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value = <...conf 1 signature...>;
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};
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};
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conf-2 {
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kernel = "kernel-2";
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fdt = "fdt-2";
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signature-1 {
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algo = "sha1,rsa2048";
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value = <...conf 1 signature...>;
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};
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};
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};
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};
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You can see that we have added hashes for all images (since they are no
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longer signed), and a signature to each configuration. In the above example,
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mkimage will sign configurations/conf-1, the kernel and fdt that are
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pointed to by the configuration (/images/kernel-1, /images/kernel-1/hash-1,
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/images/fdt-1, /images/fdt-1/hash-1) and the root structure of the image
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(so that it isn't possible to add or remove root nodes). The signature is
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written into /configurations/conf-1/signature-1/value. It can easily be
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verified later even if the FIT has been signed with other keys in the
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meantime.
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Details
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-------
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The signature node contains a property ('hashed-nodes') which lists all the
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nodes that the signature was made over. The image is walked in order and each
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tag processed as follows:
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DTB_BEGIN_NODE
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The tag and the following name are included in the signature
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if the node or its parent are present in 'hashed-nodes'
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DTB_END_NODE
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The tag is included in the signature if the node or its parent
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are present in 'hashed-nodes'
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DTB_PROPERTY
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The tag, the length word, the offset in the string table, and
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the data are all included if the current node is present in 'hashed-nodes'
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and the property name is not 'data'.
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DTB_END
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The tag is always included in the signature.
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DTB_NOP
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The tag is included in the signature if the current node is present
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in 'hashed-nodes'
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In addition, the signature contains a property 'hashed-strings' which contains
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the offset and length in the string table of the strings that are to be
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included in the signature (this is done last).
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IMPORTANT: To verify the signature outside u-boot, it is vital to not only
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calculate the hash of the image and verify the signature with that, but also to
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calculate the hashes of the kernel, fdt, and ramdisk images and check those
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match the hash values in the corresponding 'hash*' subnodes.
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Verification
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------------
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FITs are verified when loaded. After the configuration is selected a list
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of required images is produced. If there are 'required' public keys, then
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each image must be verified against those keys. This means that every image
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that might be used by the target needs to be signed with 'required' keys.
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This happens automatically as part of a bootm command when FITs are used.
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For Signed Configurations, the default verification behavior can be changed by
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the following optional property in /signature node in U-Boot's control FDT.
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required-mode
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Valid values are "any" to allow verified boot to succeed if
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the selected configuration is signed by any of the 'required' keys, and "all"
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to allow verified boot to succeed if the selected configuration is signed by
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all of the 'required' keys.
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This property can be added to a binary device tree using fdtput as shown in
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below examples::
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fdtput -t s control.dtb /signature required-mode any
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fdtput -t s control.dtb /signature required-mode all
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Enabling FIT Verification
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-------------------------
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In addition to the options to enable FIT itself, the following CONFIGs must
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be enabled:
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CONFIG_FIT_SIGNATURE
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enable signing and verification in FITs
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CONFIG_RSA
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enable RSA algorithm for signing
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CONFIG_ECDSA
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enable ECDSA algorithm for signing
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WARNING: When relying on signed FIT images with required signature check
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the legacy image format is default disabled by not defining
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CONFIG_LEGACY_IMAGE_FORMAT
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Testing
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-------
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An easy way to test signing and verification is to use the test script
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provided in test/vboot/vboot_test.sh. This uses sandbox (a special version
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of U-Boot which runs under Linux) to show the operation of a 'bootm'
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command loading and verifying images.
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A sample run is show below::
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$ make O=sandbox sandbox_config
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$ make O=sandbox
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$ O=sandbox ./test/vboot/vboot_test.sh
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Simple Verified Boot Test
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-------------------------
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Please see :doc:`verified-boot` for more information::
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/home/hs/ids/u-boot/sandbox/tools/mkimage -D -I dts -O dtb -p 2000
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Build keys
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do sha1 test
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Build FIT with signed images
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Test Verified Boot Run: unsigned signatures:: OK
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Sign images
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Test Verified Boot Run: signed images: OK
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Build FIT with signed configuration
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Test Verified Boot Run: unsigned config: OK
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Sign images
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Test Verified Boot Run: signed config: OK
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check signed config on the host
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Signature check OK
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OK
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Test Verified Boot Run: signed config: OK
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Test Verified Boot Run: signed config with bad hash: OK
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do sha256 test
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Build FIT with signed images
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Test Verified Boot Run: unsigned signatures:: OK
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Sign images
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Test Verified Boot Run: signed images: OK
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Build FIT with signed configuration
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Test Verified Boot Run: unsigned config: OK
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Sign images
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Test Verified Boot Run: signed config: OK
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check signed config on the host
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Signature check OK
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OK
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Test Verified Boot Run: signed config: OK
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Test Verified Boot Run: signed config with bad hash: OK
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Test passed
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Software signing: keydir vs keyfile
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-----------------------------------
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In the simplest case, signing is done by giving mkimage the 'keyfile'. This is
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the path to a file containing the signing key.
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The alternative is to pass the 'keydir' argument. In this case the filename of
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the key is derived from the 'keydir' and the "key-name-hint" property in the
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FIT. In this case the "key-name-hint" property is mandatory, and the key must
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exist in "<keydir>/<key-name-hint>.<ext>" Here the extension "ext" is
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specific to the signing algorithm.
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Hardware Signing with PKCS#11 or with HSM
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-----------------------------------------
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Securely managing private signing keys can challenging, especially when the
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keys are stored on the file system of a computer that is connected to the
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Internet. If an attacker is able to steal the key, they can sign malicious FIT
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images which will appear genuine to your devices.
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An alternative solution is to keep your signing key securely stored on hardware
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device like a smartcard, USB token or Hardware Security Module (HSM) and have
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them perform the signing. PKCS#11 is standard for interfacing with these crypto
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device.
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Requirements:
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- Smartcard/USB token/HSM which can work with some openssl engine
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- openssl
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For pkcs11 engine usage:
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- libp11 (provides pkcs11 engine)
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- p11-kit (recommended to simplify setup)
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- opensc (for smartcards and smartcard like USB devices)
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- gnutls (recommended for key generation, p11tool)
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For generic HSMs respective openssl engine must be installed and locateable by
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openssl. This may require setting up LD_LIBRARY_PATH if engine is not installed
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to openssl's default search paths.
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PKCS11 engine support forms "key id" based on "keydir" and with
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"key-name-hint". "key-name-hint" is used as "object" name (if not defined in
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keydir). "keydir" (if defined) is used to define (prefix for) which PKCS11 source
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is being used for lookup up for the key.
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PKCS11 engine key ids
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"pkcs11:<keydir>;object=<key-name-hint>;type=<public|private>"
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or, if keydir contains "object="
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"pkcs11:<keydir>;type=<public|private>"
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or
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"pkcs11:object=<key-name-hint>;type=<public|private>",
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Generic HSM engine support forms "key id" based on "keydir" and with
|
|
"key-name-hint". If "keydir" is specified for mkimage it is used as a prefix in
|
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"key id" and is appended with "key-name-hint".
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|
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Generic engine key ids:
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"<keydir><key-name-hint>"
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or
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"< key-name-hint>"
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In order to set the pin in the HSM, an environment variable "MKIMAGE_SIGN_PIN"
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can be specified.
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The following examples use the Nitrokey Pro using pkcs11 engine. Instructions
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for other devices may vary.
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Notes on pkcs11 engine setup:
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Make sure p11-kit, opensc are installed and that p11-kit is setup to use opensc.
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/usr/share/p11-kit/modules/opensc.module should be present on your system.
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Generating Keys On the Nitrokey::
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$ gpg --card-edit
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Reader ...........: Nitrokey Nitrokey Pro (xxxxxxxx0000000000000000) 00 00
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Application ID ...: xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
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Version ..........: 2.1
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Manufacturer .....: ZeitControl
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Serial number ....: xxxxxxxx
|
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Name of cardholder: [not set]
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Language prefs ...: de
|
|
Sex ..............: unspecified
|
|
URL of public key : [not set]
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|
Login data .......: [not set]
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Signature PIN ....: forced
|
|
Key attributes ...: rsa2048 rsa2048 rsa2048
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Max. PIN lengths .: 32 32 32
|
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PIN retry counter : 3 0 3
|
|
Signature counter : 0
|
|
Signature key ....: [none]
|
|
Encryption key....: [none]
|
|
Authentication key: [none]
|
|
General key info..: [none]
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|
|
gpg/card> generate
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|
Make off-card backup of encryption key? (Y/n) n
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|
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Please note that the factory settings of the PINs are
|
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PIN = '123456' Admin PIN = '12345678'
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You should change them using the command --change-pin
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|
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What keysize do you want for the Signature key? (2048) 4096
|
|
The card will now be re-configured to generate a key of 4096 bits
|
|
Note: There is no guarantee that the card supports the requested size.
|
|
If the key generation does not succeed, please check the
|
|
documentation of your card to see what sizes are allowed.
|
|
What keysize do you want for the Encryption key? (2048) 4096
|
|
The card will now be re-configured to generate a key of 4096 bits
|
|
What keysize do you want for the Authentication key? (2048) 4096
|
|
The card will now be re-configured to generate a key of 4096 bits
|
|
Please specify how long the key should be valid.
|
|
0 = key does not expire
|
|
<n> = key expires in n days
|
|
<n>w = key expires in n weeks
|
|
<n>m = key expires in n months
|
|
<n>y = key expires in n years
|
|
Key is valid for? (0)
|
|
Key does not expire at all
|
|
Is this correct? (y/N) y
|
|
|
|
GnuPG needs to construct a user ID to identify your key.
|
|
|
|
Real name: John Doe
|
|
Email address: john.doe@email.com
|
|
Comment:
|
|
You selected this USER-ID:
|
|
"John Doe <john.doe@email.com>"
|
|
|
|
Change (N)ame, (C)omment, (E)mail or (O)kay/(Q)uit? o
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|
|
|
|
|
Using p11tool to get the token URL:
|
|
|
|
Depending on system configuration, gpg-agent may need to be killed first::
|
|
|
|
$ p11tool --provider /usr/lib/opensc-pkcs11.so --list-tokens
|
|
Token 0:
|
|
URL: pkcs11:model=PKCS%2315%20emulated;manufacturer=ZeitControl;serial=000xxxxxxxxx;token=OpenPGP%20card%20%28User%20PIN%20%28sig%29%29
|
|
Label: OpenPGP card (User PIN (sig))
|
|
Type: Hardware token
|
|
Manufacturer: ZeitControl
|
|
Model: PKCS#15 emulated
|
|
Serial: 000xxxxxxxxx
|
|
Module: (null)
|
|
|
|
|
|
Token 1:
|
|
URL: pkcs11:model=PKCS%2315%20emulated;manufacturer=ZeitControl;serial=000xxxxxxxxx;token=OpenPGP%20card%20%28User%20PIN%29
|
|
Label: OpenPGP card (User PIN)
|
|
Type: Hardware token
|
|
Manufacturer: ZeitControl
|
|
Model: PKCS#15 emulated
|
|
Serial: 000xxxxxxxxx
|
|
Module: (null)
|
|
|
|
Use the portion of the signature token URL after "pkcs11:" as the keydir argument (-k) to mkimage below.
|
|
|
|
|
|
Use the URL of the token to list the private keys::
|
|
|
|
$ p11tool --login --provider /usr/lib/opensc-pkcs11.so --list-privkeys \
|
|
"pkcs11:model=PKCS%2315%20emulated;manufacturer=ZeitControl;serial=000xxxxxxxxx;token=OpenPGP%20card%20%28User%20PIN%20%28sig%29%29"
|
|
Token 'OpenPGP card (User PIN (sig))' with URL 'pkcs11:model=PKCS%2315%20emulated;manufacturer=ZeitControl;serial=000xxxxxxxxx;token=OpenPGP%20card%20%28User%20PIN%20%28sig%29%29' requires user PIN
|
|
Enter PIN:
|
|
Object 0:
|
|
URL: pkcs11:model=PKCS%2315%20emulated;manufacturer=ZeitControl;serial=000xxxxxxxxx;token=OpenPGP%20card%20%28User%20PIN%20%28sig%29%29;id=%01;object=Signature%20key;type=private
|
|
Type: Private key
|
|
Label: Signature key
|
|
Flags: CKA_PRIVATE; CKA_NEVER_EXTRACTABLE; CKA_SENSITIVE;
|
|
ID: 01
|
|
|
|
Use the label, in this case "Signature key" as the key-name-hint in your FIT.
|
|
|
|
Create the fitImage::
|
|
|
|
$ ./tools/mkimage -f fit-image.its fitImage
|
|
|
|
|
|
Sign the fitImage with the hardware key::
|
|
|
|
$ ./tools/mkimage -F -k \
|
|
"model=PKCS%2315%20emulated;manufacturer=ZeitControl;serial=000xxxxxxxxx;token=OpenPGP%20card%20%28User%20PIN%20%28sig%29%29" \
|
|
-K u-boot.dtb -N pkcs11 -r fitImage
|
|
|
|
|
|
Future Work
|
|
-----------
|
|
|
|
- Roll-back protection using a TPM is done using the tpm command. This can
|
|
be scripted, but we might consider a default way of doing this, built into
|
|
bootm.
|
|
|
|
|
|
Possible Future Work
|
|
--------------------
|
|
|
|
- More sandbox tests for failure modes
|
|
- Passwords for keys/certificates
|
|
- Perhaps implement OAEP
|
|
- Enhance bootm to permit scripted signature verification (so that a script
|
|
can verify an image but not actually boot it)
|
|
|
|
|
|
.. sectionauthor:: Simon Glass <sjg@chromium.org>, 1-1-13
|