Add verified boot information and test
Add a description of how to implement verified boot using signed FIT images, and a simple test which verifies operation on sandbox. The test signs a FIT image and verifies it, then signs a FIT configuration and verifies it. Then it corrupts the signature to check that this is detected. Signed-off-by: Simon Glass <sjg@chromium.org>
This commit is contained in:
Simon Glass
Tom Rini
parent
74378cf8e7
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041bca5ba3
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U-Boot Verified Boot
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====================
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Introduction
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------------
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Verified boot here means the verification of all software loaded into a
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machine during the boot process to ensure that it is authorised and correct
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for that machine.
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Verified boot extends from the moment of system reset to as far as you wish
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into the boot process. An example might be loading U-Boot from read-only
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memory, then loading a signed kernel, then using the kernel's dm-verity
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driver to mount a signed root filesystem.
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A key point is that it is possible to field-upgrade the software on machines
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which use verified boot. Since the machine will only run software that has
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been correctly signed, it is safe to read software from an updatable medium.
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It is also possible to add a secondary signed firmware image, in read-write
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memory, so that firmware can easily be upgraded in a secure manner.
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Signing
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-------
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Verified boot uses cryptographic algorithms to 'sign' software images.
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Images are signed using a private key known only to the signer, but can
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be verified using a public key. As its name suggests the public key can be
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made available without risk to the verification process. The private and
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public keys are mathematically related. For more information on how this
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works look up "public key cryptography" and "RSA" (a particular algorithm).
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The signing and verification process looks something like this:
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Signing Verification
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======= ============
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+--------------+ *
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| RSA key pair | * +---------------+
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| .key .crt | * | Public key in |
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+--------------+ +------> public key ----->| trusted place |
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| | * +---------------+
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| | * |
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v | * v
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+---------+ | * +--------------+
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| |----------+ * | |
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| signer | * | U-Boot |
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| |----------+ * | signature |--> yes/no
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+---------+ | * | verification |
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^ | * | |
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| | * +--------------+
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| | * ^
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+----------+ | * |
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| Software | +----> signed image -------------+
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| image | *
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+----------+ *
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The signature algorithm relies only on the public key to do its work. Using
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this key it checks the signature that it finds in the image. If it verifies
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then we know that the image is OK.
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The public key from the signer allows us to verify and therefore trust
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software from updatable memory.
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It is critical that the public key be secure and cannot be tampered with.
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It can be stored in read-only memory, or perhaps protected by other on-chip
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crypto provided by some modern SOCs. If the public key can ben changed, then
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the verification is worthless.
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Chaining Images
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---------------
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The above method works for a signer providing images to a run-time U-Boot.
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It is also possible to extend this scheme to a second level, like this:
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1. Master private key is used by the signer to sign a first-stage image.
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2. Master public key is placed in read-only memory.
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2. Secondary private key is created and used to sign second-stage images.
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3. Secondary public key is placed in first stage images
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4. We use the master public key to verify the first-stage image. We then
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use the secondary public key in the first-stage image to verify the second-
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state image.
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5. This chaining process can go on indefinitely. It is recommended to use a
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different key at each stage, so that a compromise in one place will not
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affect the whole change.
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Flattened Image Tree (FIT)
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--------------------------
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The FIT format is alreay widely used in U-Boot. It is a flattened device
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tree (FDT) in a particular format, with images contained within. FITs
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include hashes to verify images, so it is relatively straightforward to
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add signatures as well.
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The public key can be stored in U-Boot's CONFIG_OF_CONTROL device tree in
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a standard place. Then when a FIT it loaded it can be verified using that
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public key. Multiple keys and multiple signatures are supported.
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See signature.txt for more information.
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Simon Glass
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sjg@chromium.org
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1-1-13
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/*.dtb
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/test.fit
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/dev-keys
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/dts-v1/;
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/ {
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model = "Sandbox Verified Boot Test";
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compatible = "sandbox";
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};
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/dts-v1/;
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/ {
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model = "Sandbox Verified Boot Test";
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compatible = "sandbox";
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};
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/dts-v1/;
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/ {
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description = "Chrome OS kernel image with one or more FDT blobs";
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#address-cells = <1>;
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images {
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kernel@1 {
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data = /incbin/("test-kernel.bin");
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type = "kernel_noload";
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arch = "sandbox";
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os = "linux";
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compression = "none";
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load = <0x4>;
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entry = <0x8>;
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kernel-version = <1>;
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hash@1 {
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algo = "sha1";
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};
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};
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fdt@1 {
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description = "snow";
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data = /incbin/("sandbox-kernel.dtb");
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type = "flat_dt";
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arch = "sandbox";
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compression = "none";
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fdt-version = <1>;
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hash@1 {
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algo = "sha1";
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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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key-name-hint = "dev";
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sign-images = "fdt", "kernel";
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};
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};
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};
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};
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/dts-v1/;
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/ {
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description = "Chrome OS kernel image with one or more FDT blobs";
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#address-cells = <1>;
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images {
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kernel@1 {
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data = /incbin/("test-kernel.bin");
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type = "kernel_noload";
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arch = "sandbox";
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os = "linux";
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compression = "none";
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load = <0x4>;
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entry = <0x8>;
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kernel-version = <1>;
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signature@1 {
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algo = "sha1,rsa2048";
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key-name-hint = "dev";
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};
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};
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fdt@1 {
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description = "snow";
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data = /incbin/("sandbox-kernel.dtb");
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type = "flat_dt";
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arch = "sandbox";
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compression = "none";
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fdt-version = <1>;
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signature@1 {
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algo = "sha1,rsa2048";
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key-name-hint = "dev";
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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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};
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};
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#!/bin/sh
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#
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# Copyright (c) 2013, Google Inc.
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#
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# Simple Verified Boot Test Script
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#
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# This program is free software; you can redistribute it and/or
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# modify it under the terms of the GNU General Public License as
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# published by the Free Software Foundation; either version 2 of
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# the License, or (at your option) any later version.
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#
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# This program is distributed in the hope that it will be useful,
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# but WITHOUT ANY WARRANTY; without even the implied warranty of
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# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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# GNU General Public License for more details.
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#
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# You should have received a copy of the GNU General Public License
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# along with this program; if not, write to the Free Software
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# Foundation, Inc., 59 Temple Place, Suite 330, Boston,
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# MA 02111-1307 USA
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set -e
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# Run U-Boot and report the result
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# Args:
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# $1: Test message
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run_uboot() {
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echo -n "Test Verified Boot Run: $1: "
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${uboot} -d sandbox-u-boot.dtb >${tmp} -c '
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sb load host 0 100 test.fit;
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fdt addr 100;
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bootm 100;
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reset'
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if ! grep -q "$2" ${tmp}; then
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echo
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echo "Verified boot key check failed, output follows:"
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cat ${tmp}
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false
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else
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echo "OK"
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fi
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}
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echo "Simple Verified Boot Test"
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echo "========================="
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echo
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echo "Please see doc/uImage.FIT/verified-boot.txt for more information"
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echo
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err=0
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tmp=/tmp/vboot_test.$$
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dir=$(dirname $0)
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if [ -z ${O} ]; then
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O=.
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fi
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O=$(readlink -f ${O})
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dtc="-I dts -O dtb -p 2000"
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uboot="${O}/u-boot"
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mkimage="${O}/tools/mkimage"
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keys="${dir}/dev-keys"
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echo ${mkimage} -D "${dtc}"
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echo "Build keys"
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mkdir -p ${keys}
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# Create an RSA key pair
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openssl genrsa -F4 -out ${keys}/dev.key 2048 2>/dev/null
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# Create a certificate 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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pushd ${dir} >/dev/null
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# Compile our device tree files for kernel and U-Boot (CONFIG_OF_CONTROL)
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dtc -p 0x1000 sandbox-kernel.dts -O dtb -o sandbox-kernel.dtb
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dtc -p 0x1000 sandbox-u-boot.dts -O dtb -o sandbox-u-boot.dtb
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# Create a number kernel image with zeroes
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head -c 5000 /dev/zero >test-kernel.bin
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# Build the FIT, but don't sign anything yet
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echo Build FIT with signed images
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${mkimage} -D "${dtc}" -f sign-images.its test.fit >${tmp}
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run_uboot "unsigned signatures:" "dev-"
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# Sign images with our dev keys
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echo Sign images
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${mkimage} -D "${dtc}" -F -k dev-keys -K sandbox-u-boot.dtb -r test.fit >${tmp}
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run_uboot "signed images" "dev+"
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# Create a fresh .dtb without the public keys
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dtc -p 0x1000 sandbox-u-boot.dts -O dtb -o sandbox-u-boot.dtb
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echo Build FIT with signed configuration
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${mkimage} -D "${dtc}" -f sign-configs.its test.fit >${tmp}
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run_uboot "unsigned config" "sha1+ OK"
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# Sign images with our dev keys
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echo Sign images
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${mkimage} -D "${dtc}" -F -k dev-keys -K sandbox-u-boot.dtb -r test.fit >${tmp}
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run_uboot "signed config" "dev+"
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# Increment the first byte of the signature, which should cause failure
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sig=$(fdtget -t bx test.fit /configurations/conf@1/signature@1 value)
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newbyte=$(printf %x $((0x${sig:0:2} + 1)))
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sig="${newbyte} ${sig:2}"
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fdtput -t bx test.fit /configurations/conf@1/signature@1 value ${sig}
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run_uboot "signed config with bad hash" "Bad Data Hash"
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popd >/dev/null
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echo
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if ${ok}; then
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echo "Test passed"
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else
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echo "Test failed"
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fi
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