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>master
Simon Glass
12 years ago
committed by
Tom Rini
parent
74378cf8e7
commit
041bca5ba3
@ -0,0 +1,104 @@ |
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U-Boot Verified Boot |
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==================== |
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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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|
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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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|
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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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|
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|
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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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|
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The signing and verification process looks something like this: |
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|
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|
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Signing Verification |
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======= ============ |
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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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|
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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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|
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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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|
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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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|
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|
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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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|
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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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|
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|
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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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|
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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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|
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See signature.txt for more information. |
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|
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Simon Glass |
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sjg@chromium.org |
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1-1-13 |
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@ -0,0 +1,3 @@ |
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/*.dtb |
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/test.fit |
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/dev-keys |
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@ -0,0 +1,7 @@ |
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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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@ -0,0 +1,7 @@ |
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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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}; |
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@ -0,0 +1,45 @@ |
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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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|
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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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@ -0,0 +1,42 @@ |
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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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@ -0,0 +1,126 @@ |
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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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