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104 lines
4.5 KiB
Text
104 lines
4.5 KiB
Text
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 be 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 already 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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