a1b6b0a9c1
The patch implements secure booting for the mvebu architecture. This includes: - The addition of secure headers and all needed signatures and keys in mkimage - Commands capable of writing the board's efuses to both write the needed cryptographic data and enable the secure booting mechanism - The creation of convenience text files containing the necessary commands to write the efuses The KAK and CSK keys are expected to reside in the files kwb_kak.key and kwb_csk.key (OpenSSL 2048 bit private keys) in the top-level directory. Signed-off-by: Reinhard Pfau <reinhard.pfau@gdsys.cc> Signed-off-by: Mario Six <mario.six@gdsys.cc> Reviewed-by: Stefan Roese <sr@denx.de> Reviewed-by: Simon Glass <sjg@chromium.org> Signed-off-by: Stefan Roese <sr@denx.de>
374 lines
17 KiB
Plaintext
374 lines
17 KiB
Plaintext
The trusted boot framework on Marvell Armada 38x
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================================================
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Contents:
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1. Overview of the trusted boot
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2. Terminology
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3. Boot image layout
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4. The secured header
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5. The secured boot flow
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6. Usage example
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7. Work to be done
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8. Bibliography
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1. Overview of the trusted boot
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-------------------------------
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The Armada's trusted boot framework enables the SoC to cryptographically verify
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a specially prepared boot image. This can be used to establish a chain of trust
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from the boot firmware all the way to the OS.
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To achieve this, the Armada SoC requires a specially prepared boot image, which
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contains the relevant cryptographic data, as well as other information
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pertaining to the boot process. Furthermore, a eFuse structure (a
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one-time-writeable memory) need to be configured in the correct way.
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Roughly, the secure boot process works as follows:
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* Load the header block of the boot image, extract a special "root" public RSA
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key from it, and verify its SHA-256 hash against a SHA-256 stored in a eFuse
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field.
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* Load an array of code signing public RSA keys from the header block, and
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verify its RSA signature (contained in the header block as well) using the
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"root" RSA key.
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* Choose a code signing key, and use it to verify the header block (excluding
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the key array).
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* Verify the binary image's signature (contained in the header block) using the
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code signing key.
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* If all checks pass successfully, boot the image.
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The chain of trust is thus as follows:
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* The SHA-256 value in the eFuse field verifies the "root" public key.
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* The "root" public key verifies the code signing key array.
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* The selected code signing key verifies the header block and the binary image.
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In the special case of building a boot image containing U-Boot as the binary
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image, which employs this trusted boot framework, the following tasks need to
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be addressed:
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1. Creation of the needed cryptographic key material.
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2. Creation of a conforming boot image containing the U-Boot image as binary
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image.
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3. Burning the necessary eFuse values.
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(1) will be addressed later, (2) will be taken care of by U-Boot's build
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system (some user configuration is required, though), and for (3) the necessary
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data (essentially a series of U-Boot commands to be entered at the U-Boot
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command prompt) will be created by the build system as well.
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The documentation of the trusted boot mode is contained in part 1, chapter
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7.2.5 in the functional specification [1], and in application note [2].
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2. Terminology
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--------------
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CSK - Code Signing Key(s): An array of RSA key pairs, which
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are used to sign and verify the secured header and the
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boot loader image.
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KAK - Key Authentication Key: A RSA key pair, which is used
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to sign and verify the array of CSKs.
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Header block - The first part of the boot image, which contains the
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image's headers (also known as "headers block", "boot
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header", and "image header")
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eFuse - A one-time-writeable memory.
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BootROM - The Armada's built-in boot firmware, which is
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responsible for verifying and starting secure images.
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Boot image - The complete image the SoC's boot firmware loads
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(contains the header block and the binary image)
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Main header - The header in the header block containing information
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and data pertaining to the boot process (used for both
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the regular and secured boot processes)
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Binary image - The binary code payload of the boot image; in this
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case the U-Boot's code (also known as "source image",
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or just "image")
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Secured header - The specialized header in the header block that
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contains information and data pertaining to the
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trusted boot (also known as "security header")
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Secured boot mode - A special boot mode of the Armada SoC in which secured
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images are verified (non-secure images won't boot);
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the mode is activated by setting a eFuse field.
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Trusted debug mode - A special mode for the trusted boot that allows
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debugging of devices employing the trusted boot
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framework in a secure manner (untested in the current
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implementation).
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Trusted boot framework - The ARMADA SoC's implementation of a secure verified
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boot process.
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3. Boot image layout
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--------------------
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+-- Boot image --------------------------------------------+
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| |
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| +-- Header block --------------------------------------+ |
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| | Main header | |
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| +------------------------------------------------------+ |
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| | Secured header | |
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| +------------------------------------------------------+ |
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| | BIN header(s) | |
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| +------------------------------------------------------+ |
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| | REG header(s) | |
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| +------------------------------------------------------+ |
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| | Padding | |
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| +------------------------------------------------------+ |
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| |
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| +------------------------------------------------------+ |
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| | Binary image + checksum | |
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| +------------------------------------------------------+ |
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+----------------------------------------------------------+
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4. The secured header
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---------------------
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For the trusted boot framework, a additional header is added to the boot image.
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The following data are relevant for the secure boot:
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KAK: The KAK is contained in the secured header in the form
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of a RSA-2048 public key in DER format with a length of
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524 bytes.
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Header block signature: The RSA signature of the header block (excluding the
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CSK array), created using the selected CSK.
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Binary image signature: The RSA signature of the binary image, created using
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the selected CSK.
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CSK array: The array of the 16 CSKs as RSA-2048 public keys in DER
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format with a length of 8384 = 16 * 524 bytes.
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CSK block signature: The RSA signature of the CSK array, created using the
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KAK.
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NOTE: The JTAG delay, Box ID, and Flash ID header fields do play a role in the
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trusted boot process to enable and configure secure debugging, but they were
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not tested in the current implementation of the trusted boot in U-Boot.
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5. The secured boot flow
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------------------------
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The steps in the boot flow that are relevant for the trusted boot framework
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proceed as follows:
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1) Check if trusted boot is enabled, and perform regular boot if it is not.
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2) Load the secured header, and verify its checksum.
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3) Select the lowest valid CSK from CSK0 to CSK15.
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4) Verify the SHA-256 hash of the KAK embedded in the secured header.
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5) Verify the RSA signature of the CSK block from the secured header with the
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KAK.
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6) Verify the header block signature (which excludes the CSK block) from the
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secured header with the selected CSK.
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7) Load the binary image to the main memory and verify its checksum.
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8) Verify the binary image's RSA signature from the secured header with the
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selected CSK.
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9) Continue the boot process as in the case of the regular boot.
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NOTE: All RSA signatures are verified according to the PKCS #1 v2.1 standard
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described in [3].
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NOTE: The Box ID and Flash ID are checked after step 6, and the trusted debug
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mode may be entered there, but since this mode is untested in the current
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implementation, it is not described further.
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6. Usage example
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----------------
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### Create key material
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To employ the trusted boot framework, cryptographic key material needs to be
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created. In the current implementation, two keys are needed to build a valid
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secured boot image: The KAK private key and a CSK private key (both have to be
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2048 bit RSA keys in PEM format). Note that the usage of more than one CSK is
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currently not supported.
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NOTE: Since the public key can be generated from the private key, it is
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sufficient to store the private key for each key pair.
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OpenSSL can be used to generate the needed files kwb_kak.key and kwb_csk.key
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(the names of these files have to be configured, see the next section on
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kwbimage.cfg settings):
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openssl genrsa -out kwb_kak.key 2048
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openssl genrsa -out kwb_csk.key 2048
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The generated files have to be placed in the U-Boot root directory.
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Alternatively, instead of copying the files, symlinks to the private keys can
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be placed in the U-Boot root directory.
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WARNING: Knowledge of the KAK or CSK private key would enable an attacker to
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generate secured boot images containing arbitrary code. Hence, the private keys
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should be carefully guarded.
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### Create/Modifiy kwbimage.cfg
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The Kirkwook architecture in U-Boot employs a special board-specific
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configuration file (kwbimage.cfg), which controls various boot image settings
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that are interpreted by the BootROM, such as the boot medium. The support the
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trusted boot framework, several new options were added to faciliate
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configuration of the secured boot.
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The configuration file's layout has been retained, only the following new
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options were added:
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KAK - The name of the KAK RSA private key file in the U-Boot
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root directory, without the trailing extension of ".key".
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CSK - The name of the (active) CSK RSA private key file in the
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U-Boot root directory, without the trailing extension of
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".key".
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BOX_ID - The BoxID to be used for trusted debugging (a integer
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value).
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FLASH_ID - The FlashID to be used for trusted debugging (a integer
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value).
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JTAG_DELAY - The JTAG delay to be used for trusted debugging (a
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integer value).
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CSK_INDEX - The index of the active CSK (a integer value).
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SEC_SPECIALIZED_IMG - Flag to indicate whether to include the BoxID and FlashID
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in the image (that is, whether to use the trusted debug
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mode or not); no parameters.
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SEC_BOOT_DEV - The boot device from which the trusted boot is allowed to
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proceed, identified via a numeric ID. The tested values
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are 0x34 = NOR flash, 0x31 = SDIO/MMC card; for
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additional ID values, consult the documentation in [1].
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SEC_FUSE_DUMP - Dump the "fuse prog" commands necessary for writing the
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correct eFuse values to a text file in the U-Boot root
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directory. The parameter is the architecture for which to
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dump the commands (currently only "a38x" is supported).
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The parameter values may be hardcoded into the file, but it is also possible to
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employ a dynamic approach of creating a Autoconf-like kwbimage.cfg.in, then
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reading configuration values from Kconfig options or from the board config
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file, and generating the actual kwbimage.cfg from this template using Makefile
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mechanisms (see board/gdsys/a38x/Makefile as an example for this approach).
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### Set config options
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To enable the generation of trusted boot images, the corresponding support
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needs to be activated, and a index for the active CSK needs to be selected as
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well.
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Furthermore, eFuse writing support has to be activated in order to burn the
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eFuse structure's values (this option is just needed for programming the eFuse
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structure; production boot images may disable it).
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ARM architecture
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-> [*] Build image for trusted boot
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(0) Index of active CSK
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-> [*] Enable eFuse support
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[ ] Fake eFuse access (dry run)
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### Build and test boot image
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The creation of the boot image is done via the usual invocation of make (with a
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suitably set CROSS_COMPILE environment variable, of course). The resulting boot
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image u-boot-spl.kwb can then be tested, if so desired. The hdrparser from [5]
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can be used for this purpose. To build the tool, invoke make in the
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'tools/marvell/doimage_mv' directory of [5], which builds a stand-alone
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hdrparser executable. A test can be conducted by calling hdrparser with the
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produced boot image and the following (mandatory) parameters:
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./hdrparser -k 0 -t u-boot-spl.kwb
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Here we assume that the CSK index is 0 and the boot image file resides in the
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same directory (adapt accordingly if needed). The tool should report that all
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checksums are valid ("GOOD"), that all signature verifications succeed
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("PASSED"), and, finally, that the overall test was successful
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("T E S T S U C C E E D E D" in the last line of output).
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### Burn eFuse structure
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+----------------------------------------------------------+
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| WARNING: Burning the eFuse structure is a irreversible |
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| operation! Should wrong or corrupted values be used, the |
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| board won't boot anymore, and recovery is likely |
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| impossible! |
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+----------------------------------------------------------+
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After the build process has finished, and the SEC_FUSE_DUMP option was set in
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the kwbimage.cfg was set, a text file kwb_fuses_a38x.txt should be present in
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the U-Boot top-level directory. It contains all the necessary commands to set
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the eFuse structure to the values needed for the used KAK digest, as well as
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the CSK index, Flash ID and Box ID that were selected in kwbimage.cfg.
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Sequentially executing the commands in this file at the U-Boot command prompt
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will write these values to the eFuse structure.
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If the SEC_FUSE_DUMP option was not set, the commands needed to burn the fuses
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have to be crafted by hand. The needed fuse lines can be looked up in [1]; a
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rough overview of the process is:
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* Burn the KAK public key hash. The hash itself can be found in the file
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pub_kak_hash.txt in the U-Boot top-level directory; be careful to account for
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the endianness!
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* Burn the CSK selection, BoxID, and FlashID
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* Enable trusted boot by burning the corresponding fuse (WARNING: this must be
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the last fuse line written!)
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* Lock the unused fuse lines
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The command to employ is the "fuse prog" command previously enabled by setting
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the corresponding configuration option.
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For the trusted boot, the fuse prog command has a special syntax, since the
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ARMADA SoC demands that whole fuse lines (64 bit values) have to be written as
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a whole. The fuse prog command itself allows lists of 32 bit words to be
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written at a time, but this is translated to a series of single 32 bit write
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operations to the fuse line, where the individual 32 bit words are identified
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by a "word" counter that is increased for each write.
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To work around this restriction, we interpret each line to have three "words"
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(0-2): The first and second words are the values to be written to the fuse
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line, and the third is a lock flag, which is supposed to lock the fuse line
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when set to 1. Writes to the first and second words are memoized between
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function calls, and the fuse line is only really written and locked (on writing
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the third word) if both words were previously set, so that "incomplete" writes
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are prevented. An exception to this is a single write to the third word (index
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2) without previously writing neither the first nor the second word, which
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locks the fuse line without setting any value; this is needed to lock the
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unused fuse lines.
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As an example, to write the value 0011223344556677 to fuse line 10, we would
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use the following command:
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fuse prog -y 10 0 00112233 44556677 1
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Here 10 is the fuse line number, 0 is the index of the first word to be
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written, 00112233 and 44556677 are the values to be written to the fuse line
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(first and second word) and the trailing 1 is the value for the third word
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responsible for locking the line.
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A "lock-only" command would look like this:
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fuse prog -y 11 2 1
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Here 11 is the fuse number, 2 is the index of the first word to be written
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(notice that we only write to word 2 here; the third word for fuse line
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locking), and the 1 is the value for the word we are writing to.
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WARNING: According to application note [4], the VHV pin of the SoC must be
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connected to a 1.8V source during eFuse programming, but *must* be disconnected
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for normal operation. The AN [4] describes a software-controlled circuit (based
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on a N-channel or P-channel FET and a free GPIO pin of the SoC) to achieve
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this, but a jumper-based circuit should suffice as well. Regardless of the
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chosen circuit, the issue needs to be addressed accordingly!
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7. Work to be done
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------------------
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* Add the ability to populate more than one CSK
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* Test secure debug
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* Test on Armada XP
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8. Bibliography
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---------------
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[1] ARMADA(R) 38x Family High-Performance Single/Dual CPU System on Chip
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Functional Specification; MV-S109094-00, Rev. C; August 2, 2015,
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Preliminary
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[2] AN-383: ARMADA(R) 38x Families Secure Boot Mode Support; MV-S302501-00
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Rev. A; March 11, 2015, Preliminary
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[3] Public-Key Cryptography Standards (PKCS) #1: RSA Cryptography
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Specifications Version 2.1; February 2003;
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https://www.ietf.org/rfc/rfc3447.txt
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[4] AN-389: ARMADA(R) VHV Power; MV-S302545-00 Rev. B; January 28, 2016,
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Released
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[5] Marvell Armada 38x U-Boot support; November 25, 2015;
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https://github.com/MarvellEmbeddedProcessors/u-boot-marvell
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2017-01-05, Mario Six <mario.six@gdsys.cc>
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