Software Architecture
Arm Corstone-1000
Arm Corstone-1000 is a reference solution for IoT devices. It is part of Total Solution for IoT which consists of hardware and software reference implementation.
Corstone-1000 software plus hardware reference solution is PSA Level-2 ready certified (PSA L2 Ready) as well as System Ready IR certified(SRIR cert). More information on the Corstone-1000 subsystem product and design can be found at: Arm Corstone-1000 Software and Arm Corstone-1000 Technical Overview.
This readme explicitly focuses on the software part of the solution and provides internal details on the software components. The reference software package of the platform can be retrieved following instructions present in the user-guide document.
Design Overview
The software architecture of Corstone-1000 platform is a reference implementation of Platform Security Architecture (PSA) which provides framework to build secure IoT devices.
The base system architecture of the platform is created from three different types of systems: Secure Enclave, Host and External System. Each subsystem provides different functionality to overall SoC.
The Secure Enclave System, provides PSA Root of Trust (RoT) and cryptographic functions. It is based on a Cortex-M0+ processor, CC312 Cryptographic Accelerator and peripherals, such as watchdog and secure flash. Software running on the Secure Enclave is isolated via hardware for enhanced security. Communication with the Secure Encalve is achieved using Message Handling Units (MHUs) and shared memory. On system power on, the Secure Enclave boots first. Its software comprises of a ROM code (TF-M BL1), MCUboot BL2, and TrustedFirmware-M(TF-M) as runtime software. The software design on Secure Enclave follows Firmware Framework for M class processor (FF-M) specification.
The Host System is based on ARM Cortex-A35 processor with standardized peripherals to allow for the booting of a Linux OS. The Cortex-A35 has the TrustZone technology that allows secure and non-secure security states in the processor. The software design in the Host System follows Firmware Framework for A class processor (FF-A) specification. The boot process follows Trusted Boot Base Requirement (TBBR). The Host Subsystem is taken out of reset by the Secure Enclave system during its final stages of the initialization. The Host subsystem runs FF-A Secure Partitions(based on Trusted Services) and OPTEE-OS (OPTEE-OS) in the secure world, and U-Boot(U-Boot repo) and linux (linux repo) in the non-secure world. The communication between non-secure and the secure world is performed via FF-A messages.
An external system is intended to implement use-case specific functionality. The system is based on Cortex-M3 and run RTX RTOS. Communication between the external system and Host (Cortex-A35) can be performed using MHU as transport mechanism. The current software release supports switching on and off the external system. Support for OpenAMP-based communication is under development.
Overall, the Corstone-1000 architecture is designed to cover a range of Power, Performance, and Area (PPA) applications, and enable extension for use-case specific applications, for example, sensors, cloud connectivity, and edge computing.
Secure Boot Chain
For the security of a device, it is essential that only authorized software should run on the device. The Corstone-1000 boot uses a Secure Boot Chain process where an already authenticated image verifies and loads the following software in the chain. For the boot chain process to work, the start of the chain should be trusted, forming the Root of Trust (RoT) of the device. The RoT of the device is immutable in nature and encoded into the device by the device owner before it is deployed into the field. In Corstone-1000, the content of the ROM and CC312 OTP (One Time Programmable) memory forms the RoT.
Verification of an image can happen either by comparing the computed and stored hashes, or by checking the signature of the image if the image is signed.
It is a lengthy chain to boot the software on Corstone-1000. On power on, the Secure Enclave starts executing BL1_1 code from the ROM which is the RoT of the device. The BL1_1 is the immutable bootloader of the system, it handles the provisioning on the first boot, hardware initialization and verification of the next stage.
The BL1_2 code, hashes and keys are written into the OTP during the provisioning. The next bootstage is the BL1_2 which is copied from the OTP into the RAM. The BL1_1 also compares the BL1_2 hash with the hash saved to the OTP. The BL1_2 verifies and transfers control to the next bootstage which is the BL2. During the verification, the BL1_2 compares the BL2 image’s computed hash with the BL2 hash in the OTP. The BL2 is MCUBoot in the system. BL2 can provision additional keys on the first boot and it authenticates the initial bootloader of the host (Host Trusted Firmware-A BL2) and TF-M by checking the signatures of the images. The MCUBoot handles the image verification the following way:
Load image from a non-volatile memory to dynamic RAM.
The public key present in the image header is validated by comparing with the hash. Depending on the image, the hash of the public key is either stored in the OTP or part of the software which is being already verified in the previous stages.
The image is validated using the public key.
The execution control is passed to TF-M after the verification. TF-M being the runtime executable of the Secure Enclave which initializes itself and, at the end, brings the host CPU out of rest.
The TF-M BL1 design details and reasoning can be found in the TF-M design documents.
The Corstone-1000 has some differences compared to this design due to memory (OTP/ROM) limitations:
The provisioning bundle that contains the BL1_2 code is located in the ROM. This means the BL1_2 cannot be updated during provisioning time.
The BL1_1 handles most of the hardware initialization instead of the BL1_2. This results in a bigger BL1_1 code size than needed.
The BL1_2 does not use the post-quantum LMS verification. The BL2 is verified by comparing the computed hash to the hash which is stored in the OTP. This means the BL2 is not updatable.
Host Level Authentication
The host follows the boot standard defined in the TBBR to authenticate the secure and non-secure software.
The Firmware Image Package (FIP) packs bootloader images and other payloads into a single archive. The FIP for Corstone-1000 contains:
Trusted Boot Firmware BL2
EL3 Runtime Firmware BL31
Secure Payload BL32 (Trusted OS)
Non-Trusted Firmware BL33,
TOS_FW_CONFIG
key & content certificates
TF-M does not check the FIP signature, it only checks the Trsuted Firmware-A (TF-A) BL2’s signature in the FIP. The TF-M BL2 (MCUBoot) gets the offset for the TF-A BL2 by parsing the GUID Partition Table (GPT) to find the FIP offset, then parsing the FIP to get the offset for the TF-A BL2. Finally, MCUBoot loads and validates the TF-A BL2 image.
The implicitly trusted components are:
A SHA-256 hash of the Root of Trust Public Key (ROTPK). A development ROTPK is used and its hash embedded into the TF-A BL2 image (only for development purposes). This public key is provided by TF-A source-code.
In case of Corstone-1000, the TF-A BL2 image, can be trusted because it has been verified by the secure enclave’s BL2 (MCUBoot) before starting TF-A.
The remaining components in the Chain of Trust (CoT) are either certificates or bootloader images.
BL images authentication using the FIP certificates:
The certificates are categorized as “Key” and “Content” certificates. The key certificates are used to verify public keys which have been used to sign content certificates. The content certificates are used to store the hash of a boot loader image. An image can be authenticated by calculating its hash and matching it with the hash extracted from the content certificate.
Verification of the certificates:
The public keys defined in the Trusted Key certificate are used to verify the later certificates in the CoT process. The Trusted Key certificate is verified with the Root of Trust Public Key.
For UEFI Secure Boot, authenticated variables can be accessed from the secure flash. The feature has been integrated in U-Boot, which authenticates the images as per the UEFI specification before executing them.
Secure Services
Corstone-1000 is unique in providing a secure environment to run a secure workload. The platform has TrustZone technology in the Host subsystem but it also has hardware isolated Secure Enclave environment to run such secure workloads. In Corstone-1000, known Secure Services such as Crypto, Protected Storage, Internal Trusted Storage and Attestation are available via PSA Functional APIs in TF-M. There is no difference for a user communicating to these services which are running on a Secure Enclave instead of the secure world of the host subsystem. The below diagram presents the data flow path for such calls.
The SE Proxy SP (Secure Enclave Proxy Secure Partition) is a proxy partition managed by OPTEE which forwards such calls to the Secure Enclave. The solution relies on the RSE communication protocol which is a lightweight serialization of the psa_call() API. It can use shared memory and MHU interrupts as a doorbell for communication between two cores but currently the whole message is forwarded through the MHU channels in Corstone-1000. Corstone-1000 implements isolation level 2. Cortex-M0+ MPU (Memory Protection Unit) is used to implement isolation level 2.
For a user to define its own secure service, both the options of the host secure world or secure encalve are available. It’s a trade-off between lower latency vs higher security. Services running on a Secure Enclave are secure by real hardware isolation but have a higher latency path. In the second scenario, the services running on the secure world of the host subsystem have lower latency but virtual hardware isolation created by TrustZone technology.
Secure Firmware Update
Apart from always booting the authorized images, it is also essential that the device only accepts the authorized (signed) images in the firmware update process. Corstone-1000 supports OTA (Over the Air) firmware updates and follows Platform Security Firmware Update specification (FWU).
As standardized into FWU, the external flash is divided into two banks of which one bank has currently running images and the other bank is used for staging new images. There are four updatable units, i.e. Secure Enclave’s BL2 and TF-M, and Host’s FIP (Firmware Image Package) and Kernel Image (the initramfs bundle). The new images are accepted in the form of a UEFI capsule.
When Firmware update is triggered, U-Boot verifies the capsule by checking the capsule signature, version number and size. Then it signals the Secure Enclave that can start writing UEFI capsule into the flash.
Once this operation finishes, Secure Enclave resets the entire system. The Metadata Block in the flash has the below firmware update state machine. TF-M runs an OTA service that is responsible for accepting and updating the images in the flash. The communication between the UEFI Capsule update subsystem and the OTA service follows the same data path explained above. The OTA service writes the new images to the passive bank after successful capsule verification. It changes the state of the system to trial state and triggers the reset.
Boot loaders in Secure Enclave and Host read the Metadata block to get the information on the boot bank. In the successful trial stage, the acknowledgment from the host moves the state of the system from trial to regular. Any failure in the trial stage or system hangs leads to a system reset. This is made sure by the use of watchdog hardware. The Secure Enclave’s BL1 has the logic to identify multiple resets and eventually switch back to the previous good bank. The ability to revert to the previous bank is crucial to guarantee the availability of the device.
UEFI Runtime Support in U-Boot
Implementation of UEFI boottime and runtime APIs require variable storage. In Corstone-1000, these UEFI variables are stored in the Protected Storage service. The below diagram presents the data flow to store UEFI variables. The U-Boot implementation of the UEFI subsystem uses the U-Boot FF-A driver to communicate with the SMM Service in the secure world. The backend of the SMM service uses the proxy PS from the SE Proxy SP. From there on, the PS calls are forwarded to the Secure Enclave as explained above.
References
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