Understanding Trusted Firmware in Confidential Computing: Coconut SVSM and VirTEE 

Author:  Sal Kimmich

Trusted Firmware serves as the foundational layer in confidential computing, ensuring that the hardware and software environment’s security and integrity are maintained. Unlike regular firmware, Trusted Firmware is designed with additional security features and responsibilities to establish a Trusted Execution Environment (TEE). Here’s a deeper dive into what makes Trusted Firmware different and its role in confidential computing.

Differences Between Trusted Firmware and Regular Firmware

1. Enhanced Security Features:
   • Regular Firmware: Primarily focuses on initializing hardware components and providing basic services to the operating system.
   • Trusted Firmware: Includes enhanced security features such as cryptographic verification of firmware components, secure boot, and mechanisms to enforce hardware-based isolation of secure and non-secure execution environments.
2. Isolation and Trust:
   • Regular Firmware: Does not inherently provide mechanisms to isolate critical operations or sensitive data from the rest of the system.
   • Trusted Firmware: Establishes a TEE, isolating sensitive operations from the general-purpose operating system and protecting them from potential threats and unauthorized access.
3. Responsibility and Scope:
   • Regular Firmware: Manages standard hardware initialization and operational tasks.
   • Trusted Firmware: Manages secure initialization of hardware features, authenticates and validates software components, and provides a secure execution environment for critical tasks.

Why Trusted Firmware is Necessary

Trusted Firmware is crucial for confidential computing because it provides a secure foundation that prevents unauthorized access and tampering. Here’s why Trusted Firmware is needed and how it differs from the regular OS and firmware:

Need for Trusted OS:

• Purpose: To prevent resources from being accessed directly by the generalist OS running concurrently with it, such as preventing a user with root privileges from accessing sensitive resources.
• Security: The Trusted OS operates with higher privileges and tighter security controls, ensuring that critical operations and data are protected even if the general OS is compromised.

Differences from Normal OS:

• Size and Scope: The Trusted OS is designed to be small and secure, running with higher privileges than the general OS. For instance, in an ARMv8-a system, parts of the Trusted OS run at EL3 (highest privilege), while a hypervisor runs at EL2, and Linux at EL1.
• Purpose: The Trusted OS is not meant to replace the general OS like Linux, which is extensive and feature-rich. Instead, it secures specific resources and operations from the general OS.

Security Provided by Trusted OS:

• Threat Protection: It protects against attempts by users of the general OS to access resources managed by the Trusted OS, including both legitimate and illegitimate access attempts.
• Mechanism: It uses secure mechanisms, such as the SMC instruction, to switch between the general OS and the Trusted OS when necessary to access secure resources.

Switching Between Trusted OS and Normal World:

• Context Switching: Occurs when code running in the general OS needs to access a resource managed by the Trusted OS, such as decrypting content using a key only accessible by the Trusted OS.
• Interrupt Handling: Hardware interrupts may also trigger a switch to the Trusted OS, allowing safe handling of interrupts within the TEE context.

Example Projects

COCONUT Secure VM Service Module (SVSM)

The COCONUT Secure VM Service Module (SVSM) exemplifies Trusted Firmware in confidential computing by providing secure services and device emulations for Confidential Virtual Machines (CVMs). Key features include:

• Integration with AMD SEV-SNP: Utilizes AMD’s Secure Encrypted Virtualization with Secure Nested Paging, including the VM Privilege Level feature, to ensure robust hardware-based security.
• Secure Boot and Authentication: Ensures a secure boot process and component authentication, maintaining a trusted execution path from the firmware to the CVM.

VirTEE

VirTEE is another project that demonstrates the application of Trusted Firmware principles. It focuses on:

• Open Community Development: Collaborative development of tools for TEE bring-up, attestation, and management, supporting a wide range of virtualization platforms.
• Support for Multiple Technologies: Includes tools and libraries for AMD SEV, SEV-SNP, and Intel TDX, providing comprehensive support for secure virtualization across different hardware platforms.

Discover more about VirTEE via their project repository. 

Conclusion

Trusted Firmware is essential for establishing and maintaining secure and reliable confidential computing environments. It provides enhanced security features, isolation, and trust mechanisms that are not present in regular firmware. Projects like COCONUT-SVSM and VirTEE illustrate the practical application of Trusted Firmware principles, showcasing robust frameworks for secure virtualized environments and cross-platform confidential computing. These projects ensure the integrity and confidentiality of sensitive data and operations, advancing the field of secure computing.