SMM Code PCR Reset for Secure Kernel Initialization
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Solution Overview
Problem
The existing boot process in computing devices lacks effective detection of malicious code during the initial stages, leading to potential security risks due to the complexity of evaluating the Trusted Computing Base (TCB), which can result in insignificant deviations causing uncertainty in determining a trusted state.
Innovation Solution
The use of System Management Mode (SMM) code to reset and manage Platform Configuration Registers (PCRs) in the Trusted Platform Module (TPM), allowing for dynamic measurement and verification of the TCB, thereby simplifying the trust evaluation and enhancing security by limiting modifications to specific CPU instructions and SMM code.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If all measurements taken during the booting process are combined to define the Trusted Computing Base, then comprehensive security coverage is achieved, but the complexity of evaluating the TCB increases beyond reasonable limits
Solution Approach 1:
The patent segments the TCB evaluation by introducing a boundary measurement that divides the boot process measurements into two distinct sets: measurements taken before the boundary (first set) and measurements taken after the boundary (second set). This segmentation allows the system to selectively combine only relevant measurements from the second set with the boundary measurement itself, thereby reducing the overall TCB complexity while maintaining security coverage for critical post-boundary operations.
2Measurement precision
If the TCB includes all code executed since the machine was booted, then complete trust verification is possible, but the size of the TCB becomes excessively large and difficult to manage
Solution Approach 1:
The patent extracts and isolates the boundary measurement from the complete boot process measurements. By taking out this specific measurement point that marks a significant transition in the boot process, the system can focus TCB evaluation on the critical second set of measurements after the boundary, rather than including all measurements from the entire boot process. This extraction reduces TCB size while preserving trust verification accuracy for the most critical operational phase.
3Adaptability or versatility
If SMM code is included in the TCB with superior privileges, then comprehensive hardware control is achieved, but the security risk increases due to inability to prevent SMM execution by the operating system
Solution Approach 1:
The patent applies preliminary action by measuring and recording the SMM code execution state at the boundary measurement point before the operating system can potentially interfere with or prevent SMM execution. This preliminary measurement captures the intended SMM behavior in a trusted state, allowing the system to verify later whether SMM code executed as expected. This approach maintains hardware control capability while reducing security risk by establishing a known-good baseline for SMM operations.
Data Source
AI summary
Dynamic Root of Trust for Measurement (DRTM) mechanisms can be initiated, not by CPU-manufacturer-specific instructions, but by the execution of code in System Management Mode (SMM) that can modify the values stored in specific Platform Configuration Registers (PCRs) of a Trusted Platform Module (TPM). The SMM code can be verified prior to execution and it can be trusted based on the secure mechanisms used to update such code. The SMM code can restore a known, trusted state of the computing device and can initiate the measuring of subsequently executed code. In such a manner the Trusted Computing Base (TCB) can be limited.