CVE-2021-1104 in Instruction Set Manual
Summary
by MITRE • 08/14/2021
The RISC-V Instruction Set Manual contains a documented ambiguity for the Machine Trap Vector Base Address (MTVEC) register that may lead to a vulnerability due to the initial state of the register not being defined, potentially leading to information disclosure, data tampering and denial of service.
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Analysis
by VulDB Data Team • 08/18/2021
The vulnerability identified as CVE-2021-1104 resides within the RISC-V Instruction Set Manual and specifically targets the Machine Trap Vector Base Address (MTVEC) register functionality. This weakness represents a design ambiguity that stems from the undefined initial state of the MTVEC register, creating potential security implications for systems implementing the RISC-V architecture. The MTVEC register serves as a critical component in the trap handling mechanism of RISC-V processors, determining where the processor should jump when encountering trap conditions such as interrupts, illegal instructions, or page faults. The ambiguity arises because the specification does not clearly define what value the MTVEC register should contain upon system reset or initialization, leaving implementers with discretion in their interpretation of the register's default state.
The technical flaw manifests when systems fail to properly initialize or validate the MTVEC register state during boot processes, potentially allowing unauthorized access to trap handling mechanisms. This vulnerability creates a pathway for attackers to manipulate the processor's behavior by exploiting the undefined register state, which could result in unpredictable execution flows. When the MTVEC register contains an unexpected value, it may cause the processor to jump to unintended memory locations during trap handling, potentially enabling privilege escalation attacks or allowing malicious code to redirect execution flow. The ambiguity also affects the consistency of system behavior across different implementations, as various hardware vendors might interpret the register's initial state differently, creating potential security gaps in deployed systems.
The operational impact of this vulnerability spans multiple security domains including information disclosure, data tampering, and denial of service conditions. Attackers could exploit the undefined MTVEC register state to gain unauthorized access to sensitive system information by manipulating trap handlers to redirect execution to memory locations containing confidential data. The vulnerability also enables data tampering scenarios where malicious actors could modify system behavior through trap vector manipulation, potentially corrupting system state or executing unauthorized code sequences. Furthermore, the inconsistency in register initialization across different implementations creates a denial of service risk, as systems may exhibit unpredictable behavior when encountering trap conditions, potentially leading to system crashes or lockups that prevent normal operation.
Mitigation strategies for CVE-2021-1104 require careful attention to proper register initialization during system boot processes and implementation of robust validation mechanisms. System designers should ensure that the MTVEC register is explicitly initialized to a known safe value during system startup, typically pointing to a legitimate trap handler vector or a secure exception handling routine. Hardware implementations must be updated to clearly define and document the initial state of the MTVEC register, eliminating the ambiguity that creates this vulnerability. Software developers should implement comprehensive trap handling routines that can detect and recover from unexpected MTVEC register states, incorporating runtime checks that verify the register's validity before executing critical system operations. Additionally, system administrators should conduct thorough security assessments of RISC-V implementations to identify and remediate any instances where the MTVEC register is not properly initialized, particularly in security-sensitive applications such as embedded systems, IoT devices, or critical infrastructure components.
This vulnerability aligns with CWE-755 weakness category, specifically addressing improper initialization of resources, and relates to ATT&CK technique T1059.001 for executing malicious code through system manipulation. The ambiguity in register state definition represents a fundamental design flaw that violates the principle of least privilege and can lead to privilege escalation scenarios when exploited in conjunction with other vulnerabilities. The issue also connects to broader security concerns in the RISC-V ecosystem, where consistent interpretation of architectural specifications is crucial for maintaining security guarantees across different implementations and vendors.