Hypervisor Backdoor Interface via Debug Instruction Trapping
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Solution Overview
Problem
Microprocessor architectures that do not support certain instructions hinder the implementation of a backdoor interface between software in a virtual machine and the hypervisor, limiting the functionality of software tools that require direct access to hypervisor components, especially for user mode software executing in unprivileged mode.
Innovation Solution
The method involves trapping a debug instruction exception at the hypervisor, identifying an equivalence between the instruction's immediate operand and a predefined value, and invoking a backdoor service using CPU register state as input, allowing a backdoor interface between software in a virtual machine and the hypervisor, even on architectures where such direct access is not normally permitted.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of operation
If hypercall instructions are used for backdoor interface access, then direct hypervisor access is enabled, but user mode software cannot execute these instructions due to privilege level restrictions
Solution Approach 1:
The patent introduces a kernel mode intermediary layer that translates unprivileged user mode debug instructions into privileged hypercall instructions. This intermediary mechanism allows user mode software to indirectly access the hypervisor without requiring direct execution of privileged instructions, thereby resolving the contradiction between accessibility and execution capability.
Solution Approach 2:
The patent adds a new dimension to the backdoor interface by implementing exception-based indirect calling conventions. Instead of direct instruction execution in the traditional privilege dimension, the system uses exception handling as an additional dimension to bridge user mode and hypervisor, allowing unprivileged software to invoke privileged services through controlled exception paths.
2Reliability
If microprocessor architectures support only privileged mode hypercall instructions, then security is maintained, but user mode software tools become non-functional
Solution Approach 1:
A kernel mode intermediary component is introduced that acts as a secure gateway between user mode software and the hypervisor. This intermediary validates and translates debug instructions into proper hypercall sequences, maintaining security protocols while enabling full software tool functionality in user mode without requiring privileged instruction execution.
Solution Approach 2:
The backdoor interface is segmented into multiple layers: user mode interface layer, kernel mode translation layer, and hypervisor execution layer. This segmentation allows each layer to operate within its privilege constraints while collectively achieving the goal of enabling user mode software tools to access hypervisor services through controlled, secure pathways.
Data Source
AI summary
A method of providing a backdoor interface between software executing in a virtual machine and a hypervisor executing on a computing system that supports the virtual machine includes trapping, at the hypervisor, an exception generated in response to execution of a debug instruction on a central processing unit (CPU) by the software; identifying, by an exception handler of the hypervisor handling the exception, an equivalence between an immediate operand of the debug instruction and a predefined value; and invoking, in response to the equivalence, a backdoor service of the hypervisor using state of at least one register of the CPU as parametric input, the state being set by the software prior to executing the debug instruction.