Protecting Confidential Virtual Machines from Hardware Performance Counter Side Channels

In modern cloud platforms, it is becoming more important to preserve the privacy of guest virtual machines (VMs) from the untrusted host. To this end, Secure Encrypted Virtualization (SEV) is developed as a hardware extension to protect VMs by encrypting their memory pages and register states. Unfortunately, such confidential VMs are still vulnerable to micro-architectural side channels, and Hardware Performance Counters (HPCs) are a prominent information leakage source. To make matters worse, currently there is no systematic defense against the HPC side channels. We introduce Aegis, a unified framework for demystifying the inherent relations between the instruction execution and HPC event statistics, and defending VMs against HPC side channels with provable privacy guarantee and minimal performance overhead. Aegis consists of three modules. Application Profiler profiles the application offline and adopts information theory to quantitatively estimate the vulnerability of HPC events. Event Fuzzer leverages the fuzzing technique to automatically generate interesting inputs, i.e., instruction sequences, that can effectively alter the HPC observations. Event Obfuscator injects noisy instructions into the protected VM based on the differential privacy mechanisms for high efficiency and privacy. We present three case studies to demonstrate that Aegis can defeat different types of HPC side-channel attacks (i.e., website fingerprinting, DNN model extraction, keystroke sniffing). Evaluations show that Aegis can effectively decrease the attack accuracy from 90% to 2%, with only 3% overhead on the application execution time and 7% overhead on the CPU usage.