Network-on-Chip Microarchitecture-based Covert Channel in GPUs

As GPUs are becoming widely deployed in the cloud infrastructure to support different application domains, the security concerns of GPUs are becoming increasingly important. In particular, the support for multiprogramming in modern GPUs has led to new vulnerabilities since multiple kernels in a GPU can be executed at the same time. In this work, we propose a new microarchitectural timing covert channel for GPUs that can be established based on the shared, on-chip interconnect channels. We first reverse-engineer the organization of the on-chip networks in modern GPUs to understand the core placements throughout the GPU. The hierarchical organization of the GPU results in the sharing of interconnect bandwidth between neighboring cores. Based on this understanding, we identify how contention for the interconnect bandwidth can be exploited for a novel covert channel attack. We propose two types of interconnect-based covert channels that exploit the on-chip network hierarchy. Unlike cache-based covert channels, no states of the on-chip network need to be modified for communication in our interconnect-based covert channel and the impact of contention is very predictable. By exploiting the parallelism of GPUs, our proposed covert channel results in very high bandwidth - achieving approximately 24 Mbps of bandwidth on NVIDIA Volta GPUs and results in one of the highest known microarchitectural covert channel bandwidth.