With the rise of AI, end of Moore's law, and the digitization of public services, the demand for accelerated computing is growing. To address this demand, major cloud service providers like Amazon Web Services, Microsoft Azure, and Google Cloud Platform have incorporated FPGA instances into their infrastructure with efficient and adaptable resource allocation models. Interest is increasing in multi-tenant FPGAs, which enable multiple users to utilize FPGA resources concurrently, while the FPGA can be split into smaller sections, one per tenant. Nevertheless, it introduces significant security vulnerabilities. For instance, by configuring a malicious circuit in one tenant's section of the FPGA, attacks that cause faults or crash the entire FPGA become feasible, affecting other tenants. By splitting an FPGA into smaller fractions, a single tenant has less potential to cause catastrophic outcomes. However, in this paper, we propose another threat, which is to perform an attack where several malicious tenants coordinate an attack using an unintended covert channel. We practically verify this possibility and introduce such a synchronized and coordinated voltage drop attack from multiple malicious tenants. For synchronization, the malicious tenants use a voltage-based covert channel. Our results show that the communication is robust reaching less than 1% packet error rate and that the attack is successful and avoids state-of-the-art countermeasures.

The adoption of FPGA instances by major cloud service providers (CSPs) reflects the growing demand for accelerated and heterogeneous computing across various applications, e.g., AI. To improve the efficiency, utilization and virtualization, multi-tenant FPGAs allow multiple users to utilize FPGA resources concurrently, with each FPGA partition assigned to a separate tenant. However, this introduces significant security vulnerabilities, such as the potential for attacks by configuring a malicious circuit in one tenant's FPGA partition. One notable vulnerability is disrupting the FPGA's power distribution network, leading to faults or even crashing the entire FPGA, affecting other tenants. Usually, such an attack requires a considerable amount of resources. A naive solution would be splitting an FPGA into smaller fractions to reduce the potential for successful Power-Hammering by individual tenants and enhance the security. However, our paper demonstrates that even with smaller fractions per tenant, attacks can still occur. We propose the threat of coordinated attacks, where malicious tenants use an unintended covert channel between them. We practically validate this threat in a real cloud computing environment by introducing a synchronized and coordinated power-hammering attack from multiple malicious tenants. These tenants synchronize their actions using a voltage-based covert channel. Our results reveal the success of the attack, surpassing state-of-the-art countermeasures and detection mechanisms with a success rate exceeding 90%, compared to 30% for uncoordinated attacks.