Remote Direct Memory Access (RDMA) is a technology that allows direct access from the network to a machine's main memory without involving its CPU. RDMA offers low-latency, high-bandwidth performance and low CPU utilization. While RDMA provides massive performance boosts and has thus been adopted by several major cloud providers, security concerns have so far been neglected.

The need for RDMA NICs to bypass CPU and directly access memory results in them storing various metadata like page table entries in their on-board SRAM. When the SRAM is full, RNICs swap metadata to main memory across the PCIe bus. We exploit the resulting timing difference to establish side channels and demonstrate that these side channels can leak access patterns of victim nodes to other nodes.

We design Pythia, a set of RDMA-based remote side-channel attacks that allow an attacker on one client machine to learn how victims on other client machines access data a server exports as an in-memory data service. We reverse engineer the memory architecture of the most widely used RDMA NIC and use this knowledge to improve the efficiency of Pythia. We further extend Pythia to build side-channel attacks on Crail, a real RDMA-based key-value store application. We evaluated Pythia on four different RDMA NICs both in a laboratory and in a public cloud setting. Pythia is fast (57 mu s), accurate (97% accuracy), and can hide all its traces from the victim or the server.