Secure DIMM: Moving ORAM Primitives Closer to Memory

As more critical applications move to the cloud, there is a pressing need to provide privacy guarantees for data and computation. While cloud infrastructures are vulnerable to a variety of attacks, in this work, we focus on an attack model where an untrusted cloud operator has physical access to the server and can monitor the signals emerging from the processor socket. Even if data packets are encrypted, the sequence of addresses touched by the program serves as an information side channel. To eliminate this side channel, Oblivious RAM constructs have been investigated for decades, but continue to pose large overheads. In this work, we make the case that ORAM overheads can be significantly reduced by moving some ORAM functionality into the memory system. We first design a secure DIMM (or SDIMM) that uses commodity low-cost memory and an ASIC as a secure buffer chip. We then design two new ORAM protocols that leverage SDIMMs to reduce bandwidth, latency, and energy per ORAM access. In both protocols, each SDIMM is responsible for part of the ORAM tree. Each SDIMM performs a number of ORAM operations that are not visible to the main memory channel. By having many SDIMMs in the system, we are able to achieve highly parallel ORAM operations. The main memory channel uses its bandwidth primarily to service blocks requested by the CPU, and to perform a small subset of the many shuffle operations required by conventional ORAM. The new protocols guarantee the same obliviousness properties as Path ORAM. On a set of memory-intensive workloads, our two new ORAM protocols – Independent ORAM and Split ORAM – are able to improve performance by 1.9x and energy by 2.55x, compared to Freecursive ORAM.