Nisa Bostancı

Haocong Luo, İsmail Emir Yüksel, Ataberk Olgun et al.

We provide the first experimental demonstration of DejaVu, a phenomenon where the data previously written to DRAM cells affects DRAM's vulnerability to read disturbance. Our experimental characterization using 112 COTS DDR4 DRAM chips from all three major manufacturers shows that, compared to the baseline where we initialize the victim row by writing to it only once, 1) overwriting it with the opposite data reduces ACmin, the minimum aggressor row activation count to induce a bitflip, and 2) writing the same data twice increases ACmin. We provide two hypotheses to explain DejaVu. First, we hypothesize that overwriting the victim row with opposite data values causes under-restoration of charge in DRAM cells. Second, we hypothesize that overwriting the victim row changes charge trap states in the active region, affecting read-disturbance-induced cell leakage current. We conduct controlled characterization to provide insight into these hypotheses. We further characterize the reliability of Processing-Using-DRAM (PUD) operations with DRAM rows initialized with DejaVu patterns. Our characterization of 32-row MAJ-3 operation shows that overwriting the DRAM rows used in the operation reduces the number of bitlines that fail to reliably perform MAJ-3 by 32.7% on average compared to the baseline where rows are written only once. Based on our observations, we describe two major implications of DejaVu. We show how DRAM testing and characterization methodologies should account for DejaVu to accurately characterize read disturbance vulnerability under fixed data patterns and rigorously study data-pattern effects without unintended interference from DejaVu. We also evaluate the performance overhead of read disturbance mitigation techniques when thresholds need to be lowered to be secure against DejaVu, showing a 6.3% overhead when reducing the threshold by 20%.

Ataberk Olgun, Minesh Patel, A. Giray Yağlıkçı et al.

True random number generators (TRNG) sample random physical processes to create large amounts of random numbers for various use cases, including security-critical cryptographic primitives, scientific simulations, machine learning applications, and even recreational entertainment. Unfortunately, not every computing system is equipped with dedicated TRNG hardware, limiting the application space and security guarantees for such systems. To open the application space and enable security guarantees for the overwhelming majority of computing systems that do not necessarily have dedicated TRNG hardware, we develop QUAC-TRNG. QUAC-TRNG exploits the new observation that a carefully-engineered sequence of DRAM commands activates four consecutive DRAM rows in rapid succession. This QUadruple ACtivation (QUAC) causes the bitline sense amplifiers to non-deterministically converge to random values when we activate four rows that store conflicting data because the net deviation in bitline voltage fails to meet reliable sensing margins. We experimentally demonstrate that QUAC reliably generates random values across 136 commodity DDR4 DRAM chips from one major DRAM manufacturer. We describe how to develop an effective TRNG (QUAC-TRNG) based on QUAC. We evaluate the quality of our TRNG using NIST STS and find that QUAC-TRNG successfully passes each test. Our experimental evaluations show that QUAC-TRNG generates true random numbers with a throughput of 3.44 Gb/s (per DRAM channel), outperforming the state-of-the-art DRAM-based TRNG by 15.08x and 1.41x for basic and throughput-optimized versions, respectively. We show that QUAC-TRNG utilizes DRAM bandwidth better than the state-of-the-art, achieving up to 2.03x the throughput of a throughput-optimized baseline when scaling bus frequencies to 12 GT/s.