FlagThis

Abstract

Modern DRAM chips are vulnerable to read disturbance phenomena such as RowHammer and RowPress, which induce bitflips in DRAM rows after accessing nearby rows a certain number of times (i.e., the read disturbance threshold). ColumnDisturb is a new and fundamentally different DRAM read disturbance phenomenon. Specifically, ColumnDisturb (i) disturbs DRAM columns instead of rows, and (ii) increases the number of affected DRAM cells from those that reside in only a few neighboring rows to all cells across three consecutive DRAM subarrays. We propose ColumnKeeper, the first set of ColumnDisturb mitigations that has two variants: ColumnKeeper-D (CK-D), a deterministic mitigation mechanism, and ColumnKeeper-P (CKP), a probabilistic one. The key idea of CK-D is to take advantage of DRAMs open-bitline architecture to provide deterministic security guarantees against ColumnDisturb with low performance and energy overheads. To achieve this, CK-D employs two counters per subarray to track the number of activations affecting the odd and even columns of the subarray, respectively. When either counter reaches a predetermined threshold, CK-D refreshes one row in the corresponding subarray. The key idea of CK-P is to refresh one row in three consecutive subarrays upon a row activation in the middle subarray. This occurs with a predetermined probability and provides configurable security guarantees at low area overhead. To identify which row to refresh in each subarray, both mechanisms maintain a table with one entry per subarray that always points to the next row to refresh within the subarray. Each time either mechanism refreshes a row, it increments the corresponding entry, thereby ensuring that all rows in the subarray are refreshed in a round-robin manner. Our comprehensive evaluation shows that both mechanisms prevent ColumnDisturb bitflips at low performance, energy, and area overheads. At the current experimentally-demonstrated ColumnDisturb threshold (1M), CK-D and CK-P incur very low average single-core performance overheads of 0.15% and 0.36%, respectively, compared to a system with no ColumnDisturb mitigation. For near-future thresholds (128K), these overheads rise to a still low average of 1.70% and 2.73%, respectively. Our experimental analysis shows that mitigating ColumnDisturb with low performance overheads at low thresholds (e.g., 16K) is still possible by either adopting smaller subarray sizes or enabling subarray-level parallelism. CK-D and CK-P can be implemented with low area overheads of 0.1mm2 and 0.03mm2, respectively. ColumnKeeper is freely available at github.com/CMU-SAFARI/ColumnKeeper.