Characterizing, Exploiting, and Mitigating Vulnerabilities in MLC NAND Flash Memory Programming

Abstract: This paper summarizes our work on experimentally analyzing, exploiting, and addressing vulnerabilities in multi-level cell NAND flash memory programming, which was published in the industrial session of HPCA 2017, and examines the work's significance and future potential. Modern NAND flash memory chips use multi-level cells (MLC), which store two bits of data in each cell, to improve chip density. As MLC NAND flash memory scaled down to smaller manufacturing process technologies, manufacturers adopted a two-step programming method to improve reliability. In two-step programming, the two bits of a multi-level cell are programmed using two separate steps, in order to minimize the amount of cell-to-cell program interference induced on neighboring flash cells. In this work, we demonstrate that two-step programming exposes new reliability and security vulnerabilities in state-of-the-art MLC NAND flash memory. We experimentally characterize contemporary 1X-nm (i.e., 15--19nm) flash memory chips, and find that a partially-programmed flash cell (i.e., a cell where the second programming step has not yet been performed) is much more vulnerable to cell-to-cell interference and read disturb than a fully-programmed cell. We show that it is possible to exploit these vulnerabilities on solid-state drives (SSDs) to alter the partially-programmed data, causing (potentially malicious) data corruption. Based on our observations, we propose several new mechanisms that eliminate or mitigate these vulnerabilities in partially-programmed cells, and at the same time increase flash memory lifetime by 16%.