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USENIX ATC '22 - SoftTRR: Protect Page Tables against Rowhammer Attacks using Software-only Target Row Refresh Zhi Zhang, CSIRO’s Data61, Australia; Yueqiang Cheng, NIO Security Research; Minghua Wang, Baidu Security; Wei He and Wenhao Wang, State Key Laboratory of Information Security, Institute of Information Engineering, CAS and University of Chinese Academy of Sciences; Surya Nepal, CSIRO’s Data61, Australia; Yansong Gao, Nanjing University of Science and Technology, China; Kang Li, Baidu Security; Zhe Wang and Chenggang Wu, State Key Laboratory of Computer Architecture, Institute of Computing Technology, CAS and University of Chinese Academy of Sciences Rowhammer attacks that corrupt level-1 page tables to gain kernel privilege are the most detrimental to system security and hard to mitigate. However, recently proposed software-only mitigations are not effective against such kernel privilege escalation attacks. In this paper, we propose an effective and practical software-only defense, called SoftTRR, to protect page tables from all existing rowhammer attacks on x86. The key idea of SoftTRR is to refresh the rows occupied by page tables when a suspicious rowhammer activity is detected. SoftTRR is motivated by DRAM-chip-based target row refresh (ChipTRR) but eliminates its main security limitation (i.e., ChipTRR tracks a limited number of rows and thus can be bypassed by many-sided hammer [17]). Specifically, SoftTRR protects an unlimited number of page tables by tracking memory accesses to the rows that are in close proximity to page-table rows and refreshing the page-table rows once the tracked access count exceeds a pre-defined threshold. We implement a prototype of SoftTRR as a loadable kernel module, and evaluate its security effectiveness, performance overhead, and memory consumption. The experimental results show that SoftTRR protects page tables from real-world rowhammer attacks and incurs small performance overhead as well as memory cost. View the full USENIX ATC '22 program at https://www.usenix.org/conference/atc22/technical-sessions