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TRRScope: Understanding Target Row Refresh Mechanism for Modern DDR Protection

Advanced DDR memories are widely used in almost all electronic devices and computing systems. Therefore, Rowhammer attack, a hardware-based attack targeting DDR memory, severely threatens data security and privacy in modern computing systems. Among existing solutions, the leading and most effective example is the Target Row Refresh (TRR) mechanism. TRR is proposed as the standard protection mechanism by JEDEC [1] and is widely used in DDR4 memory. As a protection scheme, TRR will refresh the victim row once a Rowhammer attack is detected. However, recent work such as TRRespass [2] shows that TRR can be bypassed and Rowhammer attack is still possible even on the latest DDR4 memory. While it is still believed that TRR-like protection mechanisms are promising solutions against the Rowhammer attack, it becomes an urgent task to improve the security of TRR mechanisms. However, TRR implementation details are proprietary to the DDR4 manufacturers. In order to better understand TRR protection mechanisms and help improve the TRR implementation, in this paper, we propose a novel timing side-channel based reverse engineering method to understand the underlying mechanisms of TRR. We then thoroughly analyze different implementations which are integrated into various DDR4 memory chips from different manufacturers. With experimental results collected from a range of DDR4 memory chips, we prove the effectiveness of the proposed TRR recovery mechanism.