Data memory-dependent prefetchers (DMPs) are a microarchitectural optimization recently introduced in mainstream desktop and server CPUs. Although DMPs offer performance improvements, they have been shown to enable side-channel leakage of secrets, even from constant-time code that has been hardened against Spectre attacks. Unfortunately, existing defenses either incur significant performance overhead or, as we show, fail to prevent all DMP leakage. In this work, we address this gap by designing a defense that is both secure and efficient. First, we propose a classification of DMP leakage, categorize existing and novel attacks, and analyze which leakage classes are mitigated by existing defenses. We validate this analysis empirically by implementing the first open-source DMPs on a RISC-V processor, showing that the only existing defense that aims to preserve DMP performance fails to protect against entire leakage classes. Next, we propose SecDMP, a provably secure and efficient hardware-based defense. SecDMP requires secrets to be marked in memory and ensures that DMPs never encode these secrets in the microarchitecture---without compromising performance for public data. We formally prove that SecDMP introduces no leakage beyond a baseline semantics without DMPs. Notably, when combined with speculative-execution defenses, SecDMP restores complete microarchitectural security for constant-time code. Finally, our empirical evaluation shows that SecDMP prevents all DMP leakage while incurring minimal performance overhead and hardware cost.

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