DOLMA: Securing Speculation with the Principle of Transient Non-Observability

Modern processors allow attackers to leak data during transient (i.e., mis-speculated) execution through microarchitectural covert timing channels. While initial defenses were channel-specific, recent solutions employ speculative information flow control in an attempt to automatically mitigate attacks via any channel. However, we demonstrate that the current state-of-the-art defense fails to mitigate attacks using speculative stores, still allowing arbitrary data leakage during transient execution. Furthermore, we show that the state of the art does not scale to protect data in registers, incurring 30.8-63.4% overhead on SPEC 2017, depending on the threat model. We then present DOLMA, the first defense to automatically provide comprehensive protection against all known transient execution attacks. DOLMA combines a lightweight speculative information flow control scheme with a set of secure performance optimizations. By enforcing a novel principle of transient non-observability, DOLMA ensures that a time slice on a core provides a unit of isolation in the context of existing attacks. Accordingly, DOLMA can allow speculative TLB/L1 cache accesses and variable-time arithmetic without loss of security. On SPEC 2017, DOLMA achieves comprehensive protection of data in memory at 10.2-29.7% overhead, adding protection for data in registers at 22.6-42.2% overhead (8.2-21.2% less than the state of the art, with greater security).