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This paper introduces Okapi, a new hardware/software cross-layer architecturedesigned to mitigate Transient Execution Side Channel (TES) attacks in moderncomputing systems. Okapi enforces sandboxing for speculative execution,providing a hardware basis that can replace expensive speculation barriers insoftware.  At its core, Okapi allows for speculative data accesses to a memory page onlyafter the page has been accessed non-speculatively at least once by the currenttrust domain. The granularity of the trust domains can be controlled insoftware to achieve different security and performance trade-offs. Forenvironments with less stringent security needs, Okapi's features can bedeactivated to remove all performance overhead.  Without relying on any software modification, the Okapi hardware featuresalready provide full protection against TES breakout attacks at a thread-levelgranularity. This incurs an average performance overhead of only 3.17% for theSPEC CPU2017 benchmark suite.  Okapi introduces the OkapiReset instruction for additional software-levelsecurity support. This instruction allows for fine-grained sandboxing withcustom program sizes smaller than a thread, resulting in 1.68% performanceoverhead in our WebAssembly runtime experiment.  On top, Okapi provides the possibility to eliminate poisoning attacks. Forthe highest level of security, the OkapiLoad instruction prevents confidentialdata from being added to the trust domain after a sequential access, therebyenforcing weak speculative non-interference. In addition, we present a hardwareextension that limits the exploitable code space for Spectre gadgets towell-defined sections of the program. By ensuring the absence of gadgets inthese sections, developers can tailor their software towards achievingbeneficial trade-offs between the size of a trust domain and performance.

Okapi: Efficiently Safeguarding Speculative Data Accesses in Sandboxed Environments