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Spectre attacks exploit microprocessor speculative execution to read and transmit forbidden data outside the attacker's trust domain and sandbox. Recent hardware schemes allow potentially-unsafe speculative accesses but prevent the secret's transmission by delaying most access-dependent instructions even in the predominantly-common, no-attack case, which incurs performance loss and hardware complexity. Instead, we propose SafeBet which allows only, and does not delay most, safe accesses, achieving both security and high performance. SafeBet is based on the key observation that speculatively accessing a destination location is safe if the location's access by the same static trust domain has been committed previously; and potentially unsafe, otherwise. We extend this observation to handle inter trust-domain code and data interactions. SafeBet employs the Speculative Memory Access Control Table (SMACT) to track non-speculative trust domain code region-destination pairs. Disallowed accesses wait until reaching commit to trigger well-known replay, with virtually no change to the pipeline. Software simulations using SpecCPU benchmarks show that SafeBet uses an 8.3-KB SMACT per core to perform within 6% on average (63% at worst) of the unsafe baseline behind which NDA-restrictive, a previous scheme of security and hardware complexity comparable to SafeBet's, lags by 83% on average. |
