To operate efficiently across a wide range of workloads with varying power
requirements, a modern processor applies different current management
mechanisms, which briefly throttle instruction execution while they adjust
voltage and frequency to accommodate for power-hungry instructions (PHIs) in
the instruction stream. Doing so 1) reduces the power consumption of non-PHI
instructions in typical workloads and 2) optimizes system voltage regulators’
cost and area for the common use case while limiting current consumption when
executing PHIs.

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However, these mechanisms may compromise a system’s confidentiality
guarantees. In particular, we observe that multilevel side-effects of
throttling mechanisms, due to PHI-related current management mechanisms, can be
detected by two different software contexts (i.e., sender and receiver) running
on 1) the same hardware thread, 2) co-located Simultaneous Multi-Threading
(SMT) threads, and 3) different physical cores.

Based on these new observations on current management mechanisms, we develop
a new set of covert channels, IChannels, and demonstrate them in real modern
Intel processors (which span more than 70% of the entire client and server
processor market). Our analysis shows that IChannels provides more than 24x the
channel capacity of state-of-the-art power management covert channels. We
propose practical and effective mitigations to each covert channel in IChannels
by leveraging the insights we gain through a rigorous characterization of real
systems.

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