FlushTime: Towards Mitigating Flush-based Cache Attacks via Collaborating Flush Instructions and Timers on ARMv8-A.

ARMv8-A processors generally utilize optimization techniques such as multi-layer cache, out-of-order execution and branch prediction to improve performance. These optimization techniques are inevitably threatened by cache-related attacks including Flush+Reload, Flush+Flush, Meltdown, Spectre, and their variants. These attacks can break the isolation boundaries between different processes or even between user and kernel spaces. Researchers proposed many defense schemes to resist these cache-related attacks. However, they either need to modify the hardware architecture, have incomplete coverage, or introduce significant performance overhead. In this paper, we propose FlushTime, a more secure collaborative framework of cache flush instructions and generic timer on ARMv8-A. Based on the instruction/register trap mechanism of ARMv8-A, FlushTime traps cache flush instructions and generic timer from user space into kernel space, and makes them cooperate with each other in kernel space. When a flush instruction is called, the generic timer resolution will be reduced for several time slices. This collaborative mechanism can greatly mitigate the threat of all flush-based cache-related attacks. Since normal applications rarely need to obtain high resolution timestamps immediately after calling a flush instruction, FlushTime does not affect the normal operation of the system. Security and performance evaluations show that FlushTime can resist all flush-based cache-related attacks while introducing an extremely low performance overhead.