  Memory-disk synchronization is a critical technology for ensuring data
correctness, integrity, and security, especially in systems that handle
sensitive information like financial transactions and medical records. We
propose SYNC+SYNC, a group of attacks that exploit the memory-disk
synchronization primitives. SYNC+SYNC works by subtly varying the timing of
synchronization on the write buffer, offering several advantages: 1)
implemented purely in software, enabling deployment on any hardware devices; 2)
resilient against existing cache partitioning and randomization techniques; 3)
unaffected by prefetching techniques and cache replacement strategies. We
present the principles of SYNC+SYNC through the implementation of two write
covert channel protocols, using either a single file or page, and introduce
three enhanced strategies that utilize multiple files and pages. The
feasibility of these channels is demonstrated in both cross-process and
cross-sandbox scenarios across diverse operating systems (OSes). Experimental
results show that, the average rate can reach 2.036 Kb/s (with a peak rate of
14.762 Kb/s) and the error rate is 0% on Linux; when running on macOS, the
average rate achieves 10.211 Kb/s (with a peak rate of 253.022 Kb/s) and the
error rate is 0.004%. To the best of our knowledge, SYNC+SYNC is the first
high-speed write covert channel for software cache.
