  When multiple job processes are served by a single scheduler, the queueing
delays of one process are often affected by the others, resulting in a timing
side channel that leaks the arrival pattern of one process to the others. In
this work, we study such a timing side channel between a regular user and a
malicious attacker. Utilizing Shannon's mutual information as a measure of
information leakage between the user and attacker, we analyze
privacy-preserving behaviors of common work-conserving schedulers. We find that
the attacker can always learn perfectly the user's arrival process in a
longest-queue-first (LQF) scheduler. When the user's job arrival rate is very
low (near zero), first-come-first-serve (FCFS) and round robin schedulers both
completely reveal the user's arrival pattern. The near-complete information
leakage in the low-rate traffic region is proven to be reduced by half in a
work-conserving version of TDMA (WC-TDMA) scheduler, which turns out to be
privacy-optimal in the class of deterministic-working-conserving (det-WC)
schedulers, according to a universal lower bound on information leakage we
derive for all det-WC schedulers.
