Unsafe Time Handling in Smartphones

Time manipulation, typically done using gettime() and settime(), happens extensively across all software layers in smartphones, from the kernel, to the framework, to millions of apps. This paper presents the first study of a new class of software bugs on smartphones called sleep-induced time bugs (SITB). SITB happens when the phone is suspended, due to the aggressive sleeping policy adopted in smartphones, in the middle of a time critical section where time is being manipulated and delay caused by unexpected phone suspension alters the intended program behavior. We first characterize time usages in the Android kernel, framework, and 978 apps into four categories and study their vulnerabilities to system suspension. Our study shows time manipulation happens extensively in all three software layers, totaling 1047, 1737 and 7798 times, respectively, and all four usage patterns are vulnerable to SITBs. We then present a tool called KLOCK, that makes use of a set of static analyses to systematically identify sleep-induced time bugs in three of the four time usage categories. When applied to five different Android Linux kernels, KLOCK correctly flagged 63 SITBvulnerable time manipulation instances as time bugs.