EnTrust: Regulating Sensor Access by Cooperating Programs via Delegation Graphs

Modern operating systems support a cooperating program abstraction that, instead of placing all functionality into a single program, allows diverse programs to cooperate to complete tasks requested by users. However, untrusted programs may exploit such interactions to spy on users through device sensors by causing privileged system services to misuse their permissions, or to forward user requests to malicious programs inadvertently. Researchers have previously explored methods to restrict access to device sensors based on the state of the user interface that elicited the user input or based on the set of cooperating programs, but the former approach does not consider cooperating programs and the latter approach has been found to be too restrictive for many cases. In this paper, we propose EnTrust, an authorization system that tracks the processing of input events across programs for eliciting approvals from users for sensor operations. EnTrust constructs delegation graphs by linking input events to cooperation events among programs that lead to sensor operation requests, then uses such delegation graphs for eliciting authorization decisions from users. To demonstrate this approach, we implement the EnTrust authorization system for Android OS. In a laboratory study, we show that attacks can be prevented at a much higher rate (47-67% improvement) compared to the first-use approach. Our field study reveals that EnTrust only requires a user effort comparable to the first-use approach while incurring negligible performance (<1% slowdown) and memory overheads (5.5 KB per program).