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It has been demonstrated in numerous previous studies that Android and its underlying Linux operating systems do not properly isolate mobile apps to prevent cross-app side- channel attacks. Cross-app information leakage enables malicious Android apps to infer sensitive user data (e.g., passwords), or private user information (e.g., identity or location) without requiring specific permissions. Nevertheless, no prior work has ever studied these side-channel attacks on iOS-based mobile devices. One reason is that iOS does not implement procfs— the most popular side-channel attack vector; hence the previously known attacks are not feasible. In this paper, we present the first study of OS-level side-channel attacks on iOS. Specifically, we identified several new side-channel attack vectors (i.e., iOS APIs that enable cross-app information leakage); developed machine learning frameworks (i.e., classification and pattern matching) that combine multiple attack vectors to improve the accuracy of the inference attacks; demonstrated three categories of attacks that exploit these vectors and frameworks to exfiltrate sensitive user information. We have reported our findings to Apple and proposed mitigations to the attacks. Apple has incorporated some of our suggested countermeasures into iOS 11 and MacOS High Sierra 10.13 and later versions. 

Intel Software Guard Extension (SGX) protects the confidentiality and integrity of an unprivileged program running inside a secure enclave from a privileged attacker who has full control of the entire operating system (OS). Program execution inside this enclave is therefore referred to as shielded. Unfortunately, shielded execution does not protect programs from side-channel attacks by a privileged attacker. For instance, it has been shown that by changing page table entries of memory pages used by shielded execution, a malicious OS kernel could observe memory page accesses from the execution and hence infer a wide range of sensitive information about it. In fact, this page-fault side channel is only an instance of a category of side-channel attacks, here called privileged side-channel attacks, in which privileged attackers frequently preempt the shielded execution to obtain fine-grained side-channel observations. In this paper, we present Déjà Vu, a software framework that enables a shielded execution to detect such privileged side-channel attacks. Specifically, we build into shielded execution the ability to check program execution time at the granularity of paths in its control-flow graph. To provide a trustworthy source of time measurement, Déjà Vu implements a novel software reference clock that is protected by Intel Transactional Synchronization Extensions (TSX), a hardware implementation of transactional memory. Evaluations show that Déjà Vu effectively detects side-channel attacks against shielded execution and against the reference clock itself. 

Row hammer attacks exploit electrical interactions between neighboring memory cells in high-density dynamic random-access memory (DRAM) to induce memory errors. By rapidly and repeatedly accessing DRAMs with specific patterns, an adversary with limited privilege on the target machine may trigger bit flips in memory regions that he has no permission to access directly. In this paper, we explore row hammer attacks in cross-VM settings, in which a malicious VM exploits bit flips induced by row hammer attacks to crack memory isolation enforced by virtualization. To do so with high fidelity, we develop novel techniques to determine the physical address mapping in DRAM modules at runtime (to improve the effectiveness of double-sided row hammer attacks), methods to exhaustively hammer a large fraction of physical memory from a guest VM (to collect exploitable vulnerable bits), and innovative approaches to break Xen paravirtualized memory isolation (to access arbitrary physical memory of the shared machine). Our study also suggests that the demonstrated row hammer attacks are applicable in modern public clouds where Xen paravirtualization technology is adopted. This shows that the presented cross-VM row hammer attacks are of practical importance.