Modern systems assume that privileged software always behaves as expected, however, such assumptions may not hold given the prevalence of kernel vulnerabilities. One idea is to employ defenses to restrict how adversaries may exploit such vulnerabilities, such as Control-Flow Integrity (CFI), which restricts execution to a Control-Flow Graph (CFG). However, proposed applications of CFI enforcement to kernel software are too coarse-grained to restrict the adversary effectively and either fail to enforce CFI comprehensively or are very expensive.We present a mostly-automated approach for retrofitting kernel software that leverages features of such software to enable comprehensive, efficient, fine-grained CFI enforcement. We achieve this goal by leveraging two insights. We first leverage the conservative function pointer usage patterns found in the kernel source code to develop a method to compute fine-grained CFGs for kernel software. Second, we identify two opportunities for removing CFI instrumentation relative to prior optimization techniques: reusing existing kernel instrumentation and creating direct transfers, where possible. Using these insights, we show how to choose optimized defenses for kernels to handle system events, enabling comprehensive and efficient CFI enforcement.We evaluate the effectiveness of the proposed fine-grained CFI instrumentation by applying the retrofitting approach comprehensively to FreeBSD, the MINIX microkernel system, and MINIX's user-space servers, and applying this approach partly to the BitVisor hypervisor. We show that our approach eliminates over 70% of the indirect targets relative to the best current, fine-grained CFI techniques, while our optimizations reduce the instrumentation necessary to enforce coarse-grained CFI. The performance improvement due to our optimizations ranges from 51%/25% for MINIX to 12%/17% for FreeBSD for the average/maximum microbenchmark overhead. The evaluation shows that fine-grained CFI instrumentation can be computed for kernel software in practice and can be enforced more efficiently than coarse-grained CFI instrumentation.

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Abstract. Many security-sensitive programs manage resources on behalf of mutually distrusting clients. To control access to resources, authorization hooks are placed before operations on those resources. Manual hook placements by programmers are often incomplete or incorrect, leading to insecure programs. We advocate an approach that automatically identifies the set of locations to place authorization hooks that mediates all security-sensitive operations in order to enforce expected access control policies at deployment. However, one challenge is that programmers often want to minimize the effort of writing such policies. As a result, they may remove authorization hooks that they believe are unnecessary, but they may remove too many hooks, preventing the enforcement of some desirable access control policies.In this paper, we propose algorithms that automatically compute a minimal authorization hook placement that satisfies constraints that describe desirable access control policies. These authorization constraints reduce the space of enforceable access control policies; i.e., those policies that can be enforced given a hook placement that satisfies the constraints. We have built a tool that implements this authorization hook placement method, demonstrating how programmers can produce authorization hooks for real-world programs and leverage policy goalspecific constraint selectors to automatically identify many authorization constraints. Our experiments show that our technique reduces manual programmer effort by as much as 58% and produces placements that reduce the amount of policy specification by as much as 30%.

Modern distributed systems are composed from several offthe-shelf components, including operating systems, virtualization infrastructure, and application packages, upon which some custom application software (e.g., web application) is often deployed. While several commodity systems now include mandatory access control (MAC) enforcement to protect the individual components, the complexity of such MAC policies and the myriad of possible interactions among individual hosts in distributed systems makes it difficult to identify the attack paths available to adversaries. As a result, security practitioners react to vulnerabilities as adversaries uncover them, rather than proactively protecting the system's data integrity. In this paper, we develop a mostly-automated method to transform a set of commodity MAC policies into a system-wide policy that proactively protects system integrity, approximating the Clark-Wilson integrity model. The method uses the insights from the Clark-Wilson model, which requires integrity verification of security-critical data and mediation at program entrypoints, to extend existing MAC policies with the proactive mediation necessary to protect system integrity. We demonstrate the practicality of producing Clark-Wilson policies for distributed systems on a web application running on virtualized Ubuntu SELinux hosts, where our method finds: (1) that only 27 additional entrypoint mediators are sufficient to mediate the threats of remote adversaries over the entire distributed system and (2) and only 20 additional local threats require mediation to approximate Clark-Wilson integrity comprehensively. As a result, available security policies can be used as a foundation for proactive integrity protection from both local and remote threats.

Motivated by an application in network security, we investigate the following "linear" case of Directed Multicut. Let G be a directed graph which includes some distinguished vertices t 1 , . . . , t k . What is the size of the smallest edge cut which eliminates all paths from t i to t j for all i < j? We show that this problem is fixed-parameter tractable when parametrized in the cutset size p via an algorithm running in O(4 p pn 4 ) time.