Jay Ligatti

Control-flow integrity

Current software attacks often build on exploits that subvert machine-code execution. The enforcement of a basic safety property, Control-Flow Integrity (CFI), can prevent such attacks from arbitrarily controlling program behavior. CFI enforcement is simple, and its guarantees can be established formally even with respect to powerful adversaries. Moreover, CFI enforcement is practical: it is compatible with existing software and can be done efficiently using software rewriting in commodity systems. Finally, CFI provides a useful foundation for enforcing further security policies, as we demonstrate with efficient software implementations of a protected shadow call stack and of access control for memory regions.

Control-flow integrity principles, implementations, and applications

Current software attacks often build on exploits that subvert machine-code execution. The enforcement of a basic safety property, control-flow integrity (CFI), can prevent such attacks from arbitrarily controlling program behavior. CFI enforcement is simple and its guarantees can be established formally, even with respect to powerful adversaries. Moreover, CFI enforcement is practical: It is compatible with existing software and can be done efficiently using software rewriting in commodity systems. Finally, CFI provides a useful foundation for enforcing further security policies, as we demonstrate with efficient software implementations of a protected shadow call stack and of access control for memory regions.

Edit automata: enforcement mechanisms for run-time security policies

We analyze the space of security policies that can be enforced by monitoring and modifying programs at run time. Our program monitors, called edit automata, are abstract machines that examine the sequence of application program actions and transform the sequence when it deviates from a specified policy. Edit automata have a rich set of transformational powers: they may terminate an application, thereby truncating the program action stream; they may suppress undesired or dangerous actions without necessarily terminating the program; and they may also insert additional actions into the event stream.After providing a formal definition of edit automata, we develop a rigorous framework for reasoning about them and their cousins: truncation automata (which can only terminate applications), suppression automata (which can terminate applications and suppress individual actions), and insertion automata (which can terminate and insert). We give a set-theoretic characterization of the policies each sort of automaton can enforce, and we provide examples of policies that can be enforced by one sort of automaton but not another.

Composing security policies with polymer

We introduce a language and system that supports definition and composition of complex run-time security policies for Java applications. Our policies are comprised of two sorts of methods. The first is query methods that are called whenever an untrusted application tries to execute a security-sensitive action. A query method returns a suggestion indicating how the security-sensitive action should be handled. The second sort of methods are those that perform state updates as the policys suggestions are followed.The structure of our policies facilitates composition, as policies can query other policies for suggestions. In order to give programmers control over policy composition, we have designed the system so that policies, suggestions, and application events are all first-class objects that a higher-order policy may manipulate. We show how to use these programming features by developing a library of policy combinators.Our system is fully implemented, and we have defined a formal semantics for an idealized subset of the language containing all of the key features. We demonstrate the effectiveness of our system by implementing a large-scale security policy for an email client.

A theory of aspects

This paper define the semantics of MinAML, an idealized aspect-oriented programming language, by giving a type-directed translation from its user-friendly external language to its compact, well-defined core language. We argue that our framework is an effective way to give semantics to aspect-oriented programming languages in general because the translation eliminates shallow syntactic differences between related constructs and permits definition of a clean, easy-to-understand, and easy-to-reason-about core language.The core language extends the simply-typed lambda calculus with two central new abstractions: explicitly labeled program points and first-class advice. The labels serve both to trigger advice and to mark continuations that the advice may return to. These constructs are defined orthogonally to the other features of the language and we show that our abstractions can be used in both functional and object-oriented contexts. The labels are well-scoped and the language as a whole is well-typed. Consequently, programmers can use lexical scoping in the standard way to prevent aspects from interfering with local program invariants.

Run-Time Enforcement of Nonsafety Policies

A common mechanism for ensuring that software behaves securely is to monitor programs at run time and check that they dynamically adhere to constraints specified by a security policy. Whenever a program monitor detects that untrusted software is attempting to execute a dangerous action, it takes remedial steps to ensure that only safe code actually gets executed. This article improves our understanding of the space of policies enforceable by monitoring the run-time behaviors of programs. We begin by building a formal framework for analyzing policy enforcement: we precisely define policies, monitors, and enforcement. This framework allows us to prove that monitors enforce an interesting set of policies that we call the infinite renewal properties. We show how to construct a program monitor that provably enforces any reasonable infinite renewal property. We also show that the set of infinite renewal properties includes some nonsafety policies, that is, that monitors can enforce some nonsafety (including some purely liveness) policies. Finally, we demonstrate concrete examples of nonsafety policies enforceable by practical run-time monitors.

A theory of runtime enforcement, with results

This paper presents a theory of runtime enforcement based on mechanism models called MRAs (Mandatory Results Automata). MRAs can monitor and transform security-relevant actions and their results. Because previous work could not model monitors transforming results, MRAs capture realistic behaviors outside the scope of previous models. MRAs also have a simple but realistic operational semantics that makes it straightforward to define concrete MRAs. Moreover, the definitions of policies and enforcement with MRAs are significantly simpler and more expressive than those of previous models. Putting all these features together, we argue that MRAs make good general models of runtime mechanisms, upon which a theory of runtime enforcement can be based. We develop some enforceability theory by characterizing the policies MRAs can and cannot enforce.

A theory of secure control flow

Control-Flow Integrity (CFI) means that the execution of a program dynamically follows only certain paths, in accordance with a static policy. CFI can prevent attacks that, by exploiting buffer overflows and other vulnerabilities, attempt to control program behavior. This paper develops the basic theory that underlies two practical techniques for CFI enforcement, with precise formulations of hypotheses and guarantees.

Fault-tolerant typed assembly language

A transient hardware fault occurs when an energetic particle strikes a transistor, causing it to change state. Although transient faults do not permanently damage the hardware, they may corrupt computations by altering stored values and signal transfers. In this paper, we propose a new scheme for provably safe and reliable computing in the presence of transient hardware faults. In our scheme, software computations are replicated to provide redundancy while special instructions compare the independently computed results to detect errors before writing critical data. In stark contrast to any previous efforts in this area, we have analyzed our fault tolerance scheme from a formal, theoretical perspective. To be specific, first, we provide an operational semantics for our assembly language, which includes a precise formal definition of our fault model. Second, we develop an assembly-level type system designed to detect reliability problems in compiled code. Third, we provide a formal specification for program fault tolerance under the given fault model and prove that all well-typed programs are indeed fault tolerant. In addition to the formal analysis, we evaluate our detection scheme and show that it only takes 34% longer to execute than the unreliable version.

A type-theoretic interpretation of pointcuts and advice

This article defines the semantics of MinAML, an idealized aspect-oriented programming language, by giving a type-directed translation from a user-friendly external language to a compact, well-defined core language. We argue that our framework is an effective way to give semantics to aspect-oriented programming languages in general because the translation eliminates shallow syntactic differences between related constructs and permits definition of an elegant and extensible core language.The core language extends the simply-typed lambda calculus with two central new abstractions: explicitly labeled program points and first-class advice. The labels serve both to trigger advice and to mark continuations that the advice may return to. These constructs are defined orthogonally to the other features of the language and we show that our abstractions can be used in both functional and object-oriented contexts. We prove Preservation and Progress lemmas for our core language and show that the translation from MinAML source into core is type-preserving. Together these two results imply that the source language is type safe. We also consider several extensions to our basic framework including a general mechanism for analyzing the current call stack.