Proof-carrying Code Research Articles

Practical and Efficient in-Enclave Verification of Privacy Compliance.

A trusted execution environment (TEE) such as Intel Software Guard Extension (SGX) runs attestation to prove to a data owner the integrity of the initial state of an enclave, including the program to operate on her data. For this purpose, the data-processing program is supposed to be open to the owner or a trusted third party, so its functionality can be evaluated before trust being established. In the real world, however, increasingly there are application scenarios in which the program itself needs to be protected (e.g., proprietary algorithm). So its compliance with privacy policies as expected by the data owner should be verified without exposing its code. To this end, this paper presents Deflection, a new model for TEE-based delegated and flexible in-enclave code verification. Given that the conventional solutions do not work well under the resource-limited and TCB-frugal TEE, we come up with a new design inspired by Proof-Carrying Code. Our design strategically moves most of the workload to the code generator, which is responsible for producing easy-to-check code, while keeping the consumer simple. Also, the whole consumer can be made public and verified through a conventional attestation. We implemented this model on Intel SGX and demonstrate that it introduces a very small part of TCB. We also thoroughly evaluated its performance on micro- and macro- benchmarks and real-world applications, showing that the design only incurs a small overhead when enforcing several categories of security policies.

Certifying delta-oriented programs

A major design concern in modern software development frameworks is to ensure that mechanisms for updating code running on remote devices comply with given safety specifications. This paper presents a delta-oriented approach for implementing product lines where software reuse is achieved at the three levels of state-diagram modeling, C/\(\text {C}^{_{_{_{++}}}} \)source code and binary code. A safety specification is expressed on the properties of reusable software libraries that can be dynamically loaded at run time after an over-the-air update. The compilation of delta-engineered code is certified using the framework of proof-carrying code in order to guarantee safety of software updates on remote devices. An empirical evaluation of the computational cost associated with formal safety checks is done by means of experimentation.

Programs from Proofs

Today, software is traded worldwide on global markets, with apps being downloaded to smartphones within minutes or seconds. This poses, more than ever, the challenge of ensuring safety of software in the face of (1) unknown or untrusted software providers together with (2) resource-limited software consumers. The concept of Proof-Carrying Code (PCC), years ago suggested by Necula, provides one framework for securing the execution of untrusted code. PCC techniques attach safety proofs, constructed by software producers, to code. Based on the assumption that checking proofs is usually much simpler than constructing proofs, software consumers should thus be able to quickly check the safety of software. However, PCC techniques often suffer from the size of certificates (i.e., the attached proofs), making PCC techniques inefficient in practice. In this article, we introduce a new framework for the safe execution of untrusted code called Programs from Proofs (PfP). The basic assumption underlying the PfP technique is the fact that the structure of programs significantly influences the complexity of checking a specific safety property. Instead of attaching proofs to program code, the PfP technique transforms the program into an efficiently checkable form, thus guaranteeing quick safety checks for software consumers. For this transformation, the technique also uses a producer-side automatic proof of safety. More specifically, safety proving for the software producer proceeds via the construction of an abstract reachability graph (ARG) unfolding the control-flow automaton (CFA) up to the degree necessary for simple checking. To this end, we combine different sorts of software analysis: expensive analyses incrementally determining the degree of unfolding, and cheap analyses responsible for safety checking. Out of the abstract reachability graph we generate the new program. In its CFA structure, it is isomorphic to the graph and hence another, this time consumer-side, cheap analysis can quickly determine its safety. Like PCC, Programs from Proofs is a general framework instantiable with different sorts of (expensive and cheap) analysis. Here, we present the general framework and exemplify it by some concrete examples. We have implemented different instantiations on top of the configurable program analysis tool CPA checker and report on experiments, in particular on comparisons with PCC techniques.

Security monitor inlining and certification for multithreaded Java

Security monitor inlining is a technique for security policy enforcement whereby monitor functionality is injected into application code in the style of aspect-oriented programming. The intention is that the injected code enforces compliance with the policy (security), and otherwise interferes with the application as little as possible (conservativity and transparency). Such inliners are said to be correct. For sequential Java-like languages, inlining is well understood, and several provably correct inliners have been proposed. For multithreaded Java one difficulty is the need to maintain a shared monitor state. We show that this problem introduces fundamental limitations in the type of security policies that can be correctly enforced by inlining. A class of race-free policies is identified that precisely characterizes the inlineable policies by showing that inlining of a policy outside this class is either not secure or not transparent, and by exhibiting a concrete inliner for policies inside the class which is secure, conservative and transparent. The inliner is implemented for Java and applied to a number of practical application security policies. Finally, we discuss how certification in the style of proof-carrying code could be supported for inlined programs by using annotations to reduce a potentially complex verification problem for multithreaded Java bytecode to sequential verification of just the inlined code snippets.

A resource semantics and abstract machine for Safe: A functional language with regions and explicit deallocation

In this paper we summarise Safe, a first-order functional language for programming small devices and embedded systems with strict memory requirements, which has been introduced elsewhere. It has some unusual memory management features such as heap regions and explicit cell deallocation. It is targeted at a Proof Carrying Code environment, and consistently with this aim the Safe compiler provides machine checkable certificates about important safety properties such as the absence of dangling pointers and bounded memory consumption.The kernel of the paper is devoted to developing part of the Safe compiler's back-end, by deriving an appropriate abstract machine from the language semantics, by providing the code generation functions, and by formally proving that the translation is sound, both in the semantic and in the memory consumption senses.

Defending Against the Buffer Overflows: A Static Approach Using Proof-Carrying Code

Buffer overflow has been one of the most outstanding attacks in the last ten years. This kind of vulnerability may compromise the system security by various means. Existing solutions to this problem have focused on the execution environment of the malicious program rather than the hypostasis of buffer overflow and most of them try to detect buffer overflows dynamically. This paper presents the effort of applying a static analysis approach against the programs exploiting buffer overflow and the method adopted is named Proof-Carrying Code (PCC). This paper shows that: (1) it is possible to defend against most of the buffer overflow vulnerabilities with proper use of PCC; and (2) the method is well prepared to handle the coming-up variants of the buffer overflow problems. 

Dead code elimination based pointer analysis for multithreaded programs

Abstract This paper presents a new approach for optimizing multitheaded programs with pointer constructs. The approach has applications in the area of certified code (proof-carrying code) where a justification or a proof for the correctness of each optimization is required. The optimization meant here is that of dead code elimination. Towards optimizing multithreaded programs the paper presents a new operational semantics for parallel constructs like join-fork constructs, parallel loops, and conditionally spawned threads. The paper also presents a novel type system for flow-sensitive pointer analysis of multithreaded programs. This type system is extended to obtain a new type system for live-variables analysis of multithreaded programs. The live-variables type system is extended to build the third novel type system, proposed in this paper, which carries the optimization of dead code elimination. The justification mentioned above takes the form of type derivation in our approach.

Proof-Carrying Hardware Intellectual Property: A Pathway to Trusted Module Acquisition

We present a novel framework for facilitating the acquisition of provably trustworthy hardware intellectual property (IP). The proposed framework draws upon research in the field of proof-carrying code (PCC) to allow for formal yet computationally straightforward validation of security-related properties by the IP consumer. These security-related properties, agreed upon a priori by the IP vendor and consumer and codified in a temporal logic, outline the boundaries of trusted operation, without necessarily specifying the exact IP functionality. A formal proof of these properties is then crafted by the vendor and presented to the consumer alongside the hardware IP. The consumer, in turn, can easily and automatically check the correctness of the proof and, thereby, validate compliance of the hardware IP with the agreed-upon properties. We implement the proposed framework using a synthesizable subset of Verilog and a series of pertinent definitions in the Coq theorem-proving language. Finally, we demonstrate the application of this framework on a simple IP acquisition scenario, including specification of security-related properties, Verilog code for two alter- native circuit implementations, as well as proofs of their security compliance.

Secure information flow by self-composition

Information flow policies are confidentiality policies that control information leakage through program execution. A common way to enforce secure information flow is through information flow type systems. Although type systems are compositional and usually enjoy decidable type checking or inference, their extensibility is very poor: type systems need to be redefined and proved sound for each new variation of security policy and programming language for which secure information flow verification is desired.In contrast, program logics offer a general mechanism for enforcing a variety of safety policies, and for this reason are favoured in Proof Carrying Code, which is a promising security architecture for mobile code. However, the encoding of information flow policies in program logics is not straightforward because they refer to a relation between two program executions.The purpose of this paper is to investigate logical formulations of secure information flow based on the idea of self-composition, which reduces the problem of secure information flow of a program P to a safety property for a program derived from P by composing P with a renaming of itself. Self-composition enables the use of standard techniques for information flow policy verification, such as program logics and model checking, that are suitable in Proof Carrying Code infrastructures.We illustrate the applicability of self-composition in several settings, including different security policies such as non-interference and controlled forms of declassification, and programming languages including an imperative language with parallel composition, a non-deterministic language and, finally, a language with shared mutable data structures.

Software Security in Virtualized Infrastructures — The Smart Meter Example

Abstract Future infrastructures for energy, traffic, and computing will be virtualized: they will consist of decentralized, self-organizing, dynamically adaptive, and open collections of physical resources such as virtual power plants or computing clouds. Challenges to software dependability, in particular software security will be enormous. We use the example of smart power meters to discuss advanced technologies for the protection of integrity and confidentiality of software and data in virtualized infrastructures. We show that approaches based on homomorphic encryption, proof-carrying code, information flow control, deductive verification, and runtime verification are promising candidates for providing solutions to a plethora of representative challenges in the domain of virtualized infrastructures.

Silver Bullet Talks with Greg Morrisett

Gary McGraw interviews Greg Morrisett, the Allen B. Cutting Professor of Computer Science and associate dean for computer science and engineering at Harvard University. His current work is on applications of advanced type systems, model checkers, proof-carrying code, and inline reference monitors for building efficient and provably secure systems. He's also working on languages for sensor networks. Hear the full podcast of the interview at www.computer.org/security/podcasts/ or www.cigital.com/silverbullet/. The Web extra is the full transcript of the interview.

Formal reasoning about lazy-STM programs

Transactional memory (TM) is an easy-using parallel programming model that avoids common problems associated with conventional locking techniques. Several researchers have proposed a large amount of alternative hardware and software TM implementations. However, few ones focus on formal reasoning about these TM programs. In this paper, we propose a framework at assembly level for reasoning about lazy software transactional memory (STM) programs. First, we give a software TM implementation based on lightweight locks. These locks are also one part of the shared memory. Then we define the semantics of the model operationally, and the lightweight locks in transaction are non-blocking, avoiding deadlocks among transactions. Finally we design a logic -- a combination of permission accounting in separation logic and concurrent separation logic -- to verify various properties of concurrent programs based on this machine model. The whole framework is formalized using a proof-carrying-code (PCC) framework.

Semantic foundations for typed assembly languages

Typed Assembly Languages (TALs) are used to validate the safety of machine-language programs. The Foundational Proof-Carrying Code project seeks to verify the soundness of TALs using the smallest possible set of axioms: the axioms of a suitably expressive logic plus a specification of machine semantics. This article proposes general semantic foundations that permit modular proofs of the soundness of TALs. These semantic foundations include Typed Machine Language (TML), a type theory for specifying properties of low-level data with powerful and orthogonal type constructors, and L c , a compositional logic for specifying properties of machine instructions with simplified reasoning about unstructured control flow. Both of these components, whose semantics we specify using higher-order logic, are useful for proving the soundness of TALs. We demonstrate this by using TML and L c to verify the soundness of a low-level, typed assembly language, LTAL, which is the target of our core-ML-to-sparc compiler. To prove the soundness of the TML type system we have successfully applied a new approach, that of step-indexed logical relations . This approach provides the first semantic model for a type system with updatable references to values of impredicative quantified types. Both impredicative polymorphism and mutable references are essential when representing function closures in compilers with typed closure conversion, or when compiling objects to simpler typed primitives.

Formal verification of concurrent programs with read-write locks

Read-write locking is an important mechanism to improve concurrent granularity, but it is difficult to reason about the safety of concurrent programs with read-write locks. Concurrent separation logic (CSL) provides a simple but powerful technique for locally reasoning about concurrent programs with mutual exclusive locks. Unfortunately, CSL cannot be directly applied to reasoning about concurrent programs with read-write locks due to the different concurrent control mechanisms. This paper focuses on extending CSL and presenting a proof-carrying code (PCC) system for reasoning about concurrent programs with read-write locks. We extend the heap model with a writing permission set, denoted as logical heap, then define “strong separation” and “weak separation” over logical heap. Following CSL’s local-reasoning idea, we develop a novel program logic to enforce weak separations of heap between different threads and provide verification of concurrent programs with read-write locks.

Proof-Carrying Hardware: Concept and Prototype Tool Flow for Online Verification

Dynamically reconfigurable hardware combines hardware performance with software-like flexibility and finds increasing use in networked systems. The capability to load hardware modules at runtime provides these systems with an unparalleled degree of adaptivity but at the same time poses new challenges for security and safety. In this paper, we elaborate on the presentation of proof carrying hardware (PCH) as a novel approach to reconfigurable system security. PCH takes a key concept from software security, known as proof-carrying code, into the reconfigurable hardware domain. We outline the PCH concept and discuss runtime combinational equivalence checking as a first online verification problem applying the concept. We present a prototype tool flow and experimental results demonstrating the feasibility and potential of the PCH approach.

The Non-Interference Protection in BML

Many information-flow type systems have been developed that allow to control the non-interference of information between the levels of classification in the Bell-LaPadula model. We present here a translation of typing information collected for bytecode programs to a bytecode program logic. This translation uses the syntax of a bytecode specification language BML. A translation of this kind allows including the check of the non-interference property in a single, unified verification framework based on a program logic and thus can be exploited within a foundational proof-carrying code infrastructure. It also provides a flexible basis for various declassification strategies that may be useful in a particular code body.

Proof optimization for partial redundancy elimination

Partial redundancy elimination is a subtle optimization which performs common subexpression elimination and expression motion at the same time. In this paper, we use it as an example to promote and demonstrate the scalability of the technology of proof optimization. By this we mean automatic transformation of a given program’s Hoare logic proof of functional correctness or resource usage into one of the optimized program, guided by a type-derivation representation of the result of the underlying dataflow analyses. A proof optimizer is a useful tool for the producer’s side in a natural proof-carrying code scenario where programs are proved correct prior to optimizing compilation before transmission to the consumer. We present a type-systematic description of the underlying analyses and of the optimization for the while language, demonstrate that the optimization is semantically sound and improving in a formulation using type-indexed relations, and then show that these arguments can be transferred to automatic transformations of functional correctness/resource usage proofs in Hoare logics. For the improvement part, we instrument the standard semantics and Hoare logic so that evaluations of expressions become a resource.

Provably correct runtime monitoring

Runtime monitoring is an established technique to enforce a wide range of program safety and security properties. We present a formalization of monitoring and monitor inlining, for the Java Virtual Machine. Monitors are security automata given in a special-purpose monitor specification language, ConSpec. The automata operate on finite or infinite strings of calls to a fixed API, allowing local dependencies on parameter values and heap content. We use a two-level class file annotation scheme to characterize two key properties: (i) that the program is correct with respect to the monitor as a constraint on allowed program behavior, and (ii) that the program has a copy of the given monitor embedded into it. As the main application of these results we sketch a simple inlining algorithm and show how the two-level annotations can be completed to produce a fully annotated program which is valid in the standard sense of Floyd/Hoare logic. This establishes the mediation property that inlined programs are guaranteed to adhere to the intended policy. Furthermore, validity can be checked efficiently using a weakest precondition based annotation checker, thus preparing the ground for on-device checking of policy adherence in a proof-carrying code setting.

Proof-Carrying Code Based Tool for Secure Information Flow of Assembly Programs

Problem statement: How a host (the code consumer) can determine with certainty that a downloaded program received from untrusted source (the code producer) will maintain the confidentiality of the data it manipulates and it is safe to install and execute. Approach: The approach adopted for verifying that a downloaded program will not leak confidential data to unauthorized parties was based on the concept of Proof-Carrying Code (PCC). A mobile program (in its assembly form) was analyzed for information flow security based on the concept of proof-carrying code. The security policy was centered on a type system for analyzing information flows within assembly programs based on the notion of noninterference. Results: A verification tool for verifying assembly programs for information flow security was built. The tool certifies SPARC assembly programs for secure information flow by statically analyzing the program based on the idea of Proof-Carrying Code (PCC). The tool operated directly on the machine-code requiring only the inputs and outputs of the code annotated with security levels. The tool provided a windows user interface enabling the users to control the verification process. The proofs that untrusted program did not leak sensitive information were generated and checked on the host machine and if they are valid, then the untrusted program can be installed and executed safely. Conclusion: By basing proof-carrying code infrastructure on information flow analysis type-system, a sufficient assurance of protecting confidential data manipulated by the mobile program can be obtained. This assurance was come due to the fact that type systems provide a sufficient guarantee of protecting confidentiality.

Proof-Carrying Code Based Tool for Secure Information Flow of Assembly Programs

Proof-Carrying Code Based Tool for Secure Information Flow of Assembly Programs