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09:00-10:30 Session 117A: Theory and Security
Location: Maths LT1
Automata vs Linear-Programming Discounted-Sum Inclusion

ABSTRACT. The problem of {\em quantitative inclusion} formalizes the goal of comparing quantitative dimensions between systems such as worst-case execution time, resource consumption, and the like. Such systems are typically represented by formalisms such as weighted logics or weighted automata. Despite its significance in analyzing the quality of computing systems, the study of quantitative inclusion has mostly been conducted from a theoretical standpoint. In this work, we conduct the first empirical study of quantitative inclusion for discounted-sum weighted automata (\DS-inclusion, in short).

Currently, two contrasting approaches for \DS-inclusion exist: the linear-programming based \cct{DetLP} and the purely automata-theoretic \cct{BCV}. Theoretical complexity of \DetLPt is exponential in time and space while of \BCVt is \cct{PSPACE}-complete. All practical implementations of \cct{BCV}, however, are also exponential in time and space. Hence, it is not clear which of the two algorithms renders a superior implementation.

In this work we present the first implementations of these algorithms, and perform extensive experimentation to compare between the two approaches. Our empirical analysis shows how the two approaches complement each other. This is a nuanced picture that is much richer than the one obtained from the theoretical study alone.

Model checking indistinguishability of randomized security protocols
SPEAKER: Prasad Sistla

ABSTRACT. The design of security protocols is extremely subtle and vulnerable to potentially devastating flaws. As result, many tools and techniques for the automated verification of protocol designs have been developed. Unfortunately, these tools don't have the ability to model and reason about protocols with randomization, which are becoming increasingly prevalent in systems providing privacy and anonymity guarantees. The security metrics of these systems is often formulated through a notion of indistinguishability. In this paper, we give the first algorithms for model checking indistinguishability properties of randomized security protocols against the powerful threat model of a Dolev-Yao attacker. Our techniques are implemented in the Stochastic Protocol ANalayzer (SPAN) and evaluated on several examples. As part of our evaluation, we conduct the first automated analysis of an electronic voting protocol based on the 3-ballot design.

Lazy Self-Composition for Security Verification
SPEAKER: Aarti Gupta

ABSTRACT. The secure information flow problem, which checks whether low-security outputs of a program are influenced by high-security inputs, has many applications in verifying security properties in programs. In this paper we present lazyself-composition, an approach for verifying secure information flow. It is based on self-composition, where two copies of a program are created on which a safety property is checked. However, rather than an eager duplication of the given program, it uses duplication lazily to reduce the cost of verification. This lazy self-composition is guided by an interplay between symbolic taint analysis on an abstract (single copy) model and safety verification on a refined (two copy) model. We propose two verification methods based on lazy self-composition.The first is a CEGAR-style procedure, where the abstract model associated with taint analysis is refined, on demand, by using a model generated by lazy self-composition. The second is a method based on bounded model checking, where taint queries are generated dynamically during program unrolling to guide lazy self-composition and to conclude an adequate bound for correctness. We have implemented these methods on top of the SEAHORN verification platform and our evaluations show the effectiveness of lazy self-composition.

SCInfer: Refinement-based Verification of Software Countermeasures against Side-Channel Attacks
SPEAKER: Jun Zhang

ABSTRACT. Power side-channel attacks, capable of deducing secret using statistical analysis techniques, have become a serious threat to devices in cyber-physical systems and the Internet of things. Random masking is a widely used countermeasure for removing the statistical dependence between secret data and side-channel leaks. Although there are some techniques for verifying whether software code has been perfectly masked, they are limited in accuracy and scalability.

To bridge this gap, we propose a refinement-based method for formally verifying masking countermeasures. Our method is more accurate than prior syntactic type inference rule based approaches and more scalable than model-counting based approaches using SAT/SMT solvers.

Indeed, our method can be viewed as a gradual refinement of a set of carefully designed semantic type inference rules for reasoning about distribution types. These rules are kept abstract initially to allow fast deduction, and then made more concrete if necessary, i.e., when the abstract version is not able to completely resolve the verification problem.

We have implemented our method in a tool named SCInfer and evaluated it on cryptographic benchmarks including AES and MAC-Keccak. Our results show that SCInfer significantly outperforms state-of-the-art techniques in terms of both accuracy and scalability.

Symbolic Algorithms for Graphs and Markov Decision Processes with Fairness Objectives
SPEAKER: Simin Oraee

ABSTRACT. Given a model and a specification, the fundamental model-checking problem asks for algorithmic verification of whether the model satisfies the specification. We consider graphs and Markov decision processes (MDPs), which are fundamental models for reactive systems. One of the very basic specifications that arise in verification of reactive systems is the strong fairness (aka Streett) objective. Given different types of requests and corresponding grants, the objective requires that for each type, if the request event happens infinitely often, then the corresponding grant event must also happen infinitely often. All \omega-regular objectives can be expressed as Streett objectives and hence they are canonical in verification. To handle the state-space explosion, symbolic algorithms are required that operate on a succinct implicit representation of the system rather than explicitly accessing the system. While explicit algorithms for graphs and MDPs with Streett objectives have been widely studied, there has been no improvement of the basic symbolic algorithms. The worst-case numbers of symbolic steps required for the basic symbolic algorithms are as follows: quadratic for graphs and cubic for MDPs. In this work we present the first sub-quadratic symbolic algorithm for graphs with Streett objectives, and our algorithm is sub-quadratic even for MDPs. Based on our algorithmic insights we present an implementation of the new symbolic approach and show that it improves the existing approach on several academic benchmark examples.

Attracting Tangles to Solve Parity Games

ABSTRACT. Parity games have important practical applications in formal verification and synthesis, especially to solve the model-checking problem of the modal mu-calculus. They are also interesting from the theory perspective, because they are widely believed to admit a polynomial solution, but so far no such algorithm is known.

We propose a new algorithm to solve parity games based on learning tangles, which are strongly connected subgraphs for which one player has a strategy to win all cycles in the subgraph. We argue that tangles play a fundamental role in the prominent parity game solving algorithms. We show that tangle learning is competitive in practice and the fastest solver for large random games.

10:30-11:00Coffee Break
11:00-12:15 Session 119A: SAT, SMT and Decision Procedures
Location: Maths LT1
Delta-Decision Procedures for Exists-Forall Problems over the Reals
SPEAKER: Soonho Kong

ABSTRACT. Solving nonlinear SMT problems over real numbers has wide applications in robotics and AI. While significant progress is made in solving quantifier-free SMT formulas in the domain, quantified formulas have been much less investigated. We propose the first delta-complete algorithm for solving satisfiability of nonlinear SMT over real numbers with universal quantification and a wide range of nonlinear functions. Our methods combine ideas from counterexample-guided synthesis, interval constraint propagation, and local optimization. In particular, we show how special care is required in handling the interleaving of numerical and symbolic reasoning to ensure delta-completeness. In experiments, we show that the proposed algorithms can handle many new problems beyond the reach of existing SMT solvers.

Solving Quantified Bit-Vectors using Invertibility Conditions

ABSTRACT. We present a novel approach for solving quantified bit-vector formulas in Satisfiability Modulo Theories (SMT) based on computing symbolic inverses of bit-vector operators. We derive conditions that precisely characterize when bit-vector constraints are invertible for a representative set of bit-vector operators commonly supported by SMT solvers. We utilize syntax-guided synthesis techniques to aid in establishing these conditions and verify them independently by using several SMT solvers. We show that invertibility conditions can be embedded into quantifier instantiations using Hilbert choice expressions and give experimental evidence that a counterexample-guided approach for quantifier instantiation utilizing these techniques leads to performance improvements with respect to state-of-the-art solvers for quantified bit-vector constraints.

Understanding and Extending Incremental Determinization for 2QBF

ABSTRACT. Incremental determinization is a recently proposed algorithm for solving quantified Boolean formulas with one quantifier alternation. In this paper, we formalize incremental determinization as a set of inference rules to help understand the design space of similar algorithms. We then present additional inference rules that extend incremental determinization in two ways. The first extension integrates the popular CEGAR principle and the second extension allows us to expand cases and analyze them in separation. The experimental evaluation demonstrates that the extensions significantly improve the performance.

The Proof Complexity of SMT Solvers
SPEAKER: Robert Robere

ABSTRACT. The resolution proof system has been enormously helpful in deepening our understanding of conflict-driven clause-learning (CDCL) SAT solvers. In the interest of providing a similar proof complexity-theoretic analysis of satisfiability modulo theories (SMT) solvers, we introduce a generalization of resolution called Res(T). We show that many of the known results comparing resolution and CDCL solvers lift to the SMT setting, such as the result of Pipatsrisawat and Darwiche showing that CDCL solvers with ``perfect'' non-deterministic branching and an asserting clause-learning scheme can polynomially simulate general resolution. We also describe a stronger version of Res(T), Res*(T), capturing SMT solvers allowing introduction of new literals.

We analyze the theory EUF of equality with uninterpreted functions, and show that the Res*(EUF) system is able to simulate an earlier calculus introduced by Bjorner and De Moura for the purpose of analyzing DPLL(EUF). Further, we show that Res*(EUF) (and thus SMT algorithms with clause learning over EUF, new literal introduction rules and perfect branching) can simulate the Frege proof system, which is well-known to be far more powerful than resolution. Finally, we prove under the Exponential Time Hypothesis (ETH) that any reduction from EUF to SAT (such as the Ackermann reduction) must, in the worst case, produce an instance of size at least n log n from an instance of size n.

Model Generation for Quantified Formulas: A Taint-Based Approach

ABSTRACT. We focus in this paper on generating models of quantified first-order formulas over built-in theories, which is paramount in software verification and bug finding. While standard methods are either geared toward proving the absence of solution or targeted to specific theories, we propose a generic approach based on a reduction to the quantifier-free case. Our technique allows thus to reuse all the efficient machinery developed for that context. Experiments show a substantial improvement over state-of-the-art methods.

12:30-14:00Lunch Break
14:00-15:15 Session 121A: Concurrency
Location: Maths LT1
Partial Order Aware Concurrency Sampling
SPEAKER: Xinhao Yuan

ABSTRACT. We present POS, a concurrency testing approach that samples the partial order of concurrent programs. POS uses a novel prioritybased scheduling algorithm that dynamically reassigns priorities regarding the partial order information and formally ensures that each partial order will be explored with significant probability. POS is simple to implement and provides a probabilistic guarantee of error detection better than state-of-the-art sampling approaches. Evaluations show that POS is effective in covering the partial-order space of micro-benchmarks and finding concurrency bugs in real-world programs, such as Firefox’s JavaScript engine SpiderMonkey.

Reasoning About TSO Programs Using Reduction and Abstraction

ABSTRACT. We present a method for proving that a program running under the Total Store Ordering (TSO) memory model is robust, i.e., all its TSO computations are equivalent to computations under the Sequential Consistency (SC) semantics. This method is inspired by Lipton's reduction theory for proving atomicity of concurrent programs. For programs which are not robust, we introduce an abstraction mechanism that allows to construct robust programs over-approximating their TSO semantics. This enables the use of proof methods designed for the SC semantics in proving invariants that hold on the TSO semantics of a non-robust program. These techniques have been evaluated on a large set of benchmarks using the infrastructure provided by CIVL, a generic tool for reasoning about concurrent programs under the SC semantics.

Quasi-Optimal Partial Order Reduction

ABSTRACT. A dynamic partial order reduction (DPOR) algorithm is optimal when it always explores at most one representative per Mazurkiewicz trace. Existing literature suggests the reduction obtained by the non-optimal state-of-the-art Source-DPOR (SDPOR) algorithm is comparable to optimal DPOR. We show the first program with O(n) Mazurkiewicz traces where SDPOR explores O(2^n) redundant schedules and identify the cause of the blow-up as an NP-hard problem. Our main contribution is a new approach, called Quasi-Optimal POR, that can arbitrarily approximate an optimal exploration using a provided constant k. We present an implementation of our method in a new tool called DPU using specialized data structures. Experiments with DPU, including Debian packages, show that optimality is achieved with low values of k, outperforming state-of-the-art tools.

On the Completeness of Verifying Message Passing Programs under Bounded Asynchrony

ABSTRACT. We address the problem of verifying message passing programs, defined as a set of parallel processes communicating through unbounded FIFO buffers. We introduce a bounded analysis that explores a special type of computations, called k-synchronous. These computations can be viewed as (unbounded) sequences of interaction phases, each phase allowing at most k send actions (by different processes), followed by a sequence of receives corresponding to sends in the same phase. We give a procedure for deciding k-synchronizability of a program, i.e., whether every computation is equivalent (has the same happens-before relation) to one of its k-synchronous computations. We also show that reachability over k-synchronous computations and checking k-synchronizability are both PSPACE-complete. Furthermore, we introduce a class of programs called {\em flow-bounded} for which the problem of deciding whether there exists a k>0 for which the program is k-synchronizable, is decidable.

Constrained Dynamic Partial Order Reduction
SPEAKER: Miguel Isabel

ABSTRACT. The cornerstone of dynamic partial order reduction (DPOR) is the notion of independence that is used to decide whether each pair of concurrent events p and t are in a race and thus both p.t and t.p must be explored. We present constrained dynamic partial order reduction (CDPOR), an extension of the DPOR framework which is able to avoid redundant explorations based on the notion of conditional independence --the execution of p and t commutes only when certain independence constraints (ICs) are satisfied. ICs can be declared by the programmer, but importantly, we present a novel SMT-based approach to automatically synthesize ICs in a static pre-analysis. A unique feature of our approach is that we have succeeded to exploit ICs within the state-of-the-art DPOR algorithm, achieving exponential reductions over existing implementations.

15:30-16:00Coffee Break
16:00-17:45 Session 122A: CPS, Hardware, Industrial Applications
Location: Maths LT1
Formal Verification of a Vehicle-to-Vehicle (V2V) Messaging System
SPEAKER: Mark Tullsen

ABSTRACT. Vehicle-to-Vehicle (V2V) communications is a ``connected vehicles'' standard that will likely be mandated in the U.S. within the coming decade. V2V, in which automobiles broadcast to one another, promises improved safety by providing collision warnings, but it also poses a security risk. At the heart of V2V is the communication messaging system, specified in SAE J2735 using the Abstract Syntax Notation One (ASN.1) data description language. Motivated by numerous previous ASN.1 related vulnerabilities, we present the formal verification of an ASN.1 encoder/decoder pair. We describe generating an encoder/decoder pair, implemented in C, using our internally developed ASN.1 toolset. We define self-consistency for encoder/decoder pairs that approximates functional correctness without requiring a formal specification of ASN.1. We then verify self-consistency and memory-safety using symbolic simulation via the Software Analysis Workbench.

Continuous formal verification of Amazon s2n
SPEAKER: Josiah Dodds

ABSTRACT. We describe formal verification of the open source TLS implementation used in numerous Amazon services, ranging from the Amazon Simple Storage Service (S3) to the Amazon Web Services Software Development Kit (AWS SDK). A key aspect of this proof is continuous checking: to ensure that properties proved remain proved during the lifetime of the software, at each change to the code, proofs are automatically re-established with little to no interaction from the development community. Technical decisions made about the structure of the proof and tools employed were driven by this goal.

Symbolic Liveness Analysis of Real-World Software

ABSTRACT. Liveness violation bugs are notoriously hard to detect, especially due to the difficulty inherent in applying formal methods to real-world programs. We present a generic and practically useful liveness property which defines a program as being live as long as it will eventually either consume more input or terminate. We show that this property naturally maps to many different kinds of real-world programs. To demonstrate the usefulness of our liveness property, we also present an algorithm that can be efficiently implemented to dynamically find fixed points of the target program during Symbolic Execution. This extends Symbolic Execution, a well known dynamic testing technique, to find a new class of program defects, namely liveness violations, while only incurring a small runtime and memory overhead, as evidenced by our evaluation. The implementation of our method found a total of five previously undiscovered software defects in BusyBox and the GNU Coreutils. All five defects have been confirmed and fixed by the respective maintainers after shipping for years, most of them well over a decade.

Model Checking Boot Code from AWS Data Centers

ABSTRACT. This paper describes our experience with symbolic model checking in an industrial setting. We have proved that the initial boot code running in data centers at Amazon Web Services is memory safe, an essential step in establishing the security of any data center. Standard static analysis tools cannot be easily used on boot code without modification due to issues not commonly found in higher-level code, including memory-mapped device interfaces, byte-level memory access, and linker scripts. This paper describes automated solutions to these issues and their implementation in the C Bounded Model Checker (CBMC). CBMC is now the first source-level static analysis tool to extract the memory layout described in a linker script for use in its analysis.

Android Stack Systems
SPEAKER: Zhilin Wu

ABSTRACT. In this paper, we propose Android Stack Systems (ASS), a formal model to capture key mechanisms of Android multi-tasking such as activities, the back stack, launch modes, as well as task affinities. The model is based on pushdown systems with multiple stacks, and focuses on the evolution of the back stack of the Android system when interacting with activities carrying specific launch modes and task affinities. For formal analysis, we study the reachability problem of ASS. While the general problem is shown to be undecidable, we identify expressive fragments for which various verification techniques for pushdown systems or their decidable extensions are harnessed to show that the reachability problem is decidable.

Formally Verified Montgomery Multiplication

ABSTRACT. We report on a machine assisted verification of an efficient implementation of Montgomery Multiplication which is a widely used method in cryptography for efficient computation of modular exponentiation. We shortly describe the method, give a brief survey of the VeriFun system used for verification, verify a classical as well as a more recent algorithm for computing multiplicative inverses, illustrate proof-technical obstacles encountered upon verification, present the formal proofs and finally report on the effort when creating the proofs. Our work uncovered a serious fault in a state-of-the-art algorithm for computing multiplicative inverses based on Newton-Raphson iteration, thus providing further evidence for the benefit of computer-aided verification.

Inner and Outer Approximating Flowpipes for Delay Differential Equations
SPEAKER: Sylvie Putot

ABSTRACT. Delay differential equations are fundamental for modelingvnetworked control systems where the underlying network induces delay for retrieving values from sensors or delivering orders to actuators. They are notoriously difficult to integrate, as these are actually functional equations, the initial state being a function. We propose a scheme to compute inner and outer approximated flowpipes for such equations with uncertain initial states and parameters. Inner-approximated flowpipes are guaranteed to contain only reachable states, while outer-approximated flowpipes enclose all reachable states. We also introduce a notion of robust inner-approximation, which we believe opens promising perspectives for verification, beyond property falsification. The efficiency of our approach relies on the combination of Taylor models in time, with an abstraction or parameterization in space based on affine forms, or zonotopes. It also relies on an extension of the mean-value theorem, which allows us to deduce inner-approximated flowpipes, from flowpipes outerapproximating the solution of the DDE and its Jacobian with respect to constant but uncertain parameters and initial conditions. We present some experimental results obtained with our C++ implementation.