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08:30-09:00Coffee & Refreshments
09:00-10:30 Session 26A

FoMLAS Session 5

Location: Ullmann 201
Scalable Verification of GNN-based Job Schedulers

ABSTRACT. Recently, Graph Neural Networks (GNNs) have been applied for scheduling jobs over clusters, achieving better performance than hand-crafted heuristics. Despite their impressive performance, concerns remain over whether these GNN-based job schedulers meet users' expectations about other important properties, such as strategy-proofness, sharing incentive, and stability. In this work, we consider formal verification of GNN-based job schedulers. We address several domain-specific challenges such as networks that are deeper and specifications that are richer than those encountered when verifying image and NLP classifiers. We develop vegas, the first general framework for verifying both single-step and multi-step properties of these schedulers based on carefully designed algorithms that combine abstractions, refinements, solvers, and proof transfer. Our experimental results show that vegas achieves significant speed-up when verifying important properties of a state-of-the-art GNN-based scheduler compared to previous methods.

VPN: Verification of Poisoning in Neural Networks
PRESENTER: Youcheng Sun

ABSTRACT. Neural networks are successfully used in many domains including safety and security critical applications. As a result researchers have proposed formal verification techniques for verifying neural network properties. A large majority of previous efforts have focused on checking local robustness in neural networks. We instead focus on another neural network security issue, namely data poisoning, whereby an attacker inserts a trigger into a subset of the training data, in such a way that at test time, this trigger causes the classifier to predict some target class. In this paper, we show how to formulate absence of data poisoning as a property that can be checked with off-the-shelf verification tools, such as Marabou and nneum. Counterexamples of failed checks constitute potential triggers that we validate through testing. We further show that the discovered triggers are ‘transferable’ from a small model to a larger, better-trained model, allowing us to analyze state-of-the art performant models trained for image classification tasks.

Verification-Aided Deep Ensemble Selection

ABSTRACT. Deep neural networks (DNNs) have become the technology of choice for realizing a variety of complex tasks. However, as highlighted by many recent studies, even an imperceptible perturbation to a correctly classified input can lead to misclassification by a DNN. This renders DNNs vulnerable to strategic input manipulations by attackers, and also prone to oversensitivity to environmental noise.

To mitigate this phenomenon, practitioners apply joint classification by an ensemble of DNNs. By aggregating the classification outputs of different individual DNNs for the same input, ensemble-based classification reduces the risk of misclassifications due to the specific realization of the stochastic training process of any single DNN. However, the effectiveness of a DNN ensemble is highly dependent on its members not simultaneously erring on many different inputs.

In this case study, we harness recent advances in DNN verification to devise a methodology for identifying ensemble compositions that are less prone to simultaneous errors, even when the input is adversarially perturbed --- resulting in more robustly-accurate ensemble-based classification.

Our proposed framework uses a DNN verifier as a backend, and includes heuristics that help reduce the high complexity of directly verifying ensembles. More broadly, our work puts forth a novel universal objective for formal verification that can potentially improve the robustness of real-world, deep-learning-based systems across a variety of application domains.

10:30-11:00Coffee Break
11:00-12:30 Session 31A

FoMLAS Session 6

Location: Ullmann 201
Formal Specification for Learning-Enabled Autonomous Systems (Extended Abstract)
PRESENTER: Doron Peled

ABSTRACT. Formal specification provides a uniquely readable description of various aspects of a system, including its temporal behavior. This facilitates testing and sometimes also automatic verification of the system against the given specification. We present a logic-based formalism for specifying learning-enabled autonomous systems, which involve components based on neural networks. The formalism applies temporal logic with predicates for characterizing events and uses universal quantification to allow enumeration of objects. While we have applied the formalism successfully to two complex use cases, several limitations such as monitorability or lack of quantitative satisfaction also reveal further improvement potential.

Vehicle: A High-Level Language for Embedding Logical Specifications in Neural Networks
PRESENTER: Luca Arnaboldi

ABSTRACT. Verification of neural network specifications is currently an active field of research in automated theorem proving. However, the actual act of verification is merely one step in the process of constructing a verified network. Prior to verification the specification should influence the training of the network, and afterwards users may want to export the verified specification to other verification environments in order to prove a specification about a larger system that uses the network. Currently there is little consensus on how best to connect these different stages.

In this talk we will describe our proposed solution to this problem: the Vehicle specification system. Vehicle allows the user to write a single specification in a high-level human readable form, and the Vehicle compiler then compiles it down to different targets, including training frameworks, verifiers and interactive theorem provers. In this talk we will discuss the various design decisions involved in the specification language itself and hope to solicit feedback from the verification community.

(Submitted as Extended Abstract)

Differentiable Logics for Neural Network Verification
PRESENTER: Natalia Ślusarz

ABSTRACT. The rising popularity of neural networks (NNs) in recent years and their increasing prevalence in real-world applications has drawn attention to the importance of their verification. While verification is known to be computationally difficult theoretically, many techniques have been proposed for solving it in practice.

It has been observed in the literature that by default neural networks rarely satisfy logical constraints that we want to verify. A good course of action is to train the given NN to satisfy said constraint prior to verifying them. This idea is sometimes referred to as continuous verification, referring to the loop between training and verification.

Usually training with constraints is implemented by specifying a translation for a given formal logic language into loss functions. These loss functions are then used to train neural networks. Because for training purposes these functions need to be differentiable, these translations are called differentiable logics (DL).

This raises several research questions on the technical side of "training with constraints". What kind of differentiable logics are possible? What difference does a specific choice of DL make in the context of continuous verification? What are the desirable criteria for a DL viewed from the point of view of the resulting loss function? In this extended abstract we will discuss and answer these questions.

(Submitted as Extended Abstract)

12:30-14:00Lunch Break

Lunches will be held in Taub hall and in The Grand Water Research Institute.

14:00-15:30 Session 34B

FoMLAS Session 7

Location: Ullmann 201
Neural Network Verification with Proof Production

ABSTRACT. Deep neural networks (DNNs) are increasingly being employed in safety-critical systems, and there is an urgent need to guarantee their correctness. Consequently, the verification com- munity has devised multiple techniques and tools for verifying DNNs. When DNN verifiers discover an input that triggers an error, that is easy to confirm; but when they report that no error exists, there is no way to ensure that the verification tool itself is not flawed. As multiple errors have already been observed in DNN verification tools, this calls the applicability of DNN verification into question. In this work, we present a novel mechanism for enhancing Simplex-based DNN verifiers with proof production capabilities: the generation of an easy-to- check witness of unsatisfiability, which attests to the absence of errors. Our proof production is based on an efficient adaptation of the well-known Farkas’ lemma, combined with mechanisms for handling piecewise-linear functions and numerical precision errors. As a proof of concept, we implemented our technique on top of the Marabou DNN verifier. Our evaluation on a safety- critical system for airborne collision avoidance shows that proof production succeeds in almost all cases, and entails only a small overhead.

Efficient Neural Network Verification using Branch and Bound

ABSTRACT. In this talk, I will describe two recent Branch and Bound (BaB) verifiers developed by our group to ensure different safety properties of neural networks. The BaB verifiers involve two main steps: (1) recursively splitting the original verification problem into easier independent subproblems by splitting input or hidden neurons;  and (2) for each split subproblem, using fast but incomplete bound propagation techniques to compute sound estimated bounds for the outputs of the target neural network. One of the key limitations of existing BaB verifiers is computing tight relaxations of activation functions' (i.e., ReLU) nonlinearities. Our recent works (α-CROWN and β-CROWN) introduce a primal-dual approach and jointly optimize the corresponding Lagrangian multipliers for each ReLU with gradient ascent. Such an approach is highly parallelizable and avoids calls to expensive LP solvers. Our verifiers not only provide tighter output estimations than existing bound propagation methods but also can fully leverage GPUs with massive parallelization. Our verifier, α, β-CROWN (alpha-beta-CROWN), won the second International Verification of Neural Networks Competition (VNN-COMP 2021) with the highest total score. 

Bio: Suman Jana is an associate professor in the department of computer science and the data science institute at Columbia University.  His primary research interest is at the intersections of computer security and machine learning. His research has received six best paper awards, a CACM research highlight, a Google faculty fellowship, a JPMorgan Chase Faculty Research Award, an NSF CAREER award, and an ARO young investigator award.

15:30-16:00Coffee Break
16:00-17:00 Session 37B

FoMLAS Session 8

Location: Ullmann 201
Minimal Multi-Layer Modifications of Deep Neural Networks
PRESENTER: Idan Refaeli

ABSTRACT. Deep neural networks (DNNs) have become increasingly popular in recent years. However, despite their many successes, DNNs may also err and produce incorrect and potentially fatal outputs in safetycritical settings, such as autonomous driving, medical diagnosis, and airborne collision avoidance systems. Much work has been put into detecting such erroneous behavior in DNNs, e.g., via testing or verification, but removing these errors after their detection has received lesser attention. We present here a new framework, called 3M-DNN, for repairing a given DNN, which is known to err on some set of inputs. The novel repair procedure employed by 3M-DNN computes a modification to the network’s weights that corrects its behavior, and attempts to minimize this change via a sequence of calls to a backend, black-box DNN verification engine. To the best of our knowledge, our method is the first one that allows repairing the network by simultaneously modifying the weights of multiple layers. This is achieved by splitting the network into sub-networks, and applying a single-layer repairing technique to each component. We evaluated 3M-DNN on an extensive set of benchmarks, obtaining promising results.

Self-Correcting Neural Networks For Safe Classification
PRESENTER: Ravi Mangal

ABSTRACT. Classifiers learnt from data are increasingly being used as components in systems where safety is a critical concern. In this work, we present a formal notion of safety for classifiers via constraints called safe-ordering constraints. These constraints relate requirements on the order of the classes output by a classifier to conditions on its input, and are expressive enough to encode various interesting examples of classifier safety specifications from the literature. For classifiers implemented using neural networks, we also present a run-time mechanism for the enforcement of safe-ordering constraints. Our approach is based on a self-correcting layer, which provably yields safe outputs regardless of the characteristics of the classifier input. We compose this layer with an existing neural network classifier to construct a self-correcting network (SC-Net), and show that in addition to providing safe outputs, the SC-Net is guaranteed to preserve the classification accuracy of the original network whenever possible. Our approach is independent of the size and architecture of the neural network used for classification, depending only on the specified property and the dimension of the network’s output; thus it is scalable to large state-of-the-art networks. We show that our approach can be optimized for a GPU, introducing run-time overhead of less than 1ms on current hardware-even on large, widely-used networks containing hundreds of thousands of neurons and millions of parameters.

18:30-20:00Workshop Dinner (at the Technion, Taub Terrace Floor 2) - Paid event