Mario Werner

Pascal Nasahl, Robert Schilling, Mario Werner et al.

Memory vulnerabilities are a major threat to many computing systems. To effectively thwart spatial and temporal memory vulnerabilities, full logical memory safety is required. However, current mitigation techniques for memory safety are either too expensive or trade security against efficiency. One promising attempt to detect memory safety vulnerabilities in hardware is memory coloring, a security policy deployed on top of tagged memory architectures. However, due to the memory storage and bandwidth overhead of large tags, commodity tagged memory architectures usually only provide small tag sizes, thus limiting their use for security applications. Irrespective of logical memory safety, physical memory safety is a necessity in hostile environments prevalent for modern cloud computing and IoT devices. Architectures from Intel and AMD already implement transparent memory encryption to maintain confidentiality and integrity of all off-chip data. Surprisingly, the combination of both, logical and physical memory safety, has not yet been extensively studied in previous research, and a naive combination of both security strategies would accumulate both overheads. In this paper, we propose CrypTag, an efficient hardware/software co-design mitigating a large class of logical memory safety issues and providing full physical memory safety. At its core, CrypTag utilizes a transparent memory encryption engine not only for physical memory safety, but also for memory coloring at hardly any additional costs. The design avoids any overhead for tag storage by embedding memory colors in the upper bits of a pointer and using these bits as an additional input for the memory encryption. A custom compiler extension automatically leverages CrypTag to detect logical memory safety issues for commodity programs and is fully backward compatible.

Claudio Canella, Mario Werner, Daniel Gruss et al.

Software vulnerabilities in applications undermine the security of applications. By blocking unused functionality, the impact of potential exploits can be reduced. While seccomp provides a solution for filtering syscalls, it requires manual implementation of filter rules for each individual application. Recent work has investigated automated approaches for detecting and installing the necessary filter rules. However, as we show, these approaches make assumptions that are not necessary or require overly time-consuming analysis. In this paper, we propose Chestnut, an automated approach for generating strict syscall filters for Linux userspace applications with lower requirements and limitations. Chestnut comprises two phases, with the first phase consisting of two static components, i.e., a compiler and a binary analyzer, that extract the used syscalls during compilation or in an analysis of the binary. The compiler-based approach of Chestnut is up to factor 73 faster than previous approaches without affecting the accuracy adversely. On the binary analysis level, we demonstrate that the requirement of position-independent binaries of related work is not needed, enlarging the set of applications for which Chestnut is usable. In an optional second phase, Chestnut provides a dynamic refinement tool that allows restricting the set of allowed syscalls further. We demonstrate that Chestnut on average blocks 302 syscalls (86.5%) via the compiler and 288 (82.5%) using the binary-level analysis on a set of 18 widely used applications. We found that Chestnut blocks the dangerous exec syscall in 50% and 77.7% of the tested applications using the compiler- and binary-based approach, respectively. For the tested applications, Chestnut prevents exploitation of more than 62% of the 175 CVEs that target the kernel via syscalls. Finally, we perform a 6 month long-term study of a sandboxed Nginx server.

Pascal Nasahl, Robert Schilling, Mario Werner et al.

To ensure secure and trustworthy execution of applications, vendors frequently embed trusted execution environments into their systems. Here, applications are protected from adversaries, including a malicious operating system. TEEs are usually built by integrating protection mechanisms directly into the processor or by using dedicated external secure elements. However, both of these approaches only cover a narrow threat model resulting in limited security guarantees. Enclaves in the application processor typically provide weak isolation between the secure and non-secure domain, especially when considering side-channel attacks. Although secure elements do provide strong isolation, the slow communication interface to the application processor is exposed to adversaries and restricts the use cases. Independently of the used implementation approach, TEEs often lack the possibility to establish secure communication to external peripherals, and most operating systems executed inside TEEs do not provide state-of-the-art defense strategies, making them vulnerable against various attacks. We argue that TEEs implemented on the main application processor are insecure, especially when considering side-channel attacks. We demonstrate how a heterogeneous architecture can be utilized to realize a secure TEE design. We directly embed a processor into our architecture to provide strong isolation between the secure and non-secure domain. The tight coupling of TEE and REE enables HECTOR-V to provide mechanisms for establishing secure communication channels. We further introduce RISC-V Secure Co-Processor, a security-hardened processor tailored for TEEs. To secure applications executed inside the TEE, RVSCP provides control-flow integrity, rigorously restricts I/O accesses to certain execution states, and provides operating system services directly in hardware.

Robert Schilling, Mario Werner, Stefan Mangard

In typical software, many comparisons and subsequent branch operations are highly critical in terms of security. Examples include password checks, signature checks, secure boot, and user privilege checks. For embedded devices, these security-critical branches are a preferred target of fault attacks as a single bit flip or skipping a single instruction can lead to complete access to a system. In the past, numerous redundancy schemes have been proposed in order to provide control-flow-integrity (CFI) and to enable error detection on processed data. However, current countermeasures for general purpose software do not provide protection mechanisms for conditional branches. Hence, critical branches are in practice often simply duplicated. We present a generic approach to protect conditional branches, which links an encoding-based comparison result with the redundancy of CFI protection mechanisms. The presented approach can be used for all types of data encodings and CFI mechanisms and maintains their error-detection capabilities throughout all steps of a conditional branch. We demonstrate our approach by realizing an encoded comparison based on AN-codes, which is a frequently used encoding scheme to detect errors on data during arithmetic operations. We extended the LLVM compiler so that standard code and conditional branches can be protected automatically and analyze its security. Our design shows that the overhead in terms of size and runtime is lower than state-of-the-art duplication schemes.

Mario Werner, Thomas Unterluggauer, David Schaffenrath et al.

Embedded devices in the Internet of Things (IoT) face a wide variety of security challenges. For example, software attackers perform code injection and code-reuse attacks on their remote interfaces, and physical access to IoT devices allows to tamper with code in memory, steal confidential Intellectual Property (IP), or mount fault attacks to manipulate a CPU's control flow. In this work, we present Sponge-based Control Flow Protection (SCFP). SCFP is a stateful, sponge-based scheme to ensure the confidentiality of software IP and its authentic execution on IoT devices. At compile time, SCFP encrypts and authenticates software with instruction-level granularity. During execution, an SCFP hardware extension between the CPU's fetch and decode stage continuously decrypts and authenticates instructions. Sponge-based authenticated encryption in SCFP yields fine-grained control-flow integrity and thus prevents code-reuse, code-injection, and fault attacks on the code and the control flow. In addition, SCFP withstands any modification of software in memory. For evaluation, we extended a RISC-V core with SCFP and fabricated a real System on Chip (SoC). The average overhead in code size and execution time of SCFP on this design is 19.8% and 9.1%, respectively, and thus meets the requirements of embedded IoT devices.