Enabling Execute-Only Memory for COTS Binaries on Aarch64
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2017 IEEE Symposium on Security and Privacy NORAX: Enabling Execute-Only Memory for COTS Binaries on AArch64 Yaohui Chen∗ Dongli Zhang∗ Ruowen Wang† Rui Qiao∗ Ahmed M. Azab† Long Lu∗ Hayawardh Vijayakumar† Wenbo Shen† ∗ Stony Brook University † Samsung Research America Abstract—Code reuse attacks exploiting memory disclosure to attackers. Without knowing the locations of needed code or vulnerabilities can bypass all deployed mitigations. One promis- gadgets, attackers cannot build code-reuse chains. ing defense against this class of attacks is to enable execute- However, memory disclosure attacks can use information only memory (XOM) protection on top of fine-grained address space layout randomization (ASLR). However, recent works leaks in programs to de-randomize code locations, thus de- implementing XOM, despite their efficacy, only protect programs feating ASLR. Such attacks either read the program code that have been (re)built with new compiler support, leaving (direct de-randomization) or read code pointers (indirect de- commercial-off-the-shelf (COTS) binaries and source-unavailable randomization). Given that deployed ASLR techniques ran- programs unprotected. domize the load address of a large chunk of data or code, We present the design and implementation of NORAX, a leaking a single code pointer or a small sequence of code practical system that retrofits XOM into stripped COTS binaries on AArch64 platforms. Unlike previous techniques, NORAX allows attackers to identify the corresponding chunk, infer its requires neither source code nor debugging symbols. NORAX base address, and calculate the addresses of gadgets contained statically transforms existing binaries so that during runtime in the chunk. their code sections can be loaded into XOM memory pages More sophisticated fine-grained ASLR techniques [3]–[7] with embedded data relocated and data references properly updated. NORAX allows transformed binaries to leverage the aim at shuffling code blocks within the same module to make new hardware-based XOM support—a feature widely available it more difficult for attackers to guess the location of binary on AArch64 platforms (e.g., recent mobile devices) yet virtually instructions. Nevertheless, research by Snow et al. [1] proves unused due to the incompatibility of existing binaries. Further- that memory disclosure vulnerabilities can bypass the most more, NORAX is designed to co-exist with other COTS binary sophisticated ASLR techniques. hardening techniques, such as in-place randomization (IPR). We apply NORAX to the commonly used Android system binaries Therefore, a robust and effective defense against code- running on SAMSUNG Galaxy S6 and LG Nexus 5X devices. The reuse attacks should combine fine-grained ASLR with memory results show that NORAX on average slows down the execution disclosure prevention. Some recent works proposed to prevent of transformed binaries by 1.18% and increases their memory memory disclosures using compile-time techniques [8]–[10]. footprint by 2.21%, suggesting NORAX is practical for real-world Despite their effectiveness, these solutions cannot cover COTS adoption. binaries that cannot be easily recompiled and redeployed. These binaries constitute a significant portion of real-world I. INTRODUCTION applications that need protection. Modern commodity operating systems employ code in- XnR [11] is a recent work that enables executable-only tegrity protection techniques, such as data execution pre- memory (XOM [12]), which prevents code in memory from vention (DEP), to prevent traditional code injection attacks. being read as data, and in turn, blocks leaking of code Consequently, recent attacks [1], [2] increasingly leverage locations. However, XnR implements XOM at the OS level via code-reuse techniques to gain control of vulnerable programs. paging-based access control, which can cause high overhead. In code reuse attacks, a target application’s control flow is Moreover, XnR cannot directly protect COTS binaries that are manipulated in a way that snippets of existing code (called not originally built to make use of this protection. gadgets) are chained and run to carry out malicious activities. Other defenses against memory disclosure follow the idea of Knowledge of process memory layout is a key prerequisite destructive code reads [13], [14]: code is destroyed upon being for code-reuse attacks to succeed. Attackers need to know read, and therefore cannot be later executed as part of a code the exact binary instruction locations in memory to assemble reuse exploit. Unfortunately, it has been shown that destructive the chain of gadgets. Commodity operating systems widely code reads can be bypassed through code reloading [15]. In adopt address space layout randomization (ASLR), which addition, such defenses are not suitable for Android, where loads code binaries at random memory locations unpredictable all apps load system libraries at the same locations [16]. © 2017, Yaohui Chen. Under license to IEEE. 304 DOI 10.1109/SP.2017.30 Therefore, a memory read in one app enables code reuse requiring source code or debugging symbols. attacks in any other app. • We show that code-data separation problem, although In this work, we propose NORAX 1, which protects COTS undecidable in principle, is in practice achievable on binaries from code memory disclosure attacks. NORAX allow AArch64 platforms using our novel embedded data de- COTS binaries to be loaded in hardware-enforced XOM, tection algorithm. a security feature supported by recent ARM CPUs (i.e., • We perform rigorous and extensive evaluations with AArch64). Such CPUs are widely seen on today’s mobile stripped system executables and libraries on Android and devices. Without NORAX, to use the XOM feature, binaries show that NORAX is practical, effective and efficient. need to be (re)built with the necessary compiler support. This The rest of the paper is organized as follows: In § II we requirement stands in between the valuable security feature lay out the background for execute-only memory and explain and a large number of COTS binaries (e.g., all Android system the code-data separation challenges tackled by NORAX;In executables and libraries) that are already running on AArch64 § III we derive the requirements for a practical solution and CPUs but were not compiled with XOM support. NORAX then present the design of our system; In § IV we discuss in removes this requirement. It automatically patches existing bi- details the system implementation and the optimization for our naries and loads their code to XOM-enforced memory regions, reference platform Android; We then examine the correctness without affecting binaries’ normal execution. As a result, of NORAX and evaluate its performance in § V. We contrast binaries without special (re)compilation can benefit from the the related works in § VI and analyze the compatibility of hardware-backed XOM feature and be protected against code NORAX with other COTS hardening techniques and its current memory disclosure. Further, when used together with ASLR, limitations in § VII. We conclude the paper in § VIII. NORAX enables robust mitigation against code reuse attacks for COTS binaries. It is worth noting that we use Android as II. BACKGROUND the reference platform for building and evaluating NORAX. NORAX makes use of the modern MMU support in However, NORAX’s approach and techniques are generally AArch64 architecture to create execute-only memory, which applicable to other AArch64 platforms. is a hardware feature now widely available yet virtually NORAX consists of four major components: NDisassem- unused due to compatibility issues. To bridge the gap, NORAX bler, NPatcher, NLoader, and NMonitor. The first two perform reconstructs COTS binaries running on commodity Android offline binary analysis and transformation. They convert any smartphones to enforce the R ⊕ X policy. In the rest of this COTS binary built for AArch64 without XOM support into section, we explain the necessary technical background and one whose code can be protected by XOM during runtime. the challenges we face when building the system. The other two components provide supports for loading and monitoring the patched, XOM-enabled binaries during run- AArch64 eXecute-Only Memory (XOM) Support: AArch64 time. The design of NORAX tackles a fundamentally difficult defines four Exception Levels, from EL0 to EL3. EL0 has the problem: identifying data embedded in code segments, which lowest execution privilege, usually runs normal user applica- are common in ARM binaries, and relocating such data tions; EL1 is usually for hosting privileged systems, such as elsewhere so that during runtime code memory pages can be operating system kernel; EL2 is designed for hypervisor while made executable-only while allowing all embedded data to be EL3 is for secure monitor. readable. In order to enforce the instruction access permission for dif- As a evaluation, we apply NORAX to Android system ferent Exception Levels, AArch64 leverages the Unprivileged binaries running on SAMSUNG Galaxy S6 and LG Nexus eXecute Never (UXN) bit, Privileged eXecute-Never (PXN) 5X devices. The results show that NORAX on average slows bit and two AP (Access Permission) bits defined in the page down the transformed binaries by 1.18% and increases their table entry [17]. For the user space program code page, the memory footprint by 2.21%, suggesting NORAX is practical UXN bit is set to “0”, which allows the code execution at for real-world adoption. EL0, while PXN is set to “1”, which disables the execution In summary, our work makes the following contributions: in EL1. With such UXN and PXN settings, the instruction • We discover and address the gap between the highly access permissions defined by AP bits are shown in Table I. valuable XOM feature and existing binaries, which need It is easy to see that we can set the AP bits in page table but cannot use the feature without recompilation. entry to “10”, so that the kernel running in EL1 will enforce • We design and implement a comprehensive system that the execute-only permission for user space program, which is converts COTS binaries to be XOM-compatible without running in EL0.