Countering Kernel Rootkits with Lightweight Hook Protection Zhi Wang, Xuxian Jiang, Weidong Cui, Peng Ning 16th ACM Conference on Computer and Communications Security (CCS), November 2009 Presentation by Rajiv Marothu

Outline Introduction Example scenario Traditional Defense Mechanisms Motivation Design and Implementation Performance Closing Remarks

Introduction - Kernel Space The core of Operating System resides Can be accessed through systems calls Similar to running in real mode assembly language the kernel runs in kernel space, and normal programs run in user space,To keep the machine as stable as possible, you normally want only the most trusted, well-tested code to run in kernel mode/kernel space. Last Point Easy programming

Introduction - Hooking Definition - Hook Function pointers, return addresses, e.g. ext3_dir_operations->readdir Definition - Hooking Techniques used to alter or augment the behavior of an operating system, of applications, or of other software components by intercepting function calls or messages or events passed between software components.

Introduction - Rootkits Rootkit is a software program designed to gain control over a system or network. Hide presence and activities Hijack control by modifying kernel spaces Rootkits can not only hide their presence but also tamper with OS functionalities to launch various attacks. Opening backdoors Stealing private information Escalating privileges of malicious processes Disable defense mechanisms Ways of install, A rootkit is a stealthy type of malicious software (malware) designed to hide the existence of certain processes or programs from normal methods of detection and enables continued privileged access to a computer, Infects target machine typically exploiting vulnerabilities in some other applications

Example (TDL4 Rootkit) From the Rootkit.Win32.TDSS family Installs in Master Boot Record Runs before the Operating System Blocks programs from running Delivers advertisements Google redirects Source: Google/images Over 4.5 million machines were infected with it in the first three months of 2011, It was often by noted by journalists as "indestructible" in 2011 Best way to get rid of it is by replacing the MBR Previous versions (infecting drivers) could be removed with TDSS Killer from Kasperry group

Traditional Defense Approaches Three major research categories: Analysis of rootkit behavior Panorama, HookFinder, K-Tracer, and PoKeR Search common symptoms exhibited by rootkit infection Copilot, S BCFI, and Vmwatcher Preservation of kernel code integrity by preventing code from executing SecVisor, Patagonix, and NICKLE Can be bypassed by return-oriented rootkits, Hijack function pointers or return addresses,Utilize kernel code snippets

Motivation Hijacking attack on return address and function pointers In addition to the preservation of kernel code integrity, it is also equally important to safeguard relevant kernel control data By preserving the kernel control flow integrity, it enables the system to block out all rootkit infections in the first place. The act of preventing malicious rootkit codes from executing alone is not enough. This type of security can be bypassed easily. Rootkits such as the return-oriented ones, will first subvert kernel control flow and then launch the attack by only utilizing legitimate kernel code snippets.

Contributions Design, implementation, and evaluation of HookSafe Hooksafe - Hypervisor-based lightweight system that can protect thousands of kernel hooks from being hijacked by kernel rootkits. “In computing, a hypervisor, also called virtual machine monitor (VMM), is one of many virtualization techniques which allow multiple operating systems, termed guests, to run concurrently on a host computer, a feature called hardware virtualization.” Wikipedia Efficiency of defense against rootkits using HookSafe Low overhead introduced using the tool Hypervisor

Hook Safe Challenge: Protection granularity gap Hook protection requires byte granularity Hardware only provides page level protection Kernel hooks (function pointers), after initialized have frequent read access, less write access Move hooks to page-aligned memory and protect with traditional page protection Any write access can be monitored Small overhead effect

Experiment They analyzed a typical Ubuntu 8.04 server using a whole emulator called QEMU. They used 5881 Linux Kernel Hooks They found that these Kernel hooks are scattered across 41 Pages and some of them located in dynamic kernel heap

Hooks per Page Histogram Fig: Distribution of Kernel hooks in running Ubuntu system Worst case, 1 hook is allocated in a page (4092 bytes) along with other 4092 bytes of dynamic data.

Pages and Page Processing Fundamental to use non-continuous memory blocks Creates a mapping between a physical and virtual address, Provides virtual RAM Source: http://www.answers.com/topic/page-table LB is an associative cache. When a virtual address needs to be translated into a physical address, the TLB is searched first.

Problems Overview Classification of kernel rootkits Kernel Object Hooking (KOH) - hijack kernel control flow Dynamic Kernel Object Manipulation (DKOM) - modify dynamic data objects Majority of kernel rootkits are KOH rootkits (96%) KOH can gain control over kernel execution Code hooks Data hooks - most common type Kernel hooks are scattered across kernel space Prior techniques are not suitable for protecting significant amount of hooks

HookSafe Architecture Fig: Hooksafe Architecture Offline Hook Profiler Online Hook Protector

Offline hook profiler It is a component that profiles the guest kernel execution and outputs a hook access profile for each protected hook. Hook access profile will be used to enable transparent hook indirection. Kernel instructions that read or write to a hook called Hook Access Points (HAPs).  

Offline hook profiler Static analysis It is Performed on OS kernel source code, Utilize known program analysis technique to automatically collect hook access profile. More complete, but less precise. Dynamic analysis Doesn’t need OS kernel source code Run the target system on the top of an emulator and monitor every memory access to derive the hook access instruction. Allow for recording precise runtime information, but less coverage Tradeoff Coverage (static) vs Precision (dynamic) HookSafe choses precision over coverage

Offline hook profiler Implementation It is based on an open source whole system emulator QEMU uses binary translation technique which rewrites guest’s binary instruction. Then records executions of instructions that read or write memories. If instruction accesses any kernel hook it is recorded as HAP and the value. At the end, collected HAP instructions and values will be compiled as corresponding hook access profile.

Offline Hook Profiler Implementation Run in emulation and hooks are recorded with set of read/write (HAPs) and values Fig: Hook access profile

Online hook protector Its input is the Hook Access Profile. Creates a shadow copy of all protected hooks Instruments HAP instructions such that their accesses will be transparently redirected to the shadow copy. Shadow copies are moved into a centralized location to be protected from unauthorized modifications and kernel rootkits. (i.e. page level protection). Protection granularity gap problem resolved Thin Hook

Online hook protector Three processes of design: Initialization: 1. Uses a short-lived kernel module (temporary) to create shadow copy of kernel hooks and load the code for indirection layer. 2. Use the online patching that provided by the hypervisor in order to instrument HAPs in guest kernel.

Online hook protector Run-Time Read/Write Indirection Read Access: reads from the shadow hook copy and returns to HAP site. Write Access: indirection layer issues hyper call and transfers control to hypervisor for validation check. Memory protection component validates write request and update shadow hook.

Online hook protector Run-Time Tracking of Dynamically Allocated Hooks Dynamically Allocated Hooks are embedded in Dynamic Kernel Object. If one such kernel object is being allocated, a hypercall will be issued to HookSafe to create a shadow copy of the hook Another hypercall is triggered to remove the shadow copy when kernel object is released. 1. Heap

Online hook protector Implementation It is developed based on Xen Hypervisor. Hypervisor replaces the HAP instruction at runtime with ‘jmp’ instruction to allow execution flow to trampoline code in Hook indirection layer. Trampoline code collects runtime info which is used by hook redirector to determine exact kernel hook being accessed. After hook redirector processes the actual read or write on shadow hook, trampoline executes HAP specific overwritten instruction. 3. Before returning to original program.

Online hook protector Fig: Architecture of Online Hook Protection The essential idea here is to leverage a thin hook indirection layer to regulate accesses to kernel hooks. Specifically, after a guest OS boots up, we first create a shadow copy of identified hooks, and then instrument all HAPs in kernel code so that read or write accesses will be redirected to the hook indirection layer.

Online hook protector Fig: Implementation of hook indirection Variable length instructions in x86 ,jmps are padded with NOP (0x90), old instructions are re-written and moved

Evaluation In order to evaluate HookSafe’s effectiveness in preventing real-world rootkits, They used the Xen Hypervisor (version 3.3.o) to protect more than 5900 kernel hooks in Ubuntu 8.04 Linux system. There experiments with nine real-world rootkits show that Hooksafe can effectively defeat these nine rootkits attempt to hijack kernal hooks that are being protected. It prevented all of nine rootkits from modifying protected hooks and hiding themselves. This large scale protection is achieved with only 6% slow down in system performance.

Evaluation

Closing Remarks - Strengths Rootkit protection is performed dynamically i.e., without need of source code Low overhead of 6% of runtime Works with variable instruction length architecture Perform byte equivalent protection by using page protection of the hypervisor. 1. (Dynamic Analysis) 3. X86

Closing Remarks - Weakness Do not record what caused the rootkit infection. It can detect, but not defend against future attempts. When discrepancy is found it automatically assumes the original hook was compromised. Memory usage for creating shadow copies

Suggestions HookSafe should be tested on cross platforms Instead of checking discrepancy between hooks and their copy, we can try checking against a hash value to find out which is compromised 1. windows

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