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 byRajiv Marothu

University of Central Florida

Outline

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

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Introduction - Kernel Space The core of Operating System resides Can be accessed through systems calls Similar to running in real mode assembly language

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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.

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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

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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

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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

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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.

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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

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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

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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

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Hooks per Page Histogram

Fig: Distribution of Kernel hooks in running Ubuntu system

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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

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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

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HookSafe Architecture

Fig: Hooksafe Architecture

Offline Hook ProfilerOnline Hook Protector

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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).  

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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

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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.

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Offline Hook Profiler Implementation

Run in emulation and hooks are recorded with set of read/write (HAPs) and values

Fig: Hook access profile

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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

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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.

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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.

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Online hook protectorRun-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.

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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.

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Online hook protector

Fig: Architecture of Online Hook Protection 25

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Online hook protector

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Fig: Implementation of hook indirection

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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.

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Evaluation

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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.

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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

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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

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Thank You