exploring se for android confer roberts 2015 02 24 Lập trình android

As a longtime user of SELinux in server deployments, I knew its benefitsfrom a security point of view and also knew how much Android could benefit from them.. He has made contributions t

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CuuDuongThanCong.com

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www.it-ebooks.info

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Index

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Exploring SE for Android

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or transmitted in any form or by any means, without the prior written permission of thepublisher, except in the case of brief quotations embedded in critical articles or reviews.Every effort has been made in the preparation of this book to ensure the accuracy of theinformation presented However, the information contained in this book is sold withoutwarranty, either express or implied Neither the authors, nor Packt Publishing, and itsdealers and distributors will be held liable for any damages caused or alleged to be causeddirectly or indirectly by this book

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The first talk of SELinux on Android started almost as soon as Android was announced.The interest at that time was mainly shown by academic circles and developers of

SELinux itself As a longtime user of SELinux in server deployments, I knew its benefitsfrom a security point of view and also knew how much Android could benefit from them

At that time, I may have been coy about the reasons I wanted to commit some of the initialpatches to the SELinux project Looking back at the code reviews for those Android OpenSource Project (AOSP) changes, I now remember how much resistance there was in thebeginning Space on devices was at a premium, and it was considered a victory if we

could save a few kilobytes And here were the SELinux libraries and policies that

increased the system size by thirty kilobytes! The performance impact had not even beenmeasured at that time

The work continued unabated with SELinux contributors, such as Stephen Smalley,

Robert Craig, Joshua Brindle, and an author of this book, William Roberts, as well as withthe help of my coworkers Geremy Condra and Nick Kralevich at Google Slowly, throughthe herculean efforts of everyone involved, the project materialized and became more andmore complete Since Android 4.4 KitKat, SELinux is shipped in enforcing mode, and allAndroid users can benefit from the added protection that it affords

The tale doesn’t end there! Now, it’s your turn to learn This book is the first referenceavailable for the specific flavor of SELinux found in Android It’s my sincere hope thatthis book imparts the knowledge you need to understand and contribute to its continueddevelopment William Roberts has been submitting code to AOSP since the beginning ofSELinux for Android, and his and Dr Confer’s knowledge is contained in these pages It’s

up to you to read it and help write the next chapter of this saga

Kenny Root

Mountain View, CA

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William Confer has been engineering embedded and mobile systems since 1997 He has

worked for Samsung Mobile as a managing staff engineer and currently teaches computerscience at SUNY Polytechnic Institute He holds a patent in low-cost character recognitionfor extremely resource-limited devices and has multiple other patents pending for mobiletechnologies

My wife, Ása, sacrificed endlessly to help give me the space and time needed for thiswork, and I owe her more than I can say My three daughters also ensured I couldn’t

always be working on this book and distracted me in the best possible ways I couldn’t rest

if I didn’t thank all my fall 2014 students from SUNY Polytechnic Institute who put upwith me when I was sidetracked by this book Finally, and most importantly, my greatestthanks goes to my coauthor (and friend, student, and teacher), William Roberts, withoutwhom I would have to have found another

William Roberts is a software engineer who is focused on OS-level security and platform

enhancements He is one of the engineers who founded the Samsung KNOX product and

an early adopter of SE for Android He has made contributions to several open sourceprojects, such as SE for Android, the Android Open Source Project, the Linux Kernel,CyanogenMod, and OpenSC His recent interests have taken him to Smart Card

technologies and the virtualization of smart cards In his spare time, he works with Dr

simulator

I would like to thank Dr William Confer, the coauthor, for helping me write this book; hiscontributions were invaluable Also, I would like to thank my wife for supporting me andgiving me the time to do this, even though we were renovating the house Also, I wouldlike to thank my family and friends for their encouragement along the way

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Joshua Brindle is the CTO and cofounder of Quark Security Inc., a company focused on

solving mobile and cross-domain security problems Joshua has 12 years of professionalexperience in the area of development for government, academic, and open source

software that focuses on security in Linux Joshua has contributed to numerous opensource projects, both as a project maintainer and as a developer His work can be found onall SELinux systems and nearly all Linux systems Joshua’s recent experience focuses onbuilding secure mobile devices using technologies such as Security Enhancements forAndroid, mobile device, and application management

Hiromu Yakura is a student at Nada High School, Japan He is the youngest person to

hold the national information security qualification from Japan He has given lecturesabout SE for Android at many conferences He is also familiar with the security

competition, Capture the Flag (CTF), and has participated in DEF CON CTF 2014 as ateam binja

I would like to express my gratitude to my family for their understanding and support

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This book introduces the Security Enhancements (SE) for Android open source projectand walks you through the process of securing new embedded systems with SE for

Android To our knowledge, this book is the first source to document such a process in itsentirety so that students, DIY hobbyists, and engineers can create custom systems secured

by SE for Android Generally, only original equipment manufacturers (OEMs) do this, andquite commonly, the target device is a phone or tablet We truly hope our book will changethat, engaging a wide audience in development so they can use and understand these

modern security tools

We worked very hard to ensure this text is not just a step-by-step technology book

Specifically, we’ve chosen a model that directs you to fail your way to success You willfirst gain appropriate theoretical understanding of how security is gained and enforced.Then we will introduce a system that has never been secured that way (not even by us,prior to writing this book) Next, we’ll guide you through all our intelligent guesswork,embracing unexpected failures for the newly found idiosyncrasies they expose, and

eventually enforcing our custom security policies It requires you to learn to resolve

differences between major open source projects such as SELinux, SE for Android, andGoogle Android, each of which has independent goals and deployment schedules Thisprepares you to secure other devices, the process for which is always different, but

hopefully, will now be more accessible

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Chapter 1, Linux Access Controls, discusses the basics of Discretionary Access Control

(DAC), how some Android exploits leverage DAC problems, and demonstrate the needfor more robust solutions

Chapter 2, Mandatory Access Controls and SELinux, examines Mandatory Access Control

(MAC) and its manifestation in SELinux This chapter also explores tangible policy tocontrol SELinux object interaction

Chapter 9, Adding Services to Domains, emphasizes process labeling, notably the Android

services run and managed by init

Chapter 10, Placing Applications in Domains, shows you how to properly label the private

data directories of applications, as well as application runtime contexts via configurationfiles and SELinux policy

Chapter 11, Labeling Properties, demonstrates how to create and label new and existing

properties, and some of the anomalies that occur when doing so

Chapter 12, Mastering the Tool Chain, covers how the various components that control

policy on the device are actually built and created This chapter reviews the Android.mkcomponents, detailing how the heart of the build and configuration management works

Chapter 13, Getting to Enforcing Mode, utilizes all the skills you learned in the earlier

chapters to respond to audit logs from CTS and get the UDOO in enforcing mode

Appendix, The Development Environment, walks you through the necessary steps of

setting up a Linux environment suitable for you to follow all the activities in this book

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Hardware requirements include:

A UDOO-embedded development board

An 8 GB Mini SD card (while you can use a card with greater capacity, we do notrecommended it)

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This book is intended for developers and engineers who are somewhat familiar withoperating system concepts as implemented by Linux They could be hobbyists wanting tosecure their Android-powered creations, OEM engineers building handsets, or engineersfrom emerging areas where Android is seeing growth A basic background in C

programming will be helpful

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Android is an operating system composed of two distinct components The first

component is a forked mainline Linux kernel and shares almost everything in commonwith Linux The second component, which will be discussed later, is the user space

portion, which is very custom and Android specific Since the Linux kernel underpins thissystem and is responsible for the majority of access control decisions, it is the logicalplace to begin a detailed look at Android

Linux maintains data structures in the kernel for managing these permission fields, whichare accessible from user space, and ones that should be familiar to Linux and *NIX usersalike The first set of access control metadata belongs to the process, and forms a portion

of its credential set The common credentials are user and group In general, we use theterm group to mean both primary group and possible secondary group(s) You can view

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bookuser and GROUP as bookuserhand side of the output There are seven fields to consider as well Each empty field is

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Change: 2014-06-23 19:44:14.308741592 -0700

Birth: -The first access line is the most compelling It contains all the important information forthe access controls The second line is just a timestamp letting us know when the file was

bookuser as well The permission flags, (0664/-rw-rw-r ), identify the two ways that

bookuser, has permission to read from and write to hello.txt, and everyone else has

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programs), so any command we invoke should inherit our user’s permissions We canview it by issuing:

$ groups bookuser

bookuser : bookuser sudo fuse

Now that read permission is verified, let’s try write One simple way to do this is to write asimple program that writes something to the existing file In this case, we will write the

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testuser and bookuser That means when testuser accesses a file or other object (such

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As before, testuser is able to read the file The only difference is that it can now read the

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owner of an object can allow various forms of access by managing the permission bits of

testuser:

$ chown bookuser:testuser hello.txt

chown: changing ownership of `hello.txt': Operation not permitted

This did not work as we intended, but what is the issue? In Linux, only privileged

when attempting to execute that process Only processes create objects Privileged

capabilities set We will dive into the details of capabilities later For now, let’s focus onthe root

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Modify: 2014-08-23 12:47:19.123113845 -0700 Change: 2014-08-23 13:08:46.059058649 -0700 Birth: -

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in order to change the user and group of an object, you need to be privileged You can only

addressed by the capabilities model

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