CS 153 Design of Operating Systems Spring 2015 Lecture 24: Android OS

OS Abstractions 2 Operating System Hardware Applications CPUDiskRAM ProcessFile systemVirtual memory Network I/O Devices

Smartphones 3 ==

…in

What is the difference between a mobile OS and a desktop/server OS? 5

Differences l Size / form-factor u UI system design? l Resource-constrained (e.g., battery, memory) u Optimized OS (what would you do?) l Cellular and other hardware components l User has no root access u Unless OS has vulnerabilities l Security threats u App is fully sandboxed and cannot easily attack other apps 6

Android 7

Based on Linux Linux on ARM Drivers and architecture support How to port Android to a new device? Using Linux vs. Writing a new OS from scratch? Do all Linux kernel implementations work well on mobile devices? 8

Android 9

Android 10

Android 11

Differences l Size / form-factor l Resource-constrained (e.g., battery, memory) u Optimized OS (what would you do?) l Cellular and other hardware components l User has no root access u Unless OS has vulnerabilities l Security threats u Malware is fully sandboxed and cannot easily attack other apps 12

Offloading of Computation Naive offloading Speech-to-text, OCR, Apple’s Siri More sophisticated offloading - fine-grained offloading MAUI: Making Smartphones Last Longer with Code Offload Running two versions of the app on the mobile device and a powerful server Decide when/what to offload on the fly 13

Differences l Size / form-factor l Resource-constrained (e.g., battery, memory) u Optimized OS (what would you do?) l Cellular and other hardware components l User has no root access u Unless OS has vulnerabilities l Security threats u Malware is fully sandboxed and cannot easily attack other apps 14

Disk I/O FlashHard Disk Drive Random access~0.1ms5-10ms File fragment impactNoGreatly impacted Total power1/2 to 1/3 of HDDup to 15+ watts ReliabilityReliable Less reliable due to mechanical parts Write longevity Limited number of writes Less of a problem Capacity<= 1TB4TB Price$0.4 / GB$0.04 / GB 15

New Capabilities l Cellular u Make phone calls u Send/Recv SMS l GPS u Tracking l Phone number u Identification... l How to secure them? 16

Android Security 17

Android Sandbox l UID separation to protect apps from each other 18

Android Permission l Apps need permissions when they attempt to u access sensitive resource or u perform sensitive operations 19

Device-related permission 20

Non-device-related permission l Enforced in kernel through uid l Kernel code in net/ipv4/af_inet.c: #include static inline int current_has_network(void) { return in_egroup_p(AID_INET) || capable(CAP_NET_RAW); } 21

Differences l Size / form-factor l Resource-constrained (e.g., battery, memory) u Optimized OS (what would you do?) l Cellular and other hardware components l User has no root access u Unless OS has vulnerabilities l Security threats u Malware is fully sandboxed and cannot easily attack other apps 22

Android Root l No app can run with root privilege in Android u even if the user desires l Restrictions set by Google, Vendors, and Carriers u Result: bloatware, power inefficiency, lost freedom/functionality, etc. l How do we gain root back? 23

Background: Symbolic Link l On most file systems, symbolic link is supported to point to the same file content without having to copy the content l “ln –s /home/zhiyunq/ /shortcut” l /shortcut  /home/zhiyunq 24

File Permission Vulnerabilities l Works on certain Android devices u Customized by vendors such as Motorola or Samsung l Goal: Write to /data/local.prop u Add line ro.kernel.qemu=1 u But permission denied to normal app l Exploit: u rm /data/local/logs/log.txt (accessible to anyone) u ln -s /data/local.prop /data/local/logs/log.txt u What is the vulnerability? 25

Programming Model Each application is running in its own process An application can have one or more components: activities, services, broadcast receivers and content providers A task (an “application” from user’s point of view) consists of several activities from one or multiple applications An application keeps running until the system kills it because of memory shortage 26

Summary l Android OS vs. Traditional OS l Security architecture of Android l Android root exploit through file permission vulnerability 27