Dancing with dalvik

1 | Copyright © 2013, Cigital and/or its affiliates. All rights reserved. | Cigital Confidential Restricted.
THOMAS RICHARDS
Dancing with Dalvik

About me
• Thomas Richards
• Security Consultant @ Cigital, Inc
• @g13net - Twitter
• OSCP, GPEN, OSWP
• Developer
o Goofile and Pwnberry Pi
• Organizer for BsidesROC
• Presented before at GrrCON and DerbyCON
• Really enjoy tearing apart Android

Toc
• 0x1 – Intro to Dalvik
• 0x2 – Dalvik Opcode Primer
• 0x3 – Case Studies

0x01 Intro to Dalvik

What is Dalvik?
• A town in Iceland
• A lightweight register-based VM
• Optimized for embedded/mobile
platforms
o Low RAM/CPU

Dalvik optimizations
• Duplicate code is reused within DEX
• 16-bit instruction set that works directly
on local variables
o Lowers instruction count and increased
interpreter speed
• Slimmed down VM to run in less space

Stack vs register based
• Java JVM is Stack Based
o real or emulated computer that uses a pushdown stack rather
than individual machine registers to evaluate each sub-
expression in the program
• Register Based
o Dalvik uses registers as primarily units of data storage instead
of the stack. Google is hoping to accomplish 30 percent fewer
instructions as a result.
• http://stackoverflow.com/questions/2719469/why-is-the-
jvm-stack-based-and-the-dalvik-vm-register-based
• http://en.wikipedia.org/wiki/Stack_machine

Register Vs. Stack Cont.

Getting to dalvik
• Android apps are traditionally written in
Java
• Compiled into Java bytecode then
converted to Dalvik bytecode
• Java class files converted into .dex

Java vs Dalvik

Inside the APK
• AndroidManifest.xml
• Classes.dex
• Res/
• Lib/
• META-INF/

Disassembling classes.dex
• Typical tools
o Apktool
o Baksmali
• The result is several .smali files

Apktool decoding
me@Mentos ~/android/ghost $ apktool d ghost-meter.apk
I: Baksmaling...
I: Loading resource table...
I: Loaded.
I: Decoding AndroidManifest.xml with resources...
I: Loading resource table from file:
/home/me/apktool/framework/1.apk
I: Loaded.
I: Regular manifest package...
I: Decoding file-resources...
I: Decoding values */* XMLs...
I: Done.
I: Copying assets and libs...

Directories Created

Files created
me@Mentos ~/android/ghost $ ls ghost-
meter/smali/
a.smali c$a.smali c$c.smali
c$e.smali c.smali e.smali g.smali
i.smali k.smali m.smali o.smali
q.smali s.smali u.smali w.smali
b.smali c$b.smali c$d.smali com
d.smali f.smali h.smali j.smali
l.smali n.smali p.smali r.smali
t.smali v.smali x.smali
me@Mentos ~/android/ghost $

Apktool building
me@Mentos ~/android/ghost $ apktool b ghost-
meter/ ghost.apk
I: Checking whether sources has changed...
I: Smaling...
I: Checking whether resources has changed...
I: Building resources...
I: Building apk file...
me@Mentos ~/android/ghost $
• After building you must use jarsigner to sign the APK
before it can be installed!

Signing the APK
• Creating the cert
o keytool -genkey -v -keystore my-release-
key.keystore -alias alias_name -keyalg RSA
-validity 10000
• Signing the APK
o jarsigner -verbose -keystore my-release-
key.keystore GhostMeter.apk alias_name

Cydia Substrate
• Originally on iOS, recently released on
Android.
• Tool for hooking methods within an
application or the system.
• Modify runtime behavior of Android
Application without recompiling code

0x2 Dalvik Opcode Primer

Smali files
• Disassembled DEX files
• Not as scary as x86 ASM
• ASCII representation of Dalvik Opcodes
• Mostly correspond to the original .java
files

Types
V void - can only be used for return types
Z boolean
B byte
S short
C char
I int
J long (64 bits)
F float
D double (64 bits)

Helloworld example
.class public LHelloWorld;
#this is a comment
.super Ljava/lang/Object;
.method public static main([Ljava/lang/String;)V
.registers 2
sget-object v0, Ljava/lang/System;->out:Ljava/io/PrintStream;
const-string v1, "Hello World!"
invoke-virtual {v0, v1}, Ljava/io/PrintStream;->println(Ljava/lang/String;)V
return-void
.end method
http://code.google.com/p/smali/source/browse/examples/HelloWorld/HelloWorld.sm
ali

Variables
• Registers
o v0, v1, etc.
o Variables defined in the original java code
• Parameters
o p0, p1, etc
o Must be the same as defined by the method
• Both are defined at the beginning of the
method

Example
#this is a comment
.registers 2
invoke-virtual {v0, v1}, Ljava/io/PrintStream;-
>println(Ljava/lang/String;)V
return-void
.end method

Assigning variables
• const
• const-string
• Etc,
• Examples
o const v0, 0x1
o const-string v1, “This is a test”

Example
#this is a comment
.registers 2
return-void
.end method

Calling methods
• invoke-* { parameters }, method-to-call
o Static – static method
o Virtual – virtual method

Example
#this is a comment
.registers 2
return-void
.end method

Conditional statements
• If-* - If statements
o If-eq vx, vy, target – If equal
o If-nez vx, target – If vx is a nonzero
o If-eqz vx, target – If vx is zero
o If-ge vx, vy, target – if vx>=vy
o Etc
• Targets
o Shown as cond_0, cond_1

example
invoke-virtual {v0}, Ljava/lang/Class;-
>desiredAssertionStatus()Z
move-result v0
if-nez v0, :cond_0
.line 100
:cond_0
const/4 v0, 0x0
goto/16 :goto_0

Get/put
• Apps could declare constants.
• Get/Put will retrieve or store values into
those constants
• Examples:
o iput v0, v1, Lcom/greencod/pinball/gameengine/zones/Table0Zone;-
>ZORDER_TABLE:I
o iget v0, v0, Lcom/greencod/pinball/gameengine/xinterface/persistenc
e/Settings;->friction:F

Return
• Unless the method is declared as a void,
it will return a value.
• Return types:
o Return-void
o Return-object
o Return v0

0x3 case studies

Removing ads
• Ads are annoying
• Take up screen realestate
• My 4 yr old mistakenly clicks them
o “Look daddy!”

solitare
• Ad function is called inbetween deals
• Ads are annoying
o Music
o Video
• Disable calling the ads?

Short video showing ad calling

The code
• After an exhaustive search in com/mobilityware/solitare/solitare.smali:
.method public displayAd()V
.locals 1
.prologue
.line 572
iget-object v0, p0, Lcom/mobilityware/solitaire/Solitaire;-
>adControl:Lcom/mobilityware/solitaire/AdControl;
if-eqz v0, :cond_0
.line 573
invoke-virtual {v0}, Lcom/mobilityware/solitaire/AdControl;->displayAd()Z
.line 574
:cond_0
return-void
.end method

The code
• After an exhaustive search in com/mobilityware/solitare/solitare.smali:
.method public displayAd()V
.locals 1
.prologue
.line 572
if-eqz v0, :cond_0
.line 573
#invoke-virtual {v0}, Lcom/mobilityware/solitaire/AdControl;->displayAd()Z
.line 574
:cond_0
return-void
.end method

After the change is made
• Recompile the code and use apktool to
create a new APK
o You will have to sign this with your own cert
• After the APK is created:
o Remove old Solitare APK
o Install new hax0rd one

Short video showing no more ad

chess
• Most apps display the ads at all times
• When the network is disabled, the ads
don’t show
• Can we force this behavior?

Screenshot of chess with ads

Google ads
• Most apps rely on Google’s ad provider
network
• Good for us!
o Code to break this should be the same for
many apps
• How to make it look like there is no network
connectivity?
• Log Entry when Network is disabled
o adRequestWebView was null while
trying to load an ad

The original code
• From com/google/ads/c.smali:
:try_start_0
iget-object v0, p0, Lcom/google/ads/c;->f:Landroid/webkit/WebView;
if-eqz v0, :cond_0
iget-object v0, p0, Lcom/google/ads/c;->c:Lcom/google/ads/b;
if-nez v0, :cond_1
:cond_0
const-string v0, "adRequestWebView was null while trying to load an ad."
invoke-static {v0}, Lcom/google/ads/util/a;->e(Ljava/lang/String;)V
sget-object v0, Lcom/google/ads/AdRequest$ErrorCode;-
>INTERNAL_ERROR:Lcom/google/ads/AdRequest$ErrorCode

The modified code
• From com/google/ads/c.smali:
:try_start_0
iget-object v0, p0, Lcom/google/ads/c;->f:Landroid/webkit/WebView;
const v0, 0x0
if-eqz v0, :cond_0
iget-object v0, p0, Lcom/google/ads/c;->c:Lcom/google/ads/b;
if-nez v0, :cond_1
:cond_0
const-string v0, "adRequestWebView was null while trying to load an
ad."
invoke-static {v0}, Lcom/google/ads/util/a;->e(Ljava/lang/String;)V

Screenshot showing no ads

Removing Ads with Cydia
• Write once, use everywhere
• Using reflection, hook a method
• Stop ads from loading

Google Mobile Ads
• Again, very common to see used.
• Plan:
o Hook loadDataWithBaseUrl method
o Render it useless

Cydia Code
MS.hookClassLoad("com.google.ads.internal.AdWebView", new
MS.ClassLoadHook() {
public void classLoaded(Class<?> resources) {
Method loadAd;
try {
loadAd = resources.getMethod("loadDataWithBaseURL", String.class,
String.class, String.class, String.class, String.class);
} catch (NoSuchMethodException d) {
loadAd = null;
}
if (loadAd != null) {
final MS.MethodPointer old = new MS.MethodPointer();
MS.hookMethod(resources, loadAd, new MS.MethodHook() {
public Object invoked(Object _this, Object... args) throws Throwable {
return null;
}
}, old);

Screenshot of App with Ads

Screenshot after Cydia App Installed

Pinball wizard
• How about getting a high score?
• Values are most likely in the source
• Totally impress your friends with l33t
pinball skillz

Picture of game after normal score

The code
.local v12,
scorer:Lcom/greencod/gameengine/behaviours/ScorerBehaviour;
const/16 v1, 0x2c7
const/16 v3, 0x5aa
invoke-
virtual {v12, v1, v3}, Lcom/greencod/gameengine/behaviours/ScorerBe
haviour;->addScoreMessage(II)V
.line 508
const/16 v1, 0x204
const/16 v3, 0x10fe
invoke-
virtual {v12, v1, v3}, Lcom/greencod/gameengine/behaviours/ScorerBe
haviour;->addScoreMessage(II)V

Altered code
.local v12,
scorer:Lcom/greencod/gameengine/behaviours/ScorerBehaviour;
const/16 v1, 0x2c7
const/16 v3, 0xfff
invoke-
virtual {v12, v1, v3}, Lcom/greencod/gameengine/behaviours/Scorer
Behaviour;->addScoreMessage(II)V
.line 508
const/16 v1, 0x204
const/16 v3, 0xffff
invoke-
virtual {v12, v1, v3}, Lcom/greencod/gameengine/behaviours/Scorer
Behaviour;->addScoreMessage(II)V

Picture of new high score

Leaderboard

What now?
• Explore!
• Bypass restrictions?
o Very useful in mobile app assessment

References
• http://pallergabor.uw.hu/androidblog/dalvik_
opcodes.html
• http://code.google.com/p/android-apktool/
• http://code.google.com/p/smali/source/brow
se/examples/HelloWorld/HelloWorld.smali
• http://www.cydiasubstrate.com/inject/dalvik/

QUESTIONS?

Dancing with dalvik

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