##DroidProf

##Prologue A dynamic binary instrumentation toolkit for application profiling targeting on Android(L) 5.0. Hopefully, it will fly. But it is now under the POC construction. In more detail, shared library injection is the fundamental technique to support this toolkit. To profile the given application, a component of this toolkit called injector will inject a hooking library into the target. The library will hook several ART artifacts which interact with the Java class fields and methods of an application, and thus accomplish the goal for data profiling. In the current stage, the injector and the preliminary hooking library is done. But the hooking library just supports limited input/output recording for Java APIs.. Besides, a primary restriction is that the toolkit is now focusing on Intel x86 architecture.

##Prerequisite We need the fundamental Android toolchain:

• Android SDK - To create the AVD targeting on Intel x86 for experiment.
• Android NDK - To build the profiling toolkit.
  Note that, the API level should be at least 21.

##Installation The toolkit is composed of injector and hooking library, and we illustrate the installation steps respectively.
Suppose that:

• The absolute path storing DroidProf in your host machine is PATH_TO_DROIDPROF .
• The working directory in your AVD is PATH_TO_WORK.

####For Injector Firstly, we switch to PATH_TO_DROIDPROF/inject/jni to build the executable:

$ ndk-build

The executable will reside in PATH_TO_DROID_PROF/inject/libs/x86/inject

Secondly, we push the executable into AVD:

$ adb push PATH_TO_DROID_PROF/inject/libs/x86/inject PATH_TO_WORK/

And in the AVD, change the access permission of injector:

$ chmod a+x PATH_TO_WORK/inject

####For Hooking Library
Firstly, we switch to PATH_TO_DROIDPROF/payload/jni to build the library:

$ ndk-build

The library will reside in PATH_TO_DROID_PROF/payload/libs/x86/libhook.so

Secondly, we push the library into AVD:

$ adb push PATH_TO_DROID_PROF/payload/libs/x86/libhook.so PATH_TO_WORK/

And in the AVD, change the access permission of hooking library:

$ chmod a+x PATH_TO_WORK/libhook.so

##Usage

Preprocess before Experiment

Firstly, current Android enforces SELinux mandatory access control. To force an arbitrary application load and execute our hooking library, we must temporarily turn off the access control. Note that we can only switch SELinux to advisory mode (Warning but Non-Blocking) instead of complete shutdown mode.
For this, in your AVD, just type:

$ su 0 setenforce 0

Secondly, some applications may install background services which will be automatically triggered by certain Intent messages. To ensure the clean experiment and correct library injection on your target application, you have to shutdown its relevant service processes before running toolkit.
Suppose your target app is google maps, then you can try the following steps:

$ ps

Find the PID of google maps and kill it. ```sh $ kill PID ```

####Experiment Steps Essentially, Android applications are forked and initialized by zygote. To profile the complete runtime behavior of the target application, the hooking library should be injected into application process before it starts execution. Therefore, we have to rely on zygote for process fork event so that we can:

• Pause the newly forked application process
• Inject the hooking library
• Hook several critical points to monitor Java class field and method
• Restart the application process

Seems like a laborious manual effort? Do not worry! The injector already handles the effort you. All you have to do is providing the relevant command line arguments.
The usage of the injector is listed below:

./inject -z ZYGOTE_PID -a APP_PKG_KEYWORD -l /PATH_TO_WORK/libhook.so

You can acquire the zygote PID by ps command.

And for the Google maps example, the complete command should like this:

$ ./inject -z 936 -a maps -l /PATH_TO_WORK/libhook.so

If the command finishes successfully, you should see the result like:

You can verify the successful library injection by checking the process memory layout:

$ cat /proc/PID/maps

Also, the system log can help you extract the debug message printed by the constructor of the library:

$ logcat -v time -f /dev/kmsg | cat /proc/kmsg

Current Progress

• Done Triage

  • Hook several Java APIs like StringBuilder.toString() and String.indexOf(String,int). The relevant parsing for parameters and return value is done.

• To Do List

  • Plan for more Java utility APIs and Android sensitive APIs.
  • Plan for HTML formatted analysis report.

• Future Work

  • Currently, the project focuses on recording the API input/output. Other kinds of instrumentation application will be planed when the system becomes stable.

###Epilogue If you have any questions, please contact me via the mail: andy.zsshen@gmail.com
But please note that the hooking library is now under construction.
I will update it soon. :)