Android Backup Agent Arbitrary Code Execution

`Android backup agent arbitrary code execution  
---------------------------------------------  
  
The Android backup agent implementation was vulnerable to privilege  
escalation and race condition. An attacker with adb shell access could  
run arbitrary code as the system (1000) user (or any other valid  
package). The attack is tested on Android OS 4.4.4.  
  
  
The main problem is inside bindBackupAgent method in the  
ActivityManagerService.  
This method is exported through Binder and is available to call by the  
shell user, since android.permission.BACKUP is granted for it.  
  
The method has an ApplicationInfo parameter, which is unsecured (not  
cross validated through the PackageManager), so the uid member could be  
manipulated. The supplied ApplicationInfo object will be direct  
parameter for startProcessLocked().  
  
Before invoking startProcessLocked, bindBackupAgent also tries to set  
stopped state for the package.  
This call is bound to an additional permission  
(CHANGE_COMPONENT_ENABLED_STATE), which is a system permission, not even  
shell user got it.  
  
However, there is a race condition between PackageManager and  
ActivityManagerService, so this security check can by bypassed.  
  
Existence of the specified package happens first in  
mSettings.setPackageStoppedStateLPw(). If the package does not exists  
than IllegalArguemntException is thrown. (Permission would have been  
validated as next step only resulting in a SecurityException)  
  
So, if the package does not exists, IllegalArguemntException is thrown,  
which is catched by bindBackupAgent, but the execution wont stop (only a  
warning is being logged):  
  
// Backup agent is now in use, its package can't be stopped.  
try {  
AppGlobals.getPackageManager().setPackageStoppedState(  
app.packageName, false,  
UserHandle.getUserId(app.uid));  
} catch (RemoteException e) {  
} catch (IllegalArgumentException e) {  
Slog.w(TAG, "Failed trying to unstop package "  
+ app.packageName + ": " + e);  
}  
  
  
It was possible to perform the following steps in order to exploit:  
  
1. execute "pm install helloworld.apk" (with package name  
com.example.helloworld)  
  
2. with another script process logcat's output and look for  
the dexopt line (DexOpt: load 3ms, verify+opt 5ms, 161068 bytes)  
  
3. trigger execution of the bindBackupAgent system call (with uid  
spoofed to 1000 in ApplicationInfo) as soon as the dexopt line was seen  
  
  
Since this is a race condition and timing is important, it might not  
work at first. I was lucky at 3rd attempt.  
  
In this lucky scenario the package did not exists while  
setPackageStoppedStateLPw tried to find it, but then it became available  
for startPackageLocked.  
  
At this point a new process was forked by the Zygote:  
  
  
shell@grouper:/ $ ps |grep hello  
ps |grep hello  
system 6826 141 692340 17312 ffffffff 00000000 S  
com.example.helloworld  
  
  
No code was executed however, since there exists an additional security  
check in handleCreateBackupAgent in the ActivityThread:  
  
PackageInfo requestedPackage =  
getPackageManager().getPackageInfo(  
data.appInfo.packageName, 0, UserHandle.myUserId());  
if (requestedPackage.applicationInfo.uid != Process.myUid()) {  
Slog.w(TAG, "Asked to instantiate non-matching package "  
+ data.appInfo.packageName);  
return;  
}  
  
  
But the process com.example.helloserver was executed with debug flags  
(due to the simple fact that it was built by us and we built it as  
debug) so DDMS could be attached to it.  
  
To verify actual code execution, I added  
Runtime.getRuntime().exec("touch /data/app/testSystem")  
as an expression in the debugger to be evaluated by the process.  
  
The command was executed successfully:  
  
shell@grouper:/data/app $ ls -la testSystem  
ls -la testSystem  
-rw------- system system 0 2014-08-06 01:52 testSystem  
  
  
13 byte bugfix for all the above in the version control:  
https://android.googlesource.com/platform/frameworks/base/+/a8f6d1b%5E!/  
  
Lollipop is not affected, earlier Android versions are.  
  
  
Tested on: Android 4.4.4:  
Reported on: 2014-08-15  
Assigned CVE: CVE-2014-7951  
Android bug id: 15829193  
Discovered by: Imre Rad / Search-Lab Ltd.  
http://www.search-lab.hu  
http://www.securecodingacademy.com/  
`