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 {
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 the TOCTOU issue:

1. execute "pm install helloworld.apk" (with package name com.example.helloworld)

2. process logcat's output with another script 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 arrived

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

However, the custom BackupAgent class defined in this Android package was not instantiated, 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);

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, one can add 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:!/

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