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/