1 /* 2 * Copyright (C) 2009 The Android Open Source Project 3 * 4 * Licensed under the Apache License, Version 2.0 (the "License"); 5 * you may not use this file except in compliance with the License. 6 * You may obtain a copy of the License at 7 * 8 * http://www.apache.org/licenses/LICENSE-2.0 9 * 10 * Unless required by applicable law or agreed to in writing, software 11 * distributed under the License is distributed on an "AS IS" BASIS, 12 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. 13 * See the License for the specific language governing permissions and 14 * limitations under the License. 15 */ 16 17 package com.android.server; 18 19 import java.io.File; 20 import java.io.FileNotFoundException; 21 import java.io.FileOutputStream; 22 import java.io.IOException; 23 import java.io.PrintWriter; 24 25 import android.content.BroadcastReceiver; 26 import android.content.Context; 27 import android.content.Intent; 28 import android.content.IntentFilter; 29 import android.os.Binder; 30 import android.os.Environment; 31 import android.os.Handler; 32 import android.os.Message; 33 import android.os.SystemProperties; 34 import android.util.Slog; 35 36 /** 37 * A service designed to load and periodically save "randomness" 38 * for the Linux kernel RNG and to mix in data from Hardware RNG (if present) 39 * into the Linux RNG. 40 * 41 * <p>When a Linux system starts up, the entropy pool associated with 42 * {@code /dev/random} may be in a fairly predictable state. Applications which 43 * depend strongly on randomness may find {@code /dev/random} or 44 * {@code /dev/urandom} returning predictable data. In order to counteract 45 * this effect, it's helpful to carry the entropy pool information across 46 * shutdowns and startups. 47 * 48 * <p>On systems with Hardware RNG (/dev/hw_random), a block of output from HW 49 * RNG is mixed into the Linux RNG on EntropyMixer's startup and whenever 50 * EntropyMixer periodically runs to save a block of output from Linux RNG on 51 * disk. This mixing is done in a way that does not increase the Linux RNG's 52 * entropy estimate is not increased. This is to avoid having to trust/verify 53 * the quality and authenticity of the "randomness" of the HW RNG. 54 * 55 * <p>This class was modeled after the script in 56 * <a href="http://www.kernel.org/doc/man-pages/online/pages/man4/random.4.html">man 57 * 4 random</a>. 58 */ 59 public class EntropyMixer extends Binder { 60 private static final String TAG = "EntropyMixer"; 61 private static final int ENTROPY_WHAT = 1; 62 private static final int ENTROPY_WRITE_PERIOD = 3 * 60 * 60 * 1000; // 3 hrs 63 private static final long START_TIME = System.currentTimeMillis(); 64 private static final long START_NANOTIME = System.nanoTime(); 65 66 private final String randomDevice; 67 private final String hwRandomDevice; 68 private final String entropyFile; 69 70 /** 71 * Handler that periodically updates the entropy on disk. 72 */ 73 private final Handler mHandler = new Handler() { 74 @Override 75 public void handleMessage(Message msg) { 76 if (msg.what != ENTROPY_WHAT) { 77 Slog.e(TAG, "Will not process invalid message"); 78 return; 79 } 80 addHwRandomEntropy(); 81 writeEntropy(); 82 scheduleEntropyWriter(); 83 } 84 }; 85 86 private final BroadcastReceiver mBroadcastReceiver = new BroadcastReceiver() { 87 @Override 88 public void onReceive(Context context, Intent intent) { 89 writeEntropy(); 90 } 91 }; 92 93 public EntropyMixer(Context context) { 94 this(context, getSystemDir() + "/entropy.dat", "/dev/urandom", "/dev/hw_random"); 95 } 96 97 /** Test only interface, not for public use */ 98 public EntropyMixer( 99 Context context, 100 String entropyFile, 101 String randomDevice, 102 String hwRandomDevice) { 103 if (randomDevice == null) { throw new NullPointerException("randomDevice"); } 104 if (hwRandomDevice == null) { throw new NullPointerException("hwRandomDevice"); } 105 if (entropyFile == null) { throw new NullPointerException("entropyFile"); } 106 107 this.randomDevice = randomDevice; 108 this.hwRandomDevice = hwRandomDevice; 109 this.entropyFile = entropyFile; 110 loadInitialEntropy(); 111 addDeviceSpecificEntropy(); 112 addHwRandomEntropy(); 113 writeEntropy(); 114 scheduleEntropyWriter(); 115 IntentFilter broadcastFilter = new IntentFilter(Intent.ACTION_SHUTDOWN); 116 broadcastFilter.addAction(Intent.ACTION_POWER_CONNECTED); 117 broadcastFilter.addAction(Intent.ACTION_REBOOT); 118 context.registerReceiver(mBroadcastReceiver, broadcastFilter); 119 } 120 121 private void scheduleEntropyWriter() { 122 mHandler.removeMessages(ENTROPY_WHAT); 123 mHandler.sendEmptyMessageDelayed(ENTROPY_WHAT, ENTROPY_WRITE_PERIOD); 124 } 125 126 private void loadInitialEntropy() { 127 try { 128 RandomBlock.fromFile(entropyFile).toFile(randomDevice, false); 129 } catch (FileNotFoundException e) { 130 Slog.w(TAG, "No existing entropy file -- first boot?"); 131 } catch (IOException e) { 132 Slog.w(TAG, "Failure loading existing entropy file", e); 133 } 134 } 135 136 private void writeEntropy() { 137 try { 138 Slog.i(TAG, "Writing entropy..."); 139 RandomBlock.fromFile(randomDevice).toFile(entropyFile, true); 140 } catch (IOException e) { 141 Slog.w(TAG, "Unable to write entropy", e); 142 } 143 } 144 145 /** 146 * Add additional information to the kernel entropy pool. The 147 * information isn't necessarily "random", but that's ok. Even 148 * sending non-random information to {@code /dev/urandom} is useful 149 * because, while it doesn't increase the "quality" of the entropy pool, 150 * it mixes more bits into the pool, which gives us a higher degree 151 * of uncertainty in the generated randomness. Like nature, writes to 152 * the random device can only cause the quality of the entropy in the 153 * kernel to stay the same or increase. 154 * 155 * <p>For maximum effect, we try to target information which varies 156 * on a per-device basis, and is not easily observable to an 157 * attacker. 158 */ 159 private void addDeviceSpecificEntropy() { 160 PrintWriter out = null; 161 try { 162 out = new PrintWriter(new FileOutputStream(randomDevice)); 163 out.println("Copyright (C) 2009 The Android Open Source Project"); 164 out.println("All Your Randomness Are Belong To Us"); 165 out.println(START_TIME); 166 out.println(START_NANOTIME); 167 out.println(SystemProperties.get("ro.serialno")); 168 out.println(SystemProperties.get("ro.bootmode")); 169 out.println(SystemProperties.get("ro.baseband")); 170 out.println(SystemProperties.get("ro.carrier")); 171 out.println(SystemProperties.get("ro.bootloader")); 172 out.println(SystemProperties.get("ro.hardware")); 173 out.println(SystemProperties.get("ro.revision")); 174 out.println(SystemProperties.get("ro.build.fingerprint")); 175 out.println(new Object().hashCode()); 176 out.println(System.currentTimeMillis()); 177 out.println(System.nanoTime()); 178 } catch (IOException e) { 179 Slog.w(TAG, "Unable to add device specific data to the entropy pool", e); 180 } finally { 181 if (out != null) { 182 out.close(); 183 } 184 } 185 } 186 187 /** 188 * Mixes in the output from HW RNG (if present) into the Linux RNG. 189 */ 190 private void addHwRandomEntropy() { 191 try { 192 RandomBlock.fromFile(hwRandomDevice).toFile(randomDevice, false); 193 Slog.i(TAG, "Added HW RNG output to entropy pool"); 194 } catch (FileNotFoundException ignored) { 195 // HW RNG not present/exposed -- ignore 196 } catch (IOException e) { 197 Slog.w(TAG, "Failed to add HW RNG output to entropy pool", e); 198 } 199 } 200 201 private static String getSystemDir() { 202 File dataDir = Environment.getDataDirectory(); 203 File systemDir = new File(dataDir, "system"); 204 systemDir.mkdirs(); 205 return systemDir.toString(); 206 } 207 } 208
