diff options
Diffstat (limited to 'services/core/java/com/android/server/EntropyMixer.java')
| -rw-r--r-- | services/core/java/com/android/server/EntropyMixer.java | 207 |
1 files changed, 207 insertions, 0 deletions
diff --git a/services/core/java/com/android/server/EntropyMixer.java b/services/core/java/com/android/server/EntropyMixer.java new file mode 100644 index 0000000..24d8d1e --- /dev/null +++ b/services/core/java/com/android/server/EntropyMixer.java @@ -0,0 +1,207 @@ +/* + * Copyright (C) 2009 The Android Open Source Project + * + * Licensed under the Apache License, Version 2.0 (the "License"); + * you may not use this file except in compliance with the License. + * You may obtain a copy of the License at + * + * http://www.apache.org/licenses/LICENSE-2.0 + * + * Unless required by applicable law or agreed to in writing, software + * distributed under the License is distributed on an "AS IS" BASIS, + * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. + * See the License for the specific language governing permissions and + * limitations under the License. + */ + +package com.android.server; + +import java.io.File; +import java.io.FileNotFoundException; +import java.io.FileOutputStream; +import java.io.IOException; +import java.io.PrintWriter; + +import android.content.BroadcastReceiver; +import android.content.Context; +import android.content.Intent; +import android.content.IntentFilter; +import android.os.Binder; +import android.os.Environment; +import android.os.Handler; +import android.os.Message; +import android.os.SystemProperties; +import android.util.Slog; + +/** + * A service designed to load and periodically save "randomness" + * for the Linux kernel RNG and to mix in data from Hardware RNG (if present) + * into the Linux RNG. + * + * <p>When a Linux system starts up, the entropy pool associated with + * {@code /dev/random} may be in a fairly predictable state. Applications which + * depend strongly on randomness may find {@code /dev/random} or + * {@code /dev/urandom} returning predictable data. In order to counteract + * this effect, it's helpful to carry the entropy pool information across + * shutdowns and startups. + * + * <p>On systems with Hardware RNG (/dev/hw_random), a block of output from HW + * RNG is mixed into the Linux RNG on EntropyMixer's startup and whenever + * EntropyMixer periodically runs to save a block of output from Linux RNG on + * disk. This mixing is done in a way that does not increase the Linux RNG's + * entropy estimate is not increased. This is to avoid having to trust/verify + * the quality and authenticity of the "randomness" of the HW RNG. + * + * <p>This class was modeled after the script in + * <a href="http://www.kernel.org/doc/man-pages/online/pages/man4/random.4.html">man + * 4 random</a>. + */ +public class EntropyMixer extends Binder { + private static final String TAG = "EntropyMixer"; + private static final int ENTROPY_WHAT = 1; + private static final int ENTROPY_WRITE_PERIOD = 3 * 60 * 60 * 1000; // 3 hrs + private static final long START_TIME = System.currentTimeMillis(); + private static final long START_NANOTIME = System.nanoTime(); + + private final String randomDevice; + private final String hwRandomDevice; + private final String entropyFile; + + /** + * Handler that periodically updates the entropy on disk. + */ + private final Handler mHandler = new Handler() { + @Override + public void handleMessage(Message msg) { + if (msg.what != ENTROPY_WHAT) { + Slog.e(TAG, "Will not process invalid message"); + return; + } + addHwRandomEntropy(); + writeEntropy(); + scheduleEntropyWriter(); + } + }; + + private final BroadcastReceiver mBroadcastReceiver = new BroadcastReceiver() { + @Override + public void onReceive(Context context, Intent intent) { + writeEntropy(); + } + }; + + public EntropyMixer(Context context) { + this(context, getSystemDir() + "/entropy.dat", "/dev/urandom", "/dev/hw_random"); + } + + /** Test only interface, not for public use */ + public EntropyMixer( + Context context, + String entropyFile, + String randomDevice, + String hwRandomDevice) { + if (randomDevice == null) { throw new NullPointerException("randomDevice"); } + if (hwRandomDevice == null) { throw new NullPointerException("hwRandomDevice"); } + if (entropyFile == null) { throw new NullPointerException("entropyFile"); } + + this.randomDevice = randomDevice; + this.hwRandomDevice = hwRandomDevice; + this.entropyFile = entropyFile; + loadInitialEntropy(); + addDeviceSpecificEntropy(); + addHwRandomEntropy(); + writeEntropy(); + scheduleEntropyWriter(); + IntentFilter broadcastFilter = new IntentFilter(Intent.ACTION_SHUTDOWN); + broadcastFilter.addAction(Intent.ACTION_POWER_CONNECTED); + broadcastFilter.addAction(Intent.ACTION_REBOOT); + context.registerReceiver(mBroadcastReceiver, broadcastFilter); + } + + private void scheduleEntropyWriter() { + mHandler.removeMessages(ENTROPY_WHAT); + mHandler.sendEmptyMessageDelayed(ENTROPY_WHAT, ENTROPY_WRITE_PERIOD); + } + + private void loadInitialEntropy() { + try { + RandomBlock.fromFile(entropyFile).toFile(randomDevice, false); + } catch (FileNotFoundException e) { + Slog.w(TAG, "No existing entropy file -- first boot?"); + } catch (IOException e) { + Slog.w(TAG, "Failure loading existing entropy file", e); + } + } + + private void writeEntropy() { + try { + Slog.i(TAG, "Writing entropy..."); + RandomBlock.fromFile(randomDevice).toFile(entropyFile, true); + } catch (IOException e) { + Slog.w(TAG, "Unable to write entropy", e); + } + } + + /** + * Add additional information to the kernel entropy pool. The + * information isn't necessarily "random", but that's ok. Even + * sending non-random information to {@code /dev/urandom} is useful + * because, while it doesn't increase the "quality" of the entropy pool, + * it mixes more bits into the pool, which gives us a higher degree + * of uncertainty in the generated randomness. Like nature, writes to + * the random device can only cause the quality of the entropy in the + * kernel to stay the same or increase. + * + * <p>For maximum effect, we try to target information which varies + * on a per-device basis, and is not easily observable to an + * attacker. + */ + private void addDeviceSpecificEntropy() { + PrintWriter out = null; + try { + out = new PrintWriter(new FileOutputStream(randomDevice)); + out.println("Copyright (C) 2009 The Android Open Source Project"); + out.println("All Your Randomness Are Belong To Us"); + out.println(START_TIME); + out.println(START_NANOTIME); + out.println(SystemProperties.get("ro.serialno")); + out.println(SystemProperties.get("ro.bootmode")); + out.println(SystemProperties.get("ro.baseband")); + out.println(SystemProperties.get("ro.carrier")); + out.println(SystemProperties.get("ro.bootloader")); + out.println(SystemProperties.get("ro.hardware")); + out.println(SystemProperties.get("ro.revision")); + out.println(SystemProperties.get("ro.build.fingerprint")); + out.println(new Object().hashCode()); + out.println(System.currentTimeMillis()); + out.println(System.nanoTime()); + } catch (IOException e) { + Slog.w(TAG, "Unable to add device specific data to the entropy pool", e); + } finally { + if (out != null) { + out.close(); + } + } + } + + /** + * Mixes in the output from HW RNG (if present) into the Linux RNG. + */ + private void addHwRandomEntropy() { + try { + RandomBlock.fromFile(hwRandomDevice).toFile(randomDevice, false); + Slog.i(TAG, "Added HW RNG output to entropy pool"); + } catch (FileNotFoundException ignored) { + // HW RNG not present/exposed -- ignore + } catch (IOException e) { + Slog.w(TAG, "Failed to add HW RNG output to entropy pool", e); + } + } + + private static String getSystemDir() { + File dataDir = Environment.getDataDirectory(); + File systemDir = new File(dataDir, "system"); + systemDir.mkdirs(); + return systemDir.toString(); + } +} |