donut snapshot

services/java/com/android/server/EntropyService.java

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+/*
+ * 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.FileOutputStream;
+import java.io.IOException;
+import java.io.OutputStream;
+import java.io.PrintWriter;
+
+import android.os.Binder;
+import android.os.Environment;
+import android.os.Handler;
+import android.os.Message;
+import android.os.SystemProperties;
+import android.util.Log;
+
+/**
+ * A service designed to load and periodically save "randomness"
+ * for the Linux kernel.
+ *
+ * <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>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>.
+ *
+ * <p>TODO: Investigate attempting to write entropy data at shutdown time
+ * instead of periodically.
+ */
+public class EntropyService extends Binder {
+    private static final String ENTROPY_FILENAME = getSystemDir() + "/entropy.dat";
+    private static final String TAG = "EntropyService";
+    private static final int ENTROPY_WHAT = 1;
+    private static final int ENTROPY_WRITE_PERIOD = 3 * 60 * 60 * 1000;  // 3 hrs
+    private static final String RANDOM_DEV = "/dev/urandom";
+    private static final long START_TIME = System.currentTimeMillis();
+    private static final long START_NANOTIME = System.nanoTime();
+
+    /**
+     * 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) {
+                Log.e(TAG, "Will not process invalid message");
+                return;
+            }
+            writeEntropy();
+            scheduleEntropyWriter();
+        }
+    };
+
+    public EntropyService() {
+        loadInitialEntropy();
+        addDeviceSpecificEntropy();
+        writeEntropy();
+        scheduleEntropyWriter();
+    }
+
+    private void scheduleEntropyWriter() {
+        mHandler.removeMessages(ENTROPY_WHAT);
+        mHandler.sendEmptyMessageDelayed(ENTROPY_WHAT, ENTROPY_WRITE_PERIOD);
+    }
+
+    private void loadInitialEntropy() {
+        try {
+            RandomBlock.fromFile(ENTROPY_FILENAME).toFile(RANDOM_DEV);
+        } catch (IOException e) {
+            Log.w(TAG, "unable to load initial entropy (first boot?)", e);
+        }
+    }
+
+    private void writeEntropy() {
+        try {
+            RandomBlock.fromFile(RANDOM_DEV).toFile(ENTROPY_FILENAME);
+        } catch (IOException e) {
+            Log.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(RANDOM_DEV));
+            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(System.currentTimeMillis());
+            out.println(System.nanoTime());
+        } catch (IOException e) {
+            Log.w(TAG, "Unable to add device specific data to the entropy pool", e);
+        } finally {
+            if (out != null) {
+                out.close();
+            }
+        }
+    }
+
+    private static String getSystemDir() {
+        File dataDir = Environment.getDataDirectory();
+        File systemDir = new File(dataDir, "system");
+        systemDir.mkdirs();
+        return systemDir.toString();
+    }
+}