From 498c6cbd78f42d4f6d2568ead6c2697fcfaa326e Mon Sep 17 00:00:00 2001
From: Christopher Tate <ctate@google.com>
Date: Mon, 17 Nov 2014 16:09:27 -0800
Subject: [PATCH] Prevent some alarm pathologies

* Sanity-check the recurrence interval.  Some buggy apps pass seconds
  where the API expects milliseconds, with the result that the device
  pins the CPU at 100% trying to deliver alarm broadcasts every 60 ms
  or what have you.  The minimum recurrence is now 1 minute.

* Sanity-check alarms being scheduled for the immediate future.  As
  with the above this will catch people trying to schedule alarms
  in a spammy way that keeps the device from entering low-power state.
  The minimum futurity of a new alarm is now 5 seconds.

Bug 17495168

Change-Id: If8ff7d88da48960532ac21a0ba20094af9912603
---
 .../android/server/AlarmManagerService.java   | 21 ++++++++++++++++++-
 1 file changed, 20 insertions(+), 1 deletion(-)

diff --git a/services/core/java/com/android/server/AlarmManagerService.java b/services/core/java/com/android/server/AlarmManagerService.java
index 8b524ddab458c..08c47dcaa9b79 100644
--- a/services/core/java/com/android/server/AlarmManagerService.java
+++ b/services/core/java/com/android/server/AlarmManagerService.java
@@ -75,6 +75,12 @@ class AlarmManagerService extends SystemService {
     // warning message.  The time duration is in milliseconds.
     private static final long LATE_ALARM_THRESHOLD = 10 * 1000;
 
+    // Minimum futurity of a new alarm
+    private static final long MIN_FUTURITY = 5 * 1000;  // 5 seconds, in millis
+
+    // Minimum alarm recurrence interval
+    private static final long MIN_INTERVAL = 60 * 1000;  // one minute, in millis
+
     private static final int RTC_WAKEUP_MASK = 1 << RTC_WAKEUP;
     private static final int RTC_MASK = 1 << RTC;
     private static final int ELAPSED_REALTIME_WAKEUP_MASK = 1 << ELAPSED_REALTIME_WAKEUP;
@@ -696,6 +702,15 @@ class AlarmManagerService extends SystemService {
             windowLength = AlarmManager.INTERVAL_HOUR;
         }
 
+        // Sanity check the recurrence interval.  This will catch people who supply
+        // seconds when the API expects milliseconds.
+        if (interval > 0 && interval < MIN_INTERVAL) {
+            Slog.w(TAG, "Suspiciously short interval " + interval
+                    + " millis; expanding to " + (int)(MIN_INTERVAL/1000)
+                    + " seconds");
+            interval = MIN_INTERVAL;
+        }
+
         if (type < RTC_WAKEUP || type > ELAPSED_REALTIME) {
             throw new IllegalArgumentException("Invalid alarm type " + type);
         }
@@ -709,7 +724,11 @@ class AlarmManagerService extends SystemService {
         }
 
         final long nowElapsed = SystemClock.elapsedRealtime();
-        final long triggerElapsed = convertToElapsed(triggerAtTime, type);
+        final long nominalTrigger = convertToElapsed(triggerAtTime, type);
+        // Try to prevent spamming by making sure we aren't firing alarms in the immediate future
+        final long minTrigger = nowElapsed + MIN_FUTURITY;
+        final long triggerElapsed = (nominalTrigger > minTrigger) ? nominalTrigger : minTrigger;
+
         final long maxElapsed;
         if (windowLength == AlarmManager.WINDOW_EXACT) {
             maxElapsed = triggerElapsed;