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Commit 8a649c18 authored by Steve Kondik's avatar Steve Kondik Committed by Steve Kondik
Browse files

alarmmanager: Add sanity checking for elapsed_realtime alarms 

 * Try to protect against DoS by detecting if an alarm is scheduled far
   in the past and exponentially backing off.

Change-Id: I1b9e0d7b1230dd50c7ac50aaa2765820d444135e
parent 829a0471

services/core/java/com/android/server/AlarmManagerService.java

+25 −0
Original line number Diff line number Diff line
@@ -792,6 +792,31 @@ class AlarmManagerService extends SystemService { 
        }



        final long nowElapsed = SystemClock.elapsedRealtime();



        // Sanity check elapsed realtime alarms which are set in the past.

        // Doing this causes them to trigger immediately, which is a great

        // opportunity to spam. Or the user forgot to add the current time.

        // Either way, this is bad.

        if ((type == ELAPSED_REALTIME || type == ELAPSED_REALTIME_WAKEUP) &&

                interval == 0 && windowLength < 0) {

            if (nowElapsed > triggerAtTime) {

                final long orig = triggerAtTime;

                final long delta = nowElapsed - triggerAtTime;

                if (delta > triggerAtTime) {

                    triggerAtTime = nowElapsed + triggerAtTime;

                } else {

                    long offset = nowElapsed + (delta / 2);

                    if (offset < MIN_FUTURITY) {

                        offset = MIN_FUTURITY;

                    }

                    triggerAtTime = nowElapsed + offset;

                }

                Slog.w(TAG, "ELAPSED_REALTIME alarm set far in the past (" + orig +

                        "), offsetting from current time to " + triggerAtTime +

                        " from uid=" + Binder.getCallingUid() + " pid=" + Binder.getCallingPid());

            }

        }



        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;