Security Fix: Race Condition + NPE

ShellSubscriber is lazily initialized, and multiple threads can attempt
to write the same pointer since it is not initialized in threadsafe
code. Additionally, there is an NPE that crashes statsd when a null
ResultReceiver is passed in, which allows an attacker to repeatedly
crash statsd until the race condition occurs. More details, including a
proof of concept attack, are in the bug.

Bug: 141243101
Test: repro steps in bug no longer crash statsd
Test: with only the lock on iniitiallizing mShellSubscriber, statsd
still crashed but after ~7 minutes, no race condition occurred.

Change-Id: Ib56f888620497fb41d1627c07867693eb251738e

cmds/statsd/src/StatsService.cpp

@@ -266,7 +266,9 @@ status_t StatsService::onTransact(uint32_t code, const Parcel& data, Parcel* rep
                     IResultReceiver::asInterface(data.readStrongBinder());
 
             err = command(in, out, err, args, resultReceiver);
-            resultReceiver->send(err);
+            if (resultReceiver != nullptr) {
+                resultReceiver->send(err);
+            }
             return NO_ERROR;
         }
         default: { return BnStatsManager::onTransact(code, data, reply, flags); }
@@ -411,13 +413,20 @@ status_t StatsService::command(int in, int out, int err, Vector<String8>& args,
             return cmd_trigger_active_config_broadcast(out, args);
         }
         if (!args[0].compare(String8("data-subscribe"))) {
-            if (mShellSubscriber == nullptr) {
-                mShellSubscriber = new ShellSubscriber(mUidMap, mPullerManager);
+            {
+                std::lock_guard<std::mutex> lock(mShellSubscriberMutex);
+                if (mShellSubscriber == nullptr) {
+                    mShellSubscriber = new ShellSubscriber(mUidMap, mPullerManager);
+                }
             }
             int timeoutSec = -1;
             if (argCount >= 2) {
                 timeoutSec = atoi(args[1].c_str());
             }
+            if (resultReceiver == nullptr) {
+                ALOGI("Null resultReceiver given, no subscription will be started");
+                return UNEXPECTED_NULL;
+            }
             mShellSubscriber->startNewSubscription(in, out, resultReceiver, timeoutSec);
             return NO_ERROR;
         }

cmds/statsd/src/StatsService.h

@@ -432,6 +432,10 @@ private:
 
     sp<ShellSubscriber> mShellSubscriber;
 
+    /**
+     * Mutex for setting the shell subscriber
+     */
+    mutable mutex mShellSubscriberMutex;
     std::shared_ptr<LogEventQueue> mEventQueue;
 
     FRIEND_TEST(StatsLogProcessorTest, TestActivationsPersistAcrossSystemServerRestart);