Only allow the system server to connect to sync adapters

Bug: 203229608
Test: Manual test with changing the check logic + debug log
Change-Id: If18009f61360564d02dcda9b1e5fa15685e3250f
(cherry picked from commit 5827052)
(cherry picked from commit 7d1397a)
Merged-In: If18009f61360564d02dcda9b1e5fa15685e3250f

Author: Makoto Onuki

core/java/android/content/AbstractThreadedSyncAdapter.java

@@ -21,6 +21,7 @@
 import android.accounts.Account;
 import android.annotation.MainThread;
 import android.annotation.NonNull;
+import android.os.Binder;
 import android.os.Build;
 import android.os.Bundle;
 import android.os.Handler;
@@ -171,8 +172,20 @@ private Account toSyncKey(Account account) {
     }
 
     private class ISyncAdapterImpl extends ISyncAdapter.Stub {
+        private boolean isCallerSystem() {
+            final long callingUid = Binder.getCallingUid();
+            if (callingUid != Process.SYSTEM_UID) {
+                android.util.EventLog.writeEvent(0x534e4554, "203229608", -1, "");
+                return false;
+            }
+            return true;
+        }
+
         @Override
         public void onUnsyncableAccount(ISyncAdapterUnsyncableAccountCallback cb) {
+            if (!isCallerSystem()) {
+                return;
+            }
             Handler.getMain().sendMessage(obtainMessage(
                     AbstractThreadedSyncAdapter::handleOnUnsyncableAccount,
                     AbstractThreadedSyncAdapter.this, cb));
@@ -181,12 +194,16 @@ public void onUnsyncableAccount(ISyncAdapterUnsyncableAccountCallback cb) {
         @Override
         public void startSync(ISyncContext syncContext, String authority, Account account,
                 Bundle extras) {
+            if (!isCallerSystem()) {
+                return;
+            }
             if (ENABLE_LOG) {
                 if (extras != null) {
                     extras.size(); // Unparcel so its toString() will show the contents.
                 }
                 Log.d(TAG, "startSync() start " + authority + " " + account + " " + extras);
             }
+
             try {
                 final SyncContext syncContextClient = new SyncContext(syncContext);
 
@@ -242,6 +259,9 @@ public void startSync(ISyncContext syncContext, String authority, Account accoun
 
         @Override
         public void cancelSync(ISyncContext syncContext) {
+            if (!isCallerSystem()) {
+                return;
+            }
             try {
                 // synchronize to make sure that mSyncThreads doesn't change between when we
                 // check it and when we use it