base: Spoof build fingerprint for Google Play Services [1/2]

SafetyNet's CTS profile attestation checks whether Build.FINGERPRINT
matches that of the device's stock OS, which has passed CTS testing.
Spoof the fingerprint for Google Play Services to help pass SafetyNet.

NB: This code is under the gmscompat package, but it does not depend on
any code from gmscompat.

arrow edits:
 - Correct prop name for our logic. We are planning to use fp for different devices (like old oreo or follow new Pixel's).

Signed-off-by: palaych <palaych@arrowos.net>
Change-Id: I26a2498eb2e2163933303b03f6d516e5fb30fe51

core/java/android/app/Instrumentation.java

@@ -56,6 +56,7 @@
 import android.view.WindowManagerGlobal;
 
 import com.android.internal.content.ReferrerIntent;
+import com.android.internal.gmscompat.AttestationHooks;
 
 import java.io.File;
 import java.lang.annotation.Retention;
@@ -1157,6 +1158,7 @@ public Application newApplication(ClassLoader cl, String className, Context cont
         Application app = getFactory(context.getPackageName())
                 .instantiateApplication(cl, className);
         app.attach(context);
+        AttestationHooks.initApplicationBeforeOnCreate(app);
         return app;
     }
 
@@ -1174,6 +1176,7 @@ static public Application newApplication(Class<?> clazz, Context context)
             ClassNotFoundException {
         Application app = (Application)clazz.newInstance();
         app.attach(context);
+        AttestationHooks.initApplicationBeforeOnCreate(app);
         return app;
     }
 

core/java/com/android/internal/gmscompat/AttestationHooks.java

@@ -0,0 +1,65 @@
+/*
+ * Copyright (C) 2021 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.internal.gmscompat;
+
+import android.app.Application;
+import android.os.Build;
+import android.os.SystemProperties;
+import android.util.Log;
+
+import java.lang.reflect.Field;
+
+/** @hide */
+public final class AttestationHooks {
+    private static final String TAG = "GmsCompat/Attestation";
+
+    private static final String PACKAGE_GMS = "com.google.android.gms";
+
+    private static final String PRODUCT_GMS_SPOOFING_FINGERPRINT =
+            SystemProperties.get("ro.build.gms_fingerprint");
+
+    private AttestationHooks() { }
+
+    private static void setBuildField(String key, String value) {
+        try {
+            // Unlock
+            Field field = Build.class.getDeclaredField(key);
+            field.setAccessible(true);
+
+            // Edit
+            field.set(null, value);
+
+            // Lock
+            field.setAccessible(false);
+        } catch (NoSuchFieldException | IllegalAccessException e) {
+            Log.e(TAG, "Failed to spoof Build." + key, e);
+        }
+    }
+
+    private static void spoofBuildGms() {
+        // Set fingerprint for SafetyNet CTS profile
+        if (PRODUCT_GMS_SPOOFING_FINGERPRINT.length() > 0) {
+            setBuildField("FINGERPRINT", PRODUCT_GMS_SPOOFING_FINGERPRINT);
+        }
+    }
+
+    public static void initApplicationBeforeOnCreate(Application app) {
+        if (PACKAGE_GMS.equals(app.getPackageName())) {
+            spoofBuildGms();
+        }
+    }
+}