From 90433d3f1295c92035b65d42908aead62ea0c8da Mon Sep 17 00:00:00 2001
From: Curtis Belmonte <curtislb@google.com>
Date: Wed, 7 Oct 2020 15:51:09 -0700
Subject: [PATCH] DO NOT MERGE Check fingerprint client against top activity in
 auth callback

Due to a race condition with activity task stack broadcasts, it's
currently possible for fingerprint authentication to succeed for a
non-top activity. This means, for example, that a malicious overlay
could be drawn in order to mislead the user about what they are
authenticating for.

This commit addresses the issue by adding a check to the biometric
authentication client interface that ensures the authenticating
activity is on top at the time of authentication. Otherwise, the
pending authentication will fail, as if an incorrect biometric had
been presented.

Test: Follow steps from b/159249069:
1. Install com.pro100svitlo.fingerprintauthdemo from the Play store.
2. Install the PoC attack app from b/159249069.
3. Start the PoC attack app and press the "Launch PoC attack" button.
4. Use fingerprint to authenticate while the overlay is showing.

Before: Authentication succeeds, and a new activity is launched.
After: Authentication fails, and no new activity is launched.

Bug: 159249069
Change-Id: I3a810cd7e6b97333f648c978e44242662342ec57
Merged-In: I289d67e5c7055ed60f7a96725c523d07cd047b23
---
 .../biometrics/AuthenticationClient.java      | 53 +++++++++++++++++++
 1 file changed, 53 insertions(+)

diff --git a/services/core/java/com/android/server/biometrics/AuthenticationClient.java b/services/core/java/com/android/server/biometrics/AuthenticationClient.java
index ef1b574c29a8d..b4d74e0900f96 100644
--- a/services/core/java/com/android/server/biometrics/AuthenticationClient.java
+++ b/services/core/java/com/android/server/biometrics/AuthenticationClient.java
@@ -16,16 +16,22 @@
 
 package com.android.server.biometrics;
 
+import android.app.ActivityManager;
+import android.app.ActivityTaskManager;
+import android.content.ComponentName;
 import android.content.Context;
+import android.content.pm.ApplicationInfo;
 import android.hardware.biometrics.BiometricAuthenticator;
 import android.hardware.biometrics.BiometricConstants;
 import android.hardware.biometrics.BiometricsProtoEnums;
 import android.os.IBinder;
 import android.os.RemoteException;
 import android.security.KeyStore;
+import android.util.EventLog;
 import android.util.Slog;
 
 import java.util.ArrayList;
+import java.util.List;
 
 /**
  * A class to keep track of the authentication state for a given client.
@@ -148,7 +154,54 @@ public abstract class AuthenticationClient extends ClientMonitor {
                     + ", requireConfirmation: " + mRequireConfirmation
                     + ", user: " + getTargetUserId());
 
+            // Ensure authentication only succeeds if the client activity is on top or is keyguard.
+            boolean isBackgroundAuth = false;
+            if (authenticated && !Utils.isKeyguard(getContext(), getOwnerString())) {
+                try {
+                    final List<ActivityManager.RunningTaskInfo> tasks =
+                            ActivityTaskManager.getService().getTasks(1);
+                    if (tasks == null || tasks.isEmpty()) {
+                        Slog.e(TAG, "No running tasks reported");
+                        isBackgroundAuth = true;
+                    } else {
+                        final ComponentName topActivity = tasks.get(0).topActivity;
+                        if (topActivity == null) {
+                            Slog.e(TAG, "Unable to get top activity");
+                            isBackgroundAuth = true;
+                        } else {
+                            final String topPackage = topActivity.getPackageName();
+                            if (!topPackage.contentEquals(getOwnerString())) {
+                                Slog.e(TAG, "Background authentication detected, top: " + topPackage
+                                        + ", client: " + this);
+                                isBackgroundAuth = true;
+                            }
+                        }
+                    }
+                } catch (RemoteException e) {
+                    Slog.e(TAG, "Unable to get running tasks", e);
+                    isBackgroundAuth = true;
+                }
+            }
+
+            // Fail authentication if we can't confirm the client activity is on top.
+            if (isBackgroundAuth) {
+                Slog.e(TAG, "Failing possible background authentication");
+                authenticated = false;
+
+                // SafetyNet logging for exploitation attempts of b/159249069.
+                final ApplicationInfo appInfo = getContext().getApplicationInfo();
+                EventLog.writeEvent(0x534e4554, "159249069", appInfo != null ? appInfo.uid : -1,
+                        "Attempted background authentication");
+            }
+
             if (authenticated) {
+                // SafetyNet logging for b/159249069 if constraint is violated.
+                if (isBackgroundAuth) {
+                    final ApplicationInfo appInfo = getContext().getApplicationInfo();
+                    EventLog.writeEvent(0x534e4554, "159249069", appInfo != null ? appInfo.uid : -1,
+                            "Successful background authentication!");
+                }
+
                 mAlreadyDone = true;
 
                 if (listener != null) {