mdm: add Android syspolicy handler

Updates tailscale/corp#18202

Adds a syspolicy handler for Android in cmd/tailscale. This allows the Go code to use the syspolicy package to read values set by a system administrator using the Android RestrictionsManager.

Out of the box, this adds supports for a number of MDM policies that are fully integrated on the Go side, such as `ExitNodeID` (forced exit node functionality).

Signed-off-by: Andrea Gottardo <andrea@gottardo.me>

android/src/main/java/com/tailscale/ipn/App.java

@@ -18,6 +18,7 @@ import android.content.ContentValues;
 import android.content.Context;
 import android.content.Intent;
 import android.content.IntentFilter;
+import android.content.RestrictionsManager;
 import android.content.SharedPreferences;
 import android.content.pm.PackageManager;
 import android.content.pm.PackageInfo;
@@ -57,6 +58,7 @@ import java.util.ArrayList;
 import java.util.Collections;
 import java.util.List;
 import java.util.Locale;
+import java.util.Objects;
 
 import androidx.core.app.NotificationCompat;
 import androidx.core.app.NotificationManagerCompat;
@@ -67,6 +69,12 @@ import androidx.security.crypto.EncryptedSharedPreferences;
 import androidx.security.crypto.MasterKey;
 
 import androidx.browser.customtabs.CustomTabsIntent;
+
+import com.tailscale.ipn.mdm.AlwaysNeverUserDecidesSetting;
+import com.tailscale.ipn.mdm.BooleanSetting;
+import com.tailscale.ipn.mdm.MDMSettings;
+import com.tailscale.ipn.mdm.ShowHideSetting;
+import com.tailscale.ipn.mdm.StringSetting;
 import com.tailscale.ipn.ui.service.IpnManager;
 
 import org.gioui.Gio;
@@ -105,15 +113,15 @@ public class App extends Application {
 
 		createNotificationChannel(NOTIFY_CHANNEL_ID, "Notifications", NotificationManagerCompat.IMPORTANCE_DEFAULT);
 		createNotificationChannel(STATUS_CHANNEL_ID, "VPN Status", NotificationManagerCompat.IMPORTANCE_LOW);
-		createNotificationChannel(FILE_CHANNEL_ID, "File transfers", NotificationManagerCompat.IMPORTANCE_DEFAULT);	
-	
+		createNotificationChannel(FILE_CHANNEL_ID, "File transfers", NotificationManagerCompat.IMPORTANCE_DEFAULT);
+
 		_application = this;
 	}
 
 	// requestNetwork attempts to find the best network that matches the passed NetworkRequest. It is possible that
 	// this might return an unusuable network, eg a captive portal.
 	private void setAndRegisterNetworkCallbacks() {
-		connectivityManager.requestNetwork(dns.getDNSConfigNetworkRequest(), new ConnectivityManager.NetworkCallback(){			
+		connectivityManager.requestNetwork(dns.getDNSConfigNetworkRequest(), new ConnectivityManager.NetworkCallback(){
 			@Override
 			public void onAvailable(Network network){
 				super.onAvailable(network);
@@ -423,4 +431,42 @@ public class App extends Application {
 		UiModeManager mm = (UiModeManager)getSystemService(UI_MODE_SERVICE);
 		return mm.getCurrentModeType() == Configuration.UI_MODE_TYPE_TELEVISION;
 	}
+
+	/*
+	The following methods are called by the syspolicy handler from Go via JNI.
+	 */
+
+	boolean getSyspolicyBooleanValue(String key) {
+		RestrictionsManager manager = (RestrictionsManager) this.getSystemService(Context.RESTRICTIONS_SERVICE);
+		MDMSettings mdmSettings = new MDMSettings(manager);
+		BooleanSetting setting = BooleanSetting.valueOf(key);
+		return mdmSettings.get(setting);
+	}
+
+	String getSyspolicyStringValue(String key) {
+		RestrictionsManager manager = (RestrictionsManager) this.getSystemService(Context.RESTRICTIONS_SERVICE);
+		MDMSettings mdmSettings = new MDMSettings(manager);
+
+        // Before looking for a StringSetting matching the given key, Go could also be
+        // asking us for either a AlwaysNeverUserDecidesSetting or a ShowHideSetting.
+        // Check the enum cases for these two before looking for a StringSetting.
+        try {
+            AlwaysNeverUserDecidesSetting anuSetting = AlwaysNeverUserDecidesSetting.valueOf(key);
+            return mdmSettings.get(anuSetting).getValue();
+        } catch (IllegalArgumentException eanu) { // AlwaysNeverUserDecidesSetting does not exist
+            try {
+                ShowHideSetting showHideSetting = ShowHideSetting.valueOf(key);
+                return mdmSettings.get(showHideSetting).getValue();
+            } catch (IllegalArgumentException esh) {
+                try {
+                    StringSetting stringSetting = StringSetting.valueOf(key);
+                    String value = mdmSettings.get(stringSetting);
+                    return Objects.requireNonNullElse(value, "");
+                } catch (IllegalArgumentException estr) {
+                    android.util.Log.d("MDM", key+" is not defined on Android. Returning empty.");
+                    return "";
+                }
+            }
+        }
+	}
 }

cmd/tailscale/main.go

@@ -50,6 +50,7 @@ import (
 	"tailscale.com/tailcfg"
 	"tailscale.com/types/logid"
 	"tailscale.com/types/netmap"
+	"tailscale.com/util/syspolicy"
 	"tailscale.com/wgengine/router"
 )
 
@@ -281,6 +282,7 @@ func main() {
 
 	a.store = newStateStore(a.jvm, a.appCtx)
 	interfaces.RegisterInterfaceGetter(a.getInterfaces)
+	syspolicy.RegisterHandler(androidHandler{a: a})
 	go func() {
 		ctx := context.Background()
 		if err := a.runBackend(ctx); err != nil {

cmd/tailscale/syspolicy_handler.go

@@ -0,0 +1,66 @@
+// Copyright (c) 2024 Tailscale Inc & AUTHORS All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package main
+
+import (
+	"log"
+
+	"github.com/tailscale/tailscale-android/cmd/jni"
+	"tailscale.com/util/syspolicy"
+)
+
+// androidHandler is a syspolicy handler for the Android version of the Tailscale client,
+// which lets the main networking code read values set via the Android RestrictionsManager.
+type androidHandler struct {
+	a *App
+}
+
+func (h androidHandler) ReadString(key string) (string, error) {
+	if key == "" {
+		return "", syspolicy.ErrNoSuchKey
+	}
+	retVal := ""
+	err := jni.Do(h.a.jvm, func(env *jni.Env) error {
+		cls := jni.GetObjectClass(env, h.a.appCtx)
+		m := jni.GetMethodID(env, cls, "getSyspolicyStringValue", "(Ljava/lang/String;)Ljava/lang/String;")
+		strObj, err := jni.CallObjectMethod(env, h.a.appCtx, m, jni.Value(jni.JavaString(env, key)))
+		if err != nil {
+			return err
+		}
+		retVal = jni.GoString(env, jni.String(strObj))
+		return nil
+	})
+	if err != nil {
+		log.Printf("syspolicy: failed to get string value via JNI: %v", err)
+	}
+	return retVal, err
+}
+
+func (h androidHandler) ReadBoolean(key string) (bool, error) {
+	if key == "" {
+		return false, syspolicy.ErrNoSuchKey
+	}
+	retVal := false
+	err := jni.Do(h.a.jvm, func(env *jni.Env) error {
+		cls := jni.GetObjectClass(env, h.a.appCtx)
+		m := jni.GetMethodID(env, cls, "getSyspolicyBooleanValue", "(Ljava/lang/String;)Z")
+		b, err := jni.CallBooleanMethod(env, h.a.appCtx, m, jni.Value(jni.JavaString(env, key)))
+		retVal = b
+		return err
+	})
+	if err != nil {
+		log.Printf("syspolicy: failed to get bool value via JNI: %v", err)
+	}
+	return retVal, err
+}
+
+func (h androidHandler) ReadUInt64(key string) (uint64, error) {
+	if key == "" {
+		return 0, syspolicy.ErrNoSuchKey
+	}
+	// TODO(angott): drop ReadUInt64 everywhere. We are not using it.
+	log.Fatalf("ReadUInt64 is not implemented on Android")
+	return 0, nil
+}