src/com.example.android.mediabrowserservice/PackageValidator.java

/*
 * Copyright (C) 2014 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.example.android.mediabrowserservice;

import android.content.Context;
import android.content.pm.PackageInfo;
import android.content.pm.PackageManager;
import android.content.res.XmlResourceParser;
import android.os.Process;
import android.util.Base64;

import com.example.android.mediabrowserservice.utils.LogHelper;

import org.xmlpull.v1.XmlPullParserException;

import java.io.IOException;
import java.util.ArrayList;
import java.util.HashMap;
import java.util.Map;

/**
 * Validates that the calling package is authorized to browse a
 * {@link android.service.media.MediaBrowserService}.
 *
 * The list of allowed signing certificates and their corresponding package names is defined in
 * res/xml/allowed_media_browser_callers.xml.
 */
public class PackageValidator {
    private static final String TAG = LogHelper.makeLogTag(PackageValidator.class);

    /**
     * Map allowed callers' certificate keys to the expected caller information.
     *
     */
    private final Map<String, ArrayList<CallerInfo>> mValidCertificates;

    public PackageValidator(Context ctx) {
        mValidCertificates = readValidCertificates(ctx.getResources().getXml(
            R.xml.allowed_media_browser_callers));
    }

    private Map<String, ArrayList<CallerInfo>> readValidCertificates(XmlResourceParser parser) {
        HashMap<String, ArrayList<CallerInfo>> validCertificates = new HashMap<>();
        try {
            int eventType = parser.next();
            while (eventType != XmlResourceParser.END_DOCUMENT) {
                if (eventType == XmlResourceParser.START_TAG
                        && parser.getName().equals("signing_certificate")) {

                    String name = parser.getAttributeValue(null, "name");
                    String packageName = parser.getAttributeValue(null, "package");
                    boolean isRelease = parser.getAttributeBooleanValue(null, "release", false);
                    String certificate = parser.nextText().replaceAll("\\s|\\n", "");

                    CallerInfo info = new CallerInfo(name, packageName, isRelease, certificate);

                    ArrayList<CallerInfo> infos = validCertificates.get(certificate);
                    if (infos == null) {
                        infos = new ArrayList<>();
                        validCertificates.put(certificate, infos);
                    }
                    LogHelper.v(TAG, "Adding allowed caller: ", info.name,
                        " package=", info.packageName, " release=", info.release,
                        " certificate=", certificate);
                    infos.add(info);
                }
                eventType = parser.next();
            }
        } catch (XmlPullParserException | IOException e) {
            LogHelper.e(TAG, e, "Could not read allowed callers from XML.");
        }
        return validCertificates;
    }

    /**
     * @return false if the caller is not authorized to get data from this MediaBrowserService
     */
    @SuppressWarnings("BooleanMethodIsAlwaysInverted")
    public boolean isCallerAllowed(Context context, String callingPackage, int callingUid) {
        // Always allow calls from the framework, self app or development environment.
        if (Process.SYSTEM_UID == callingUid || Process.myUid() == callingUid) {
            return true;
        }
        PackageManager packageManager = context.getPackageManager();
        PackageInfo packageInfo;
        try {
            packageInfo = packageManager.getPackageInfo(
                    callingPackage, PackageManager.GET_SIGNATURES);
        } catch (PackageManager.NameNotFoundException e) {
            LogHelper.w(TAG, e, "Package manager can't find package: ", callingPackage);
            return false;
        }
        if (packageInfo.signatures.length != 1) {
            LogHelper.w(TAG, "Caller has more than one signature certificate!");
            return false;
        }
        String signature = Base64.encodeToString(
            packageInfo.signatures[0].toByteArray(), Base64.NO_WRAP);

        // Test for known signatures:
        ArrayList<CallerInfo> validCallers = mValidCertificates.get(signature);
        if (validCallers == null) {
            LogHelper.v(TAG, "Signature for caller ", callingPackage, " is not valid: \n"
                , signature);
            if (mValidCertificates.isEmpty()) {
                LogHelper.w(TAG, "The list of valid certificates is empty. Either your file ",
                        "res/xml/allowed_media_browser_callers.xml is empty or there was an error ",
                        "while reading it. Check previous log messages.");
            }
            return false;
        }

        // Check if the package name is valid for the certificate:
        StringBuffer expectedPackages = new StringBuffer();
        for (CallerInfo info: validCallers) {
            if (callingPackage.equals(info.packageName)) {
                LogHelper.v(TAG, "Valid caller: ", info.name, "  package=", info.packageName,
                    " release=", info.release);
                return true;
            }
            expectedPackages.append(info.packageName).append(' ');
        }

        LogHelper.i(TAG, "Caller has a valid certificate, but its package doesn't match any ",
            "expected package for the given certificate. Caller's package is ", callingPackage,
            ". Expected packages as defined in res/xml/allowed_media_browser_callers.xml are (",
            expectedPackages, "). This caller's certificate is: \n", signature);

        return false;
    }

    private final static class CallerInfo {
        final String name;
        final String packageName;
        final boolean release;
        final String signingCertificate;

        public CallerInfo(String name, String packageName, boolean release,
                          String signingCertificate) {
            this.name = name;
            this.packageName = packageName;
            this.release = release;
            this.signingCertificate = signingCertificate;
        }
    }
}