DO NOT MERGE: ActivityManager#killBackgroundProcesses can kill caller's own app only

unless it's a system app.

Bug: 239423414
Bug: 223376078
Test: atest CtsAppTestCases:ActivityManagerTest
(cherry picked from https://googleplex-android-review.googlesource.com/q/commit:fa94ce5c7738e449cb6bd68c77af4858018e49e0)
Merged-In: Iac6baa889965b8ffecd9a43179a4c96632ad1d02
Change-Id: Iac6baa889965b8ffecd9a43179a4c96632ad1d02

core/java/android/app/ActivityManager.java

diff --git a/core/java/android/app/ActivityManager.java b/core/java/android/app/ActivityManager.java
index 5d1d225..68a42d1 100644
--- a/core/java/android/app/ActivityManager.java
+++ b/core/java/android/app/ActivityManager.java
@@ -3934,6 +3934,9 @@
      * processes to reclaim memory; the system will take care of restarting
      * these processes in the future as needed.
      *
+     * <p class="note">Third party applications can only use this API to kill their own processes.
+     * </p>
+     *
      * @param packageName The name of the package whose processes are to
      * be killed.
      */

core/res/AndroidManifest.xml

diff --git a/core/res/AndroidManifest.xml b/core/res/AndroidManifest.xml
index 7439b2f..cb00011 100644
--- a/core/res/AndroidManifest.xml
+++ b/core/res/AndroidManifest.xml
@@ -3123,7 +3123,11 @@
         android:protectionLevel="normal" />
 
     <!-- Allows an application to call
-        {@link android.app.ActivityManager#killBackgroundProcesses}.
+         {@link android.app.ActivityManager#killBackgroundProcesses}.
+
+         <p class="note">Third party applications can only use this API to kill their own
+         processes.</p>
+
          <p>Protection level: normal
     -->
     <permission android:name="android.permission.KILL_BACKGROUND_PROCESSES"

services/core/java/com/android/server/am/ActivityManagerService.java

diff --git a/services/core/java/com/android/server/am/ActivityManagerService.java b/services/core/java/com/android/server/am/ActivityManagerService.java
index a70a01f..cd221a8 100644
--- a/services/core/java/com/android/server/am/ActivityManagerService.java
+++ b/services/core/java/com/android/server/am/ActivityManagerService.java
@@ -3837,8 +3837,20 @@
             Slog.w(TAG, msg);
             throw new SecurityException(msg);
         }
+        final int callingUid = Binder.getCallingUid();
+        final int callingPid = Binder.getCallingPid();
+        final int callingAppId = UserHandle.getAppId(callingUid);
 
-        userId = mUserController.handleIncomingUser(Binder.getCallingPid(), Binder.getCallingUid(),
+        ProcessRecord proc;
+        synchronized (mPidsSelfLocked) {
+            proc = mPidsSelfLocked.get(callingPid);
+        }
+        final boolean hasKillAllPermission = PERMISSION_GRANTED == checkPermission(
+                android.Manifest.permission.FORCE_STOP_PACKAGES, callingPid, callingUid)
+                || UserHandle.isCore(callingUid)
+                || (proc != null && proc.info.isSystemApp());
+
+        userId = mUserController.handleIncomingUser(callingPid, callingUid,
                 userId, true, ALLOW_FULL_ONLY, "killBackgroundProcesses", null);
         final int[] userIds = mUserController.expandUserId(userId);
 
@@ -3853,7 +3865,7 @@
                                     targetUserId));
                 } catch (RemoteException e) {
                 }
-                if (appId == -1) {
+                if (appId == -1 || (!hasKillAllPermission && appId != callingAppId)) {
                     Slog.w(TAG, "Invalid packageName: " + packageName);
                     return;
                 }
@@ -3881,6 +3893,22 @@
             throw new SecurityException(msg);
         }
 
+        final int callingUid = Binder.getCallingUid();
+        final int callingPid = Binder.getCallingPid();
+
+        ProcessRecord proc;
+        synchronized (mPidsSelfLocked) {
+            proc = mPidsSelfLocked.get(callingPid);
+        }
+        if (callingUid >= FIRST_APPLICATION_UID
+                && (proc == null || !proc.info.isSystemApp())) {
+            final String msg = "Permission Denial: killAllBackgroundProcesses() from pid="
+                    + callingPid + ", uid=" + callingUid + " is not allowed";
+            Slog.w(TAG, msg);
+            // Silently return to avoid existing apps from crashing.
+            return;
+        }
+
         final long callingId = Binder.clearCallingIdentity();
         try {
             synchronized (this) {