Laxify SecureSM to allow creation of the JDK's innocuous threads (#77789)

Co-authored-by: Elastic Machine <[email protected]>

Author: ChrisHegarty

libs/secure-sm/src/main/java/org/elasticsearch/secure_sm/SecureSM.java

@@ -154,6 +154,12 @@ private void debugThreadGroups(final ThreadGroup caller, final ThreadGroup targe
     private static final Permission MODIFY_THREAD_PERMISSION = new RuntimePermission("modifyThread");
     private static final Permission MODIFY_ARBITRARY_THREAD_PERMISSION = new ThreadPermission("modifyArbitraryThread");
 
+    // Returns true if the given thread is an instance of the JDK's InnocuousThread.
+    private static boolean isInnocuousThread(Thread t) {
+        final Class<?> c = t.getClass();
+        return c.getModule() == Object.class.getModule() && c.getName().equals("jdk.internal.misc.InnocuousThread");
+    }
+
     protected void checkThreadAccess(Thread t) {
         Objects.requireNonNull(t);
 
@@ -166,7 +172,7 @@ protected void checkThreadAccess(Thread t) {
 
         if (target == null) {
             return;    // its a dead thread, do nothing.
-        } else if (source.parentOf(target) == false) {
+        } else if (source.parentOf(target) == false && isInnocuousThread(t) == false) {
             checkPermission(MODIFY_ARBITRARY_THREAD_PERMISSION);
         }
     }