Debugger: Prevent Abort on Invalid Virtual Method

If the debugger is asked to invoke an interface or virtual
method on a class the mono_object_get_virtual_method_internal
will assert & abort (or crash).

mono_object_get_virtual_method_internal assumes that
it is being asked to invoke a virtual method on an instance
that implements it.  If that's not true it will assert & abort or
access invalid memory if assertions are disabled.

To protect against this we check if the this_arg can be cast to
the interface or base class type.  If it can then we should be
safe to call mono_object_get_virtual_method_internal.

Author: scott-ferguson-unity

mono/mini/debugger-agent.c

@@ -6545,6 +6545,10 @@ do_invoke_method (DebuggerTlsData *tls, Buffer *buf, InvokeData *invoke, guint8
 			PRINT_DEBUG_MSG (1, "[%p] Error: Interface method invoked without this argument.\n", (gpointer) (gsize) mono_native_thread_id_get ());
 			return ERR_INVALID_ARGUMENT;
 		}
+		if (!mono_object_isinst(this_arg, m->klass)) {
+			PRINT_DEBUG_MSG (1, "[%p] Error: Interface method invoked on object that doesn't implement the interface.\n", (gpointer) (gsize) mono_native_thread_id_get ());
+			return ERR_INVALID_ARGUMENT;
+		}
 		m = mono_object_get_virtual_method_internal (this_arg, m);
 		/* Transform this to the format the rest of the code expects it to be */
 		if (m_class_is_valuetype (m->klass)) {
@@ -6556,6 +6560,10 @@ do_invoke_method (DebuggerTlsData *tls, Buffer *buf, InvokeData *invoke, guint8
 			PRINT_DEBUG_MSG (1, "[%p] Error: invoke with INVOKE_FLAG_VIRTUAL flag set without this argument.\n", (gpointer) (gsize) mono_native_thread_id_get ());
 			return ERR_INVALID_ARGUMENT;
 		}
+		if (!mono_object_isinst(this_arg, m->klass)) {
+			PRINT_DEBUG_MSG (1, "[%p] Error: invoke with INVOKE_FLAG_VIRTUAL flag on object that object that doesn't implement the method.\n", (gpointer) (gsize) mono_native_thread_id_get ());
+			return ERR_INVALID_ARGUMENT;
+		}
 		m = mono_object_get_virtual_method_internal (this_arg, m);
 		if (m_class_is_valuetype (m->klass)) {
 			this_buf = (guint8 *)g_alloca (mono_class_instance_size (m->klass));