diff --git a/drivers/usb/serial/safe_serial.c b/drivers/usb/serial/safe_serial.c
index 238b54993446..d267a31dcccf 100644
--- a/drivers/usb/serial/safe_serial.c
+++ b/drivers/usb/serial/safe_serial.c
@@ -259,6 +259,7 @@ static int safe_prepare_write_buffer(struct usb_serial_port *port,
 static int safe_startup(struct usb_serial *serial)
 {
 	struct usb_interface_descriptor	*desc;
+	int bulk_out_size;
 
 	if (serial->dev->descriptor.bDeviceClass != CDC_DEVICE_CLASS)
 		return -ENODEV;
@@ -279,6 +280,16 @@ static int safe_startup(struct usb_serial *serial)
 	default:
 		return -EINVAL;
 	}
+
+	/*
+	 * The bulk-out buffer needs to be large enough for the two-byte
+	 * trailer in safe mode, but assume anything smaller than eight bytes
+	 * is broken.
+	 */
+	bulk_out_size = serial->port[0]->bulk_out_size;
+	if (bulk_out_size > 0 && bulk_out_size < 8)
+		return -EINVAL;
+
 	return 0;
 }