USB: cdc-acm: handle broken union descriptors

Handle broken union functional descriptors where the master-interface
doesn't exist or where its class is of neither Communication or Data
type (as required by the specification) by falling back to
"combined-interface" probing.

Note that this still allows for handling union descriptors with switched
interfaces.

This specifically makes the Whistler radio scanners TRX series devices
work with the driver without adding further quirks to the device-id
table.

Reported-by: Daniel Caujolle-Bert <f1rmb.daniel@gmail.com>
Tested-by: Daniel Caujolle-Bert <f1rmb.daniel@gmail.com>
Acked-by: Oliver Neukum <oneukum@suse.com>
Signed-off-by: Johan Hovold <johan@kernel.org>
Link: https://lore.kernel.org/r/20200921135951.24045-3-johan@kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/usb/class/cdc-acm.c

@@ -1241,9 +1241,21 @@ static int acm_probe(struct usb_interface *intf,
 			}
 		}
 	} else {
+		int class = -1;
+
 		data_intf_num = union_header->bSlaveInterface0;
 		control_interface = usb_ifnum_to_if(usb_dev, union_header->bMasterInterface0);
 		data_interface = usb_ifnum_to_if(usb_dev, data_intf_num);
+
+		if (control_interface)
+			class = control_interface->cur_altsetting->desc.bInterfaceClass;
+
+		if (class != USB_CLASS_COMM && class != USB_CLASS_CDC_DATA) {
+			dev_dbg(&intf->dev, "Broken union descriptor, assuming single interface\n");
+			combined_interfaces = 1;
+			control_interface = data_interface = intf;
+			goto look_for_collapsed_interface;
+		}
 	}
 
 	if (!control_interface || !data_interface) {