can: gs_usb: fix time stamp counter initialization

In the Linux kernel, the following vulnerability has been resolved:

can: gs_usb: fix time stamp counter initialization

If the gs_usb device driver is unloaded (or unbound) before the

interface is shut down, the USB stack first calls the struct

usb_driver::disconnect and then the struct net_device_ops::ndo_stop

callback.

In gs_usb_disconnect() all pending bulk URBs are killed, i.e. no more

RX'ed CAN frames are send from the USB device to the host. Later in

gs_can_close() a reset control message is send to each CAN channel to

remove the controller from the CAN bus. In this race window the USB

device can still receive CAN frames from the bus and internally queue

them to be send to the host.

At least in the current version of the candlelight firmware, the queue

of received CAN frames is not emptied during the reset command. After

loading (or binding) the gs_usb driver, new URBs are submitted during

the struct net_device_ops::ndo_open callback and the candlelight

firmware starts sending its already queued CAN frames to the host.

However, this scenario was not considered when implementing the

hardware timestamp function. The cycle counter/time counter

infrastructure is set up (gs_usb_timestamp_init()) after the USBs are

submitted, resulting in a NULL pointer dereference if

timecounter_cyc2time() (via the call chain:

gs_usb_receive_bulk_callback() -> gs_usb_set_timestamp() ->

gs_usb_skb_set_timestamp()) is called too early.

Move the gs_usb_timestamp_init() function before the URBs are

submitted to fix this problem.

For a comprehensive solution, we need to consider gs_usb devices with

more than 1 channel. The cycle counter/time counter infrastructure is

setup per channel, but the RX URBs are per device. Once gs_can_open()

of _a_ channel has been called, and URBs have been submitted, the

gs_usb_receive_bulk_callback() can be called for _all_ available

channels, even for channels that are not running, yet. As cycle

counter/time counter has not set up, this will again lead to a NULL

pointer dereference.

Convert the cycle counter/time counter from a "per channel" to a "per

device" functionality. Also set it up, before submitting any URBs to

the device.

Further in gs_usb_receive_bulk_callback(), don't process any URBs for

not started CAN channels, only resubmit the URB.