HID: intel-ish-hid: Fix kernel panic during warm reset

During warm reset device->fw_client is set to NULL. If a bus driver is
registered after this NULL setting and before new firmware clients are
enumerated by ISHTP, kernel panic will result in the function
ishtp_cl_bus_match(). This is because of reference to
device->fw_client->props.protocol_name.

ISH firmware after getting successfully loaded, sends a warm reset
notification to remove all clients from the bus and sets
device->fw_client to NULL. Until kernel v5.15, all enabled ISHTP kernel
module drivers were loaded right after any of the first ISHTP device was
registered, regardless of whether it was a matched or an unmatched
device. This resulted in all drivers getting registered much before the
warm reset notification from ISH.

Starting kernel v5.16, this issue got exposed after the change was
introduced to load only bus drivers for the respective matching devices.
In this scenario, cros_ec_ishtp device and cros_ec_ishtp driver are
registered after the warm reset device fw_client NULL setting.
cros_ec_ishtp driver_register() triggers the callback to
ishtp_cl_bus_match() to match ISHTP driver to the device and causes kernel
panic in guid_equal() when dereferencing fw_client NULL pointer to get
protocol_name.

Fixes: f155dfea ("platform/x86: isthp_eclite: only load for matching devices")
Fixes: facfe0a4 ("platform/chrome: chros_ec_ishtp: only load for matching devices")
Fixes: 0d0cccc0 ("HID: intel-ish-hid: hid-client: only load for matching devices")
Fixes: 44e2a58c ("HID: intel-ish-hid: fw-loader: only load for matching devices")
Cc: <stable@vger.kernel.org> # 5.16+
Signed-off-by: Tanu Malhotra <tanu.malhotra@intel.com>
Tested-by: Shaunak Saha <shaunak.saha@intel.com>
Acked-by: Srinivas Pandruvada <srinivas.pandruvada@linux.intel.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>

drivers/hid/intel-ish-hid/ishtp/bus.c

@@ -241,8 +241,8 @@ static int ishtp_cl_bus_match(struct device *dev, struct device_driver *drv)
 	struct ishtp_cl_device *device = to_ishtp_cl_device(dev);
 	struct ishtp_cl_driver *driver = to_ishtp_cl_driver(drv);
 
-	return guid_equal(&driver->id[0].guid,
-			  &device->fw_client->props.protocol_name);
+	return(device->fw_client ? guid_equal(&driver->id[0].guid,
+	       &device->fw_client->props.protocol_name) : 0);
 }
 
 /**