can: etas_es58x: fix potential NULL pointer dereference on udev->serial

The driver assumed that es58x_dev->udev->serial could never be NULL.
While this is true on commercially available devices, an attacker
could spoof the device identity providing a NULL USB serial number.
That would trigger a NULL pointer dereference.

Add a check on es58x_dev->udev->serial before accessing it.

Reported-by: yan kang <kangyan91@outlook.com>
Reported-by: yue sun <samsun1006219@gmail.com>
Closes: https://lore.kernel.org/linux-can/SY8P300MB0421E0013C0EBD2AA46BA709A1F42@SY8P300MB0421.AUSP300.PROD.OUTLOOK.COM/
Fixes: 9f06631 ("can: etas_es58x: export product information through devlink_ops::info_get()")
Signed-off-by: Vincent Mailhol <mailhol.vincent@wanadoo.fr>
Link: https://patch.msgid.link/20250204154859.9797-2-mailhol.vincent@wanadoo.fr
Cc: stable@vger.kernel.org
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>

Author: vincent-mailhol

drivers/net/can/usb/etas_es58x/es58x_devlink.c

@@ -248,7 +248,11 @@ static int es58x_devlink_info_get(struct devlink *devlink,
 			return ret;
 	}
 
-	return devlink_info_serial_number_put(req, es58x_dev->udev->serial);
+	if (es58x_dev->udev->serial)
+		ret = devlink_info_serial_number_put(req,
+						     es58x_dev->udev->serial);
+
+	return ret;
 }
 
 const struct devlink_ops es58x_dl_ops = {