net: Catch invalid index in XPS mapping

When setting the XPS value of a TX queue, warn the user once if the
index of the queue is greater than the number of allocated TX queues.

Previously, this scenario went uncaught. In the best case, it resulted
in unnecessary allocations. In the worst case, it resulted in
out-of-bounds memory references through calls to `netdev_get_tx_queue(
dev, index)`. Therefore, it is important to inform the user but not
worth returning an error and risk downing the netdevice.
Signed-off-by: Nick Child <nnac123@linux.ibm.com>
Reviewed-by: Piotr Raczynski <piotr.raczynski@intel.com>
Link: https://lore.kernel.org/r/20230321150725.127229-1-nnac123@linux.ibm.comSigned-off-by: Jakub Kicinski <kuba@kernel.org>

net/core/dev.c

@@ -2535,6 +2535,8 @@ int __netif_set_xps_queue(struct net_device *dev, const unsigned long *mask,
 	struct xps_map *map, *new_map;
 	unsigned int nr_ids;
 
+	WARN_ON_ONCE(index >= dev->num_tx_queues);
+
 	if (dev->num_tc) {
 		/* Do not allow XPS on subordinate device directly */
 		num_tc = dev->num_tc;