net-sysfs: take the rtnl lock when accessing xps_rxqs_map and num_tc

Accesses to dev->xps_rxqs_map (when using dev->num_tc) should be
protected by the rtnl lock, like we do for netif_set_xps_queue. I didn't
see an actual bug being triggered, but let's be safe here and take the
rtnl lock while accessing the map in sysfs.

Fixes: 8af2c06f ("net-sysfs: Add interface for Rx queue(s) map per Tx queue")
Signed-off-by: Antoine Tenart <atenart@kernel.org>
Reviewed-by: Alexander Duyck <alexanderduyck@fb.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

net/core/net-sysfs.c

@@ -1429,22 +1429,29 @@ static struct netdev_queue_attribute xps_cpus_attribute __ro_after_init
 
 static ssize_t xps_rxqs_show(struct netdev_queue *queue, char *buf)
 {
+	int j, len, ret, num_tc = 1, tc = 0;
 	struct net_device *dev = queue->dev;
 	struct xps_dev_maps *dev_maps;
 	unsigned long *mask, index;
-	int j, len, num_tc = 1, tc = 0;
 
 	index = get_netdev_queue_index(queue);
 
+	if (!rtnl_trylock())
+		return restart_syscall();
+
 	if (dev->num_tc) {
 		num_tc = dev->num_tc;
 		tc = netdev_txq_to_tc(dev, index);
-		if (tc < 0)
-			return -EINVAL;
+		if (tc < 0) {
+			ret = -EINVAL;
+			goto err_rtnl_unlock;
+		}
 	}
 	mask = bitmap_zalloc(dev->num_rx_queues, GFP_KERNEL);
-	if (!mask)
-		return -ENOMEM;
+	if (!mask) {
+		ret = -ENOMEM;
+		goto err_rtnl_unlock;
+	}
 
 	rcu_read_lock();
 	dev_maps = rcu_dereference(dev->xps_rxqs_map);
@@ -1470,10 +1477,16 @@ static ssize_t xps_rxqs_show(struct netdev_queue *queue, char *buf)
 out_no_maps:
 	rcu_read_unlock();
 
+	rtnl_unlock();
+
 	len = bitmap_print_to_pagebuf(false, buf, mask, dev->num_rx_queues);
 	bitmap_free(mask);
 
 	return len < PAGE_SIZE ? len : -EINVAL;
+
+err_rtnl_unlock:
+	rtnl_unlock();
+	return ret;
 }
 
 static ssize_t xps_rxqs_store(struct netdev_queue *queue, const char *buf,