net: fix race on decreasing number of TX queues

netif_set_real_num_tx_queues() can be called when netdev is up.
That usually happens when user requests change of number of
channels/rings with ethtool -L.  The procedure for changing
the number of queues involves resetting the qdiscs and setting
dev->num_tx_queues to the new value.  When the new value is
lower than the old one, extra care has to be taken to ensure
ordering of accesses to the number of queues vs qdisc reset.

Currently the queues are reset before new dev->num_tx_queues
is assigned, leaving a window of time where packets can be
enqueued onto the queues going down, leading to a likely
crash in the drivers, since most drivers don't check if TX
skbs are assigned to an active queue.

Fixes: e6484930d7c7 ("net: allocate tx queues in register_netdevice")
Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/core/dev.c

@@ -2382,8 +2382,11 @@ EXPORT_SYMBOL(netdev_set_num_tc);
  */
 int netif_set_real_num_tx_queues(struct net_device *dev, unsigned int txq)
 {
+	bool disabling;
 	int rc;
 
+	disabling = txq < dev->real_num_tx_queues;
+
 	if (txq < 1 || txq > dev->num_tx_queues)
 		return -EINVAL;
 
@@ -2399,15 +2402,19 @@ int netif_set_real_num_tx_queues(struct net_device *dev, unsigned int txq)
 		if (dev->num_tc)
 			netif_setup_tc(dev, txq);
 
-		if (txq < dev->real_num_tx_queues) {
+		dev->real_num_tx_queues = txq;
+
+		if (disabling) {
+			synchronize_net();
 			qdisc_reset_all_tx_gt(dev, txq);
 #ifdef CONFIG_XPS
 			netif_reset_xps_queues_gt(dev, txq);
 #endif
 		}
+	} else {
+		dev->real_num_tx_queues = txq;
 	}
 
-	dev->real_num_tx_queues = txq;
 	return 0;
 }
 EXPORT_SYMBOL(netif_set_real_num_tx_queues);