Patch "net: fix race on decreasing number of TX queues" has been added to the 4.9-stable tree

This is a note to let you know that I've just added the patch titled

net: fix race on decreasing number of TX queues

to the 4.9-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
net-fix-race-on-decreasing-number-of-tx-queues.patch
and it can be found in the queue-4.9 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: Jakub Kicinski <[email protected]>
Date: Mon, 12 Feb 2018 21:35:31 -0800
Subject: net: fix race on decreasing number of TX queues

From: Jakub Kicinski <[email protected]>

[ Upstream commit ac5b70198adc25c73fba28de4f78adcee8f6be0b ]

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.

---
 net/core/dev.c |   11 +++++++++--
 1 file changed, 9 insertions(+), 2 deletions(-)

@@ -2199,8 +2199,11 @@ EXPORT_SYMBOL(netif_set_xps_queue);
  */
 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;
 

@@ -2216,15 +2219,19 @@ int netif_set_real_num_tx_queues(struct
 		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);

Patches currently in stable-queue which might be from [email protected] are

queue-4.9/net-fix-race-on-decreasing-number-of-tx-queues.patch