[NET]: Fix race in dev_close(). (Bug 9750)

There is a race in Linux kernel file net/core/dev.c, function dev_close.
The function calls function dev_deactivate, which calls function
dev_watchdog_down that deletes the watchdog timer. However, after that, a
driver can call netif_carrier_ok, which calls function
__netdev_watchdog_up that can add the watchdog timer again. Function
unregister_netdevice calls function dev_shutdown that traps the bug
!timer_pending(&dev->watchdog_timer). Moving dev_deactivate after
netif_running() has been cleared prevents function netif_carrier_on
from calling __netdev_watchdog_up and adding the watchdog timer again.
Signed-off-by: NMatti Linnanvuori <mattilinnanvuori@yahoo.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/core/dev.c

@@ -1071,8 +1071,6 @@ int dev_close(struct net_device *dev)
 	 */
 	call_netdevice_notifiers(NETDEV_GOING_DOWN, dev);
 
-	dev_deactivate(dev);
-
 	clear_bit(__LINK_STATE_START, &dev->state);
 
 	/* Synchronize to scheduled poll. We cannot touch poll list,
@@ -1083,6 +1081,8 @@ int dev_close(struct net_device *dev)
 	 */
 	smp_mb__after_clear_bit(); /* Commit netif_running(). */
 
+	dev_deactivate(dev);
+
 	/*
 	 *	Call the device specific close. This cannot fail.
 	 *	Only if device is UP