net: dsa: microchip: Added the condition for scheduling ksz_mib_read_work

When the ksz module is installed and removed using rmmod, kernel crashes
with null pointer dereferrence error. During rmmod, ksz_switch_remove
function tries to cancel the mib_read_workqueue using
cancel_delayed_work_sync routine and unregister switch from dsa.

During dsa_unregister_switch it calls ksz_mac_link_down, which in turn
reschedules the workqueue since mib_interval is non-zero.
Due to which queue executed after mib_interval and it tries to access
dp->slave. But the slave is unregistered in the ksz_switch_remove
function. Hence kernel crashes.

To avoid this crash, before canceling the workqueue, resetted the
mib_interval to 0.

v1 -> v2:
-Removed the if condition in ksz_mib_read_work

Fixes: 469b390e1ba3 ("net: dsa: microchip: use delayed_work instead of timer + work")
Signed-off-by: Arun Ramadoss <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

drivers/net/dsa/microchip/ksz_common.c

···
 void ksz_switch_remove(struct ksz_device *dev)
 {
 	/* timer started */
-	if (dev->mib_read_interval)
+	if (dev->mib_read_interval) {
+		dev->mib_read_interval = 0;
 		cancel_delayed_work_sync(&dev->mib_read);
+	}
 
 	dev->dev_ops->exit(dev);
 	dsa_unregister_switch(dev->ds);