net: dsa: microchip: fix race condition

Between queuing the delayed work and finishing the setup of the dsa
ports, the process may sleep in request_module() (via
phy_device_create()) and the queued work may be executed prior to the
switch net devices being registered. In ksz_mib_read_work(), a NULL
dereference will happen within netof_carrier_ok(dp->slave).

Not queuing the delayed work in ksz_init_mib_timer() makes things even
worse because the work will now be queued for immediate execution
(instead of 2000 ms) in ksz_mac_link_down() via
dsa_port_link_register_of().

Call tree:
ksz9477_i2c_probe()
\--ksz9477_switch_register()
   \--ksz_switch_register()
      +--dsa_register_switch()
      |  \--dsa_switch_probe()
      |     \--dsa_tree_setup()
      |        \--dsa_tree_setup_switches()
      |           +--dsa_switch_setup()
      |           |  +--ksz9477_setup()
      |           |  |  \--ksz_init_mib_timer()
      |           |  |     |--/* Start the timer 2 seconds later. */
      |           |  |     \--schedule_delayed_work(&dev->mib_read, msecs_to_jiffies(2000));
      |           |  \--__mdiobus_register()
      |           |     \--mdiobus_scan()
      |           |        \--get_phy_device()
      |           |           +--get_phy_id()
      |           |           \--phy_device_create()
      |           |              |--/* sleeping, ksz_mib_read_work() can be called meanwhile */
      |           |              \--request_module()
      |           |
      |           \--dsa_port_setup()
      |              +--/* Called for non-CPU ports */
      |              +--dsa_slave_create()
      |              |  +--/* Too late, ksz_mib_read_work() may be called beforehand */
      |              |  \--port->slave = ...
      |             ...
      |              +--Called for CPU port */
      |              \--dsa_port_link_register_of()
      |                 \--ksz_mac_link_down()
      |                    +--/* mib_read must be initialized here */
      |                    +--/* work is already scheduled, so it will be executed after 2000 ms */
      |                    \--schedule_delayed_work(&dev->mib_read, 0);
      \-- /* here port->slave is setup properly, scheduling the delayed work should be safe */

Solution:
1. Do not queue (only initialize) delayed work in ksz_init_mib_timer().
2. Only queue delayed work in ksz_mac_link_down() if init is completed.
3. Queue work once in ksz_switch_register(), after dsa_register_switch()
has completed.

Fixes: 7c6ff470 ("net: dsa: microchip: add MIB counter reading support")
Signed-off-by: Christian Eggers <ceggers@arri.de>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Reviewed-by: Vladimir Oltean <olteanv@gmail.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

drivers/net/dsa/microchip/ksz_common.c

@@ -103,14 +103,8 @@ void ksz_init_mib_timer(struct ksz_device *dev)
 
 	INIT_DELAYED_WORK(&dev->mib_read, ksz_mib_read_work);
 
-	/* Read MIB counters every 30 seconds to avoid overflow. */
-	dev->mib_read_interval = msecs_to_jiffies(30000);
-
 	for (i = 0; i < dev->mib_port_cnt; i++)
 		dev->dev_ops->port_init_cnt(dev, i);
-
-	/* Start the timer 2 seconds later. */
-	schedule_delayed_work(&dev->mib_read, msecs_to_jiffies(2000));
 }
 EXPORT_SYMBOL_GPL(ksz_init_mib_timer);
 
@@ -143,6 +137,8 @@ void ksz_mac_link_down(struct dsa_switch *ds, int port, unsigned int mode,
 
 	/* Read all MIB counters when the link is going down. */
 	p->read = true;
+	/* timer started */
+	if (dev->mib_read_interval)
 		schedule_delayed_work(&dev->mib_read, 0);
 }
 EXPORT_SYMBOL_GPL(ksz_mac_link_down);
@@ -450,6 +446,12 @@ int ksz_switch_register(struct ksz_device *dev,
 		return ret;
 	}
 
+	/* Read MIB counters every 30 seconds to avoid overflow. */
+	dev->mib_read_interval = msecs_to_jiffies(30000);
+
+	/* Start the MIB timer. */
+	schedule_delayed_work(&dev->mib_read, 0);
+
 	return 0;
 }
 EXPORT_SYMBOL(ksz_switch_register);