[PATCH] net: dsa: microchip: fix race condition

[Christian Eggers <ceggers@arri.de>]

[ Upstream commit 8098bd69bc4e925070313b1b95d03510f4f24738 ]

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: 7c6ff470aa86 ("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>
---
This is a back port for 5.4. The original version has been applied to 5.8/5.9
a few hours ago.

Please decide whether the Reviewed-By: and Singed-off-by: tags shall be kept
or removed when forwarding this to stable.

regards
Christian

 drivers/net/dsa/microchip/ksz_common.c | 18 ++++++++++--------
 1 file changed, 10 insertions(+), 8 deletions(-)

diff --git a/drivers/net/dsa/microchip/ksz_common.c b/drivers/net/dsa/microchip/ksz_common.c
index 7fabc0e3d807..b1a9d1012fc4 100644
--- a/drivers/net/dsa/microchip/ksz_common.c
+++ b/drivers/net/dsa/microchip/ksz_common.c
@@ -107,18 +107,11 @@ void ksz_init_mib_timer(struct ksz_device *dev)
 {
 	int i;
 
-	/* Read MIB counters every 30 seconds to avoid overflow. */
-	dev->mib_read_interval = msecs_to_jiffies(30000);
-
 	INIT_WORK(&dev->mib_read, ksz_mib_read_work);
 	timer_setup(&dev->mib_read_timer, mib_monitor, 0);
 
 	for (i = 0; i < dev->mib_port_cnt; i++)
 		dev->dev_ops->port_init_cnt(dev, i);
-
-	/* Start the timer 2 seconds later. */
-	dev->mib_read_timer.expires = jiffies + msecs_to_jiffies(2000);
-	add_timer(&dev->mib_read_timer);
 }
 EXPORT_SYMBOL_GPL(ksz_init_mib_timer);
 
@@ -152,7 +145,9 @@ void ksz_adjust_link(struct dsa_switch *ds, int port,
 	/* Read all MIB counters when the link is going down. */
 	if (!phydev->link) {
 		p->read = true;
-		schedule_work(&dev->mib_read);
+		/* timer started */
+		if (dev->mib_read_interval)
+			schedule_work(&dev->mib_read);
 	}
 	mutex_lock(&dev->dev_mutex);
 	if (!phydev->link)
@@ -464,6 +459,13 @@ 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. */
+	dev->mib_read_timer.expires = jiffies;
+	add_timer(&dev->mib_read_timer);
+
 	return 0;
 }
 EXPORT_SYMBOL(ksz_switch_register);
-- 
Christian Eggers
Embedded software developer

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