[PATCH net 2/2] net: dsa: mxl862xx: fix use-after-free of DSA ports in crc_err_work

[Daniel Golle]

Upon an MDIO CRC error mxl862xx_crc_err_work_fn() walks the DSA ports
and closes the CPU port conduits:

	dsa_switch_for_each_cpu_port(dp, priv->ds)
		dev_close(dp->conduit);

mxl862xx_remove() unregisters the switch before cancelling this work:

	set_bit(MXL862XX_FLAG_WORK_STOPPED, &priv->flags);
	cancel_delayed_work_sync(&priv->stats_work);
	dsa_unregister_switch(ds);
	mxl862xx_host_shutdown(priv);

dsa_unregister_switch() frees the dsa_port objects. If a CRC error
schedules the work during teardown it can run after the ports have been
freed and dereference freed memory.

Guard the port walk with MXL862XX_FLAG_WORK_STOPPED, which is already set
before dsa_unregister_switch(). DSA tears the ports down under
rtnl_lock(), so checking the flag under rtnl_lock() means the work either
runs before teardown and sees valid ports, or runs afterwards, observes
the flag and skips the walk. This mirrors the host_flood_work handler,
which skips torn-down ports under rtnl_lock().

Link: https://sashiko.dev/#/patchset/cover.1780968180.git.daniel%40makrotopia.org?part=2
Fixes: a319d0c8c8ce ("net: dsa: mxl862xx: add CRC for MDIO communication")
Signed-off-by: Daniel Golle <daniel@xxxxxxxxxxxxxx>
---
 drivers/net/dsa/mxl862xx/mxl862xx-host.c | 11 ++++++-----
 1 file changed, 6 insertions(+), 5 deletions(-)

diff --git a/drivers/net/dsa/mxl862xx/mxl862xx-host.c b/drivers/net/dsa/mxl862xx/mxl862xx-host.c
index 882c5d960941..4acd216f7cc0 100644
--- a/drivers/net/dsa/mxl862xx/mxl862xx-host.c
+++ b/drivers/net/dsa/mxl862xx/mxl862xx-host.c
@@ -41,12 +41,13 @@ static void mxl862xx_crc_err_work_fn(struct work_struct *work)
 						  crc_err_work);
 	struct dsa_port *dp;
 
-	dev_warn(&priv->mdiodev->dev,
-		 "MDIO CRC error detected, shutting down all ports\n");
-
 	rtnl_lock();
-	dsa_switch_for_each_cpu_port(dp, priv->ds)
-		dev_close(dp->conduit);
+	if (!test_bit(MXL862XX_FLAG_WORK_STOPPED, &priv->flags)) {
+		dev_warn(&priv->mdiodev->dev,
+			 "MDIO CRC error detected, shutting down all ports\n");
+		dsa_switch_for_each_cpu_port(dp, priv->ds)
+			dev_close(dp->conduit);
+	}
 	rtnl_unlock();
 
 	clear_bit(MXL862XX_FLAG_CRC_ERR, &priv->flags);
-- 
2.54.0