net: dsa: sja1105: fix use-after-free after calling of_find_compatible_node, or worse

It seems that of_find_compatible_node has a weird calling convention in
which it calls of_node_put() on the "from" node argument, instead of
leaving that up to the caller. This comes from the fact that
of_find_compatible_node with a non-NULL "from" argument it only supposed
to be used as the iterator function of for_each_compatible_node(). OF
iterator functions call of_node_get on the next OF node and of_node_put()
on the previous one.

When of_find_compatible_node calls of_node_put, it actually never
expects the refcount to drop to zero, because the call is done under the
atomic devtree_lock context, and when the refcount drops to zero it
triggers a kobject and a sysfs file deletion, which assume blocking
context.

So any driver call to of_find_compatible_node is probably buggy because
an unexpected of_node_put() takes place.

What should be done is to use the of_get_compatible_child() function.

Fixes: 5a8f097 ("net: dsa: sja1105: register the MDIO buses for 100base-T1 and 100base-TX")
Link: https://lore.kernel.org/netdev/20210814010139.kzryimmp4rizlznt@skbuf/
Suggested-by: Frank Rowand <[email protected]>
Suggested-by: Rob Herring <[email protected]>
Signed-off-by: Vladimir Oltean <[email protected]>
Signed-off-by: David S. Miller <[email protected]>

Author: vladimiroltean

drivers/net/dsa/sja1105/sja1105_mdio.c

@@ -284,8 +284,7 @@ static int sja1105_mdiobus_base_tx_register(struct sja1105_private *priv,
 	struct mii_bus *bus;
 	int rc = 0;
 
-	np = of_find_compatible_node(mdio_node, NULL,
-				     "nxp,sja1110-base-tx-mdio");
+	np = of_get_compatible_child(mdio_node, "nxp,sja1110-base-tx-mdio");
 	if (!np)
 		return 0;
 
@@ -339,8 +338,7 @@ static int sja1105_mdiobus_base_t1_register(struct sja1105_private *priv,
 	struct mii_bus *bus;
 	int rc = 0;
 
-	np = of_find_compatible_node(mdio_node, NULL,
-				     "nxp,sja1110-base-t1-mdio");
+	np = of_get_compatible_child(mdio_node, "nxp,sja1110-base-t1-mdio");
 	if (!np)
 		return 0;