net: arc_emac: Fix use after free in arc_mdio_probe()

If bus->state is equal to MDIOBUS_ALLOCATED, mdiobus_free(bus) will free
the "bus". But bus->name is still used in the next line, which will lead
to a use after free.

We can fix it by putting the name in a local variable and make the
bus->name point to the rodata section "name",then use the name in the
error message without referring to bus to avoid the uaf.

Fixes: 95b5fc0 ("net: arc_emac: Make use of the helper function dev_err_probe()")
Signed-off-by: Jianglei Nie <niejianglei2021@163.com>
Link: https://lore.kernel.org/r/20220309121824.36529-1-niejianglei2021@163.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

drivers/net/ethernet/arc/emac_mdio.c

@@ -132,6 +132,7 @@ int arc_mdio_probe(struct arc_emac_priv *priv)
 {
 	struct arc_emac_mdio_bus_data *data = &priv->bus_data;
 	struct device_node *np = priv->dev->of_node;
+	const char *name = "Synopsys MII Bus";
 	struct mii_bus *bus;
 	int error;
 
@@ -142,7 +143,7 @@ int arc_mdio_probe(struct arc_emac_priv *priv)
 	priv->bus = bus;
 	bus->priv = priv;
 	bus->parent = priv->dev;
-	bus->name = "Synopsys MII Bus";
+	bus->name = name;
 	bus->read = &arc_mdio_read;
 	bus->write = &arc_mdio_write;
 	bus->reset = &arc_mdio_reset;
@@ -167,7 +168,7 @@ int arc_mdio_probe(struct arc_emac_priv *priv)
 	if (error) {
 		mdiobus_free(bus);
 		return dev_err_probe(priv->dev, error,
-				     "cannot register MDIO bus %s\n", bus->name);
+				     "cannot register MDIO bus %s\n", name);
 	}
 
 	return 0;