net: dsa: Fix use after free in dsa_switch_remove()

The order in which the ports are deleted from the list and freed and the
call to dsa_switch_remove() is done is reversed, which leads to an
use after free condition. Reverse the two: first tear down the ports and
switch from the fabric, then free the ports associated with that switch
fabric.

Fixes: 05f294a ("net: dsa: allocate ports on touch")
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Reviewed-by: Vivien Didelot <vivien.didelot@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/dsa/dsa2.c

@@ -874,12 +874,13 @@ static void dsa_switch_remove(struct dsa_switch *ds)
 	struct dsa_switch_tree *dst = ds->dst;
 	struct dsa_port *dp, *next;
 
+	dsa_tree_teardown(dst);
+
 	list_for_each_entry_safe(dp, next, &dst->ports, list) {
 		list_del(&dp->list);
 		kfree(dp);
 	}
 
-	dsa_tree_teardown(dst);
 	dsa_tree_put(dst);
 }