net/sched: cls_u32: fix refcount leak in the error path of u32_change()

when users replace cls_u32 filters with new ones having wrong parameters,
so that u32_change() fails to validate them, the kernel doesn't roll-back
correctly, and leaves semi-configured rules.

Fix this in u32_walk(), avoiding a call to the walker function on filters
that don't have a match rule connected. The side effect is, these "empty"
filters are not even dumped when present; but that shouldn't be a problem
as long as we are restoring the original behaviour, where semi-configured
filters were not even added in the error path of u32_change().

Fixes: 6676d5e4 ("net: sched: set dedicated tcf_walker flag when tp is empty")
Signed-off-by: Davide Caratti <dcaratti@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/sched/cls_u32.c

@@ -1108,10 +1108,33 @@ static int u32_change(struct net *net, struct sk_buff *in_skb,
 	return err;
 }
 
+static bool u32_hnode_empty(struct tc_u_hnode *ht, bool *non_root_ht)
+{
+	int i;
+
+	if (!ht)
+		return true;
+	if (!ht->is_root) {
+		*non_root_ht = true;
+		return false;
+	}
+	if (*non_root_ht)
+		return false;
+	if (ht->refcnt < 2)
+		return true;
+
+	for (i = 0; i <= ht->divisor; i++) {
+		if (rtnl_dereference(ht->ht[i]))
+			return false;
+	}
+	return true;
+}
+
 static void u32_walk(struct tcf_proto *tp, struct tcf_walker *arg,
 		     bool rtnl_held)
 {
 	struct tc_u_common *tp_c = tp->data;
+	bool non_root_ht = false;
 	struct tc_u_hnode *ht;
 	struct tc_u_knode *n;
 	unsigned int h;
@@ -1124,6 +1147,8 @@ static void u32_walk(struct tcf_proto *tp, struct tcf_walker *arg,
 	     ht = rtnl_dereference(ht->next)) {
 		if (ht->prio != tp->prio)
 			continue;
+		if (u32_hnode_empty(ht, &non_root_ht))
+			return;
 		if (arg->count >= arg->skip) {
 			if (arg->fn(tp, ht, arg) < 0) {
 				arg->stop = 1;