net_sched: cls_route: remove from list when handle is 0

When a route filter is replaced and the old filter has a 0 handle, the old
one won't be removed from the hashtable, while it will still be freed.

The test was there since before commit 1109c00547 ("net: sched: RCU
cls_route"), when a new filter was not allocated when there was an old one.
The old filter was reused and the reinserting would only be necessary if an
old filter was replaced. That was still wrong for the same case where the
old handle was 0.

Remove the old filter from the list independently from its handle value.

This fixes CVE-2022-2588, also reported as ZDI-CAN-17440.

Reported-by: Zhenpeng Lin <zplin@u.northwestern.edu>
Signed-off-by: Thadeu Lima de Souza Cascardo <cascardo@canonical.com>
Reviewed-by: Kamal Mostafa <kamal@canonical.com>
Cc: <stable@vger.kernel.org>
Acked-by: Jamal Hadi Salim <jhs@mojatatu.com>
Link: https://lore.kernel.org/r/20220809170518.164662-1-cascardo@canonical.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

net/sched/cls_route.c

@@ -526,7 +526,7 @@ static int route4_change(struct net *net, struct sk_buff *in_skb,
 	rcu_assign_pointer(f->next, f1);
 	rcu_assign_pointer(*fp, f);
 
-	if (fold && fold->handle && f->handle != fold->handle) {
+	if (fold) {
 		th = to_hash(fold->handle);
 		h = from_hash(fold->handle >> 16);
 		b = rtnl_dereference(head->table[th]);