cls_flow: use tcf_exts_get_net() before call_rcu()

Hold netns refcnt before call_rcu() and release it after
the tcf_exts_destroy() is done.

Note, on ->destroy() path we have to respect the return value
of tcf_exts_get_net(), on other paths it should always return
true, so we don't need to care.

Cc: Lucas Bates <lucasb@mojatatu.com>
Cc: Jamal Hadi Salim <jhs@mojatatu.com>
Cc: Jiri Pirko <jiri@resnulli.us>
Signed-off-by: Cong Wang <xiyou.wangcong@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/sched/cls_flow.c

@@ -372,15 +372,21 @@ static const struct nla_policy flow_policy[TCA_FLOW_MAX + 1] = {
 	[TCA_FLOW_PERTURB]	= { .type = NLA_U32 },
 };
 
-static void flow_destroy_filter_work(struct work_struct *work)
+static void __flow_destroy_filter(struct flow_filter *f)
 {
-	struct flow_filter *f = container_of(work, struct flow_filter, work);
-
-	rtnl_lock();
 	del_timer_sync(&f->perturb_timer);
 	tcf_exts_destroy(&f->exts);
 	tcf_em_tree_destroy(&f->ematches);
+	tcf_exts_put_net(&f->exts);
 	kfree(f);
+}
+
+static void flow_destroy_filter_work(struct work_struct *work)
+{
+	struct flow_filter *f = container_of(work, struct flow_filter, work);
+
+	rtnl_lock();
+	__flow_destroy_filter(f);
 	rtnl_unlock();
 }
 
@@ -552,8 +558,10 @@ static int flow_change(struct net *net, struct sk_buff *in_skb,
 
 	*arg = fnew;
 
-	if (fold)
+	if (fold) {
+		tcf_exts_get_net(&fold->exts);
 		call_rcu(&fold->rcu, flow_destroy_filter);
+	}
 	return 0;
 
 err2:
@@ -570,6 +578,7 @@ static int flow_delete(struct tcf_proto *tp, void *arg, bool *last)
 	struct flow_filter *f = arg;
 
 	list_del_rcu(&f->list);
+	tcf_exts_get_net(&f->exts);
 	call_rcu(&f->rcu, flow_destroy_filter);
 	*last = list_empty(&head->filters);
 	return 0;
@@ -594,7 +603,10 @@ static void flow_destroy(struct tcf_proto *tp)
 
 	list_for_each_entry_safe(f, next, &head->filters, list) {
 		list_del_rcu(&f->list);
+		if (tcf_exts_get_net(&f->exts))
 			call_rcu(&f->rcu, flow_destroy_filter);
+		else
+			__flow_destroy_filter(f);
 	}
 	kfree_rcu(head, rcu);
 }