netfilter: flowtable: GC pushes back packets to classic path

Since 41f2c7c ("net/sched: act_ct: Fix promotion of offloaded
unreplied tuple"), flowtable GC pushes back flows with IPS_SEEN_REPLY
back to classic path in every run, ie. every second. This is because of
a new check for NF_FLOW_HW_ESTABLISHED which is specific of sched/act_ct.

In Netfilter's flowtable case, NF_FLOW_HW_ESTABLISHED never gets set on
and IPS_SEEN_REPLY is unreliable since users decide when to offload the
flow before, such bit might be set on at a later stage.

Fix it by adding a custom .gc handler that sched/act_ct can use to
deal with its NF_FLOW_HW_ESTABLISHED bit.

Fixes: 41f2c7c ("net/sched: act_ct: Fix promotion of offloaded unreplied tuple")
Reported-by: Vladimir Smelhaus <vl.sm@email.cz>
Reviewed-by: Paul Blakey <paulb@nvidia.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

include/net/netfilter/nf_flow_table.h

@@ -53,6 +53,7 @@ struct nf_flowtable_type {
 	struct list_head		list;
 	int				family;
 	int				(*init)(struct nf_flowtable *ft);
+	bool				(*gc)(const struct flow_offload *flow);
 	int				(*setup)(struct nf_flowtable *ft,
 						 struct net_device *dev,
 						 enum flow_block_command cmd);

net/netfilter/nf_flow_table_core.c

@@ -316,12 +316,6 @@ void flow_offload_refresh(struct nf_flowtable *flow_table,
 }
 EXPORT_SYMBOL_GPL(flow_offload_refresh);
 
-static bool nf_flow_is_outdated(const struct flow_offload *flow)
-{
-	return test_bit(IPS_SEEN_REPLY_BIT, &flow->ct->status) &&
-		!test_bit(NF_FLOW_HW_ESTABLISHED, &flow->flags);
-}
-
 static inline bool nf_flow_has_expired(const struct flow_offload *flow)
 {
 	return nf_flow_timeout_delta(flow->timeout) <= 0;
@@ -407,12 +401,18 @@ nf_flow_table_iterate(struct nf_flowtable *flow_table,
 	return err;
 }
 
+static bool nf_flow_custom_gc(struct nf_flowtable *flow_table,
+			      const struct flow_offload *flow)
+{
+	return flow_table->type->gc && flow_table->type->gc(flow);
+}
+
 static void nf_flow_offload_gc_step(struct nf_flowtable *flow_table,
 				    struct flow_offload *flow, void *data)
 {
 	if (nf_flow_has_expired(flow) ||
 	    nf_ct_is_dying(flow->ct) ||
-	    nf_flow_is_outdated(flow))
+	    nf_flow_custom_gc(flow_table, flow))
 		flow_offload_teardown(flow);
 
 	if (test_bit(NF_FLOW_TEARDOWN, &flow->flags)) {

net/sched/act_ct.c

@@ -278,7 +278,14 @@ static int tcf_ct_flow_table_fill_actions(struct net *net,
 	return err;
 }
 
+static bool tcf_ct_flow_is_outdated(const struct flow_offload *flow)
+{
+	return test_bit(IPS_SEEN_REPLY_BIT, &flow->ct->status) &&
+	       !test_bit(NF_FLOW_HW_ESTABLISHED, &flow->flags);
+}
+
 static struct nf_flowtable_type flowtable_ct = {
+	.gc		= tcf_ct_flow_is_outdated,
 	.action		= tcf_ct_flow_table_fill_actions,
 	.owner		= THIS_MODULE,
 };