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Date:   Tue, 17 May 2022 11:18:30 +0200
From:   Pablo Neira Ayuso <>
Subject: [PATCH nf] netfilter: flowtable: fix TCP flow teardown

This patch addresses three possible problems:

1. ct gc may race to undo the timeout adjustment of the packet path, leaving
   the conntrack entry in place with the internal offload timeout (one day).

2. ct gc removes the ct because the IPS_OFFLOAD_BIT is not set and the CLOSE
   timeout is reached before the flow offload del.

3. tcp ct is always set to ESTABLISHED with a very long timeout
   in flow offload teardown/delete even though the state might be already
   CLOSED. Also as a remark we cannot assume that the FIN or RST packet
   is hitting flow table teardown as the packet might get bumped to the
   slow path in nftables.

This patch resets IPS_OFFLOAD_BIT from flow_offload_teardown(), so
conntrack handles the tcp rst/fin packet which triggers the CLOSE/FIN
state transition.

Moreover, teturn the connection's ownership to conntrack upon teardown
by clearing the offload flag and fixing the established timeout value.
The flow table GC thread will asynchonrnously free the flow table and
hardware offload entries.

Before this patch, the IPS_OFFLOAD_BIT remained set for expired flows on
which is also misleading since the flow is back to classic conntrack

If nf_ct_delete() removes the entry from the conntrack table, then it
calls nf_ct_put() which decrements the refcnt. This is not a problem
because the flowtable holds a reference to the conntrack object from
flow_offload_alloc() path which is released via flow_offload_free().

This patch also updates nft_flow_offload to skip packets in SYN_RECV
state. Since we might miss or bump packets to slow path, we do not know
what will happen there while we are still in SYN_RECV, this patch
postpones offload up to the next packet which also aligns to the
existing behaviour in tc-ct.

flow_offload_teardown() does not reset the existing tcp state from
flow_offload_fixup_tcp() to ESTABLISHED anymore, packets bump to slow
path might have already update the state to CLOSE/FIN.

Joint work with Oz and Sven.

Fixes: 1e5b2471bcc4 ("netfilter: nf_flow_table: teardown flow timeout race")
Signed-off-by: Pablo Neira Ayuso <>
Everyone happy with this? Please add your Signed-off-by tag.

I'm showing as the author in this patch, but this is basically a mix and
match of Oz's and Sven's patches.

Thanks a lot for your patience !

 net/netfilter/nf_flow_table_core.c | 33 +++++++-----------------------
 net/netfilter/nft_flow_offload.c   |  3 ++-
 2 files changed, 9 insertions(+), 27 deletions(-)

diff --git a/net/netfilter/nf_flow_table_core.c b/net/netfilter/nf_flow_table_core.c
index 20b4a14e5d4e..ebdf5332e838 100644
--- a/net/netfilter/nf_flow_table_core.c
+++ b/net/netfilter/nf_flow_table_core.c
@@ -179,12 +179,11 @@ EXPORT_SYMBOL_GPL(flow_offload_route_init);
 static void flow_offload_fixup_tcp(struct ip_ct_tcp *tcp)
 	tcp->seen[0].td_maxwin = 0;
 	tcp->seen[1].td_maxwin = 0;
-static void flow_offload_fixup_ct_timeout(struct nf_conn *ct)
+static void flow_offload_fixup_ct(struct nf_conn *ct)
 	struct net *net = nf_ct_net(ct);
 	int l4num = nf_ct_protonum(ct);
@@ -193,7 +192,9 @@ static void flow_offload_fixup_ct_timeout(struct nf_conn *ct)
 	if (l4num == IPPROTO_TCP) {
 		struct nf_tcp_net *tn = nf_tcp_pernet(net);
-		timeout = tn->timeouts[TCP_CONNTRACK_ESTABLISHED];
+		flow_offload_fixup_tcp(&ct->proto.tcp);
+		timeout = tn->timeouts[ct->proto.tcp.state];
 		timeout -= tn->offload_timeout;
 	} else if (l4num == IPPROTO_UDP) {
 		struct nf_udp_net *tn = nf_udp_pernet(net);
@@ -211,18 +212,6 @@ static void flow_offload_fixup_ct_timeout(struct nf_conn *ct)
 		WRITE_ONCE(ct->timeout, nfct_time_stamp + timeout);
-static void flow_offload_fixup_ct_state(struct nf_conn *ct)
-	if (nf_ct_protonum(ct) == IPPROTO_TCP)
-		flow_offload_fixup_tcp(&ct->proto.tcp);
-static void flow_offload_fixup_ct(struct nf_conn *ct)
-	flow_offload_fixup_ct_state(ct);
-	flow_offload_fixup_ct_timeout(ct);
 static void flow_offload_route_release(struct flow_offload *flow)
 	nft_flow_dst_release(flow, FLOW_OFFLOAD_DIR_ORIGINAL);
@@ -361,22 +350,14 @@ static void flow_offload_del(struct nf_flowtable *flow_table,
-	clear_bit(IPS_OFFLOAD_BIT, &flow->ct->status);
-	if (nf_flow_has_expired(flow))
-		flow_offload_fixup_ct(flow->ct);
-	else
-		flow_offload_fixup_ct_timeout(flow->ct);
 void flow_offload_teardown(struct flow_offload *flow)
+	clear_bit(IPS_OFFLOAD_BIT, &flow->ct->status);
 	set_bit(NF_FLOW_TEARDOWN, &flow->flags);
-	flow_offload_fixup_ct_state(flow->ct);
+	flow_offload_fixup_ct(flow->ct);
@@ -466,7 +447,7 @@ static void nf_flow_offload_gc_step(struct nf_flowtable *flow_table,
 	if (nf_flow_has_expired(flow) ||
 	    nf_ct_is_dying(flow->ct) ||
-		set_bit(NF_FLOW_TEARDOWN, &flow->flags);
+		flow_offload_teardown(flow);
 	if (test_bit(NF_FLOW_TEARDOWN, &flow->flags)) {
 		if (test_bit(NF_FLOW_HW, &flow->flags)) {
diff --git a/net/netfilter/nft_flow_offload.c b/net/netfilter/nft_flow_offload.c
index 187b8cb9a510..6612ad8f1565 100644
--- a/net/netfilter/nft_flow_offload.c
+++ b/net/netfilter/nft_flow_offload.c
@@ -298,7 +298,8 @@ static void nft_flow_offload_eval(const struct nft_expr *expr,
 		tcph = skb_header_pointer(pkt->skb, nft_thoff(pkt),
 					  sizeof(_tcph), &_tcph);
-		if (unlikely(!tcph || tcph->fin || tcph->rst))
+		if (unlikely(!tcph || tcph->fin || tcph->rst ||
+			     !nf_conntrack_tcp_established(&ct->proto.tcp)))
 			goto out;

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