[PATCH nf] netfilter: nft_flow_offload: do not offload multicast

[Koral Ilgun <koralilgun@gmail.com>]

From: Koral Ilgun <koral.ilgun@adtran.com>

A flow holds one output per direction, which it resolves when it is
added. The outputs of a multicast route change when listeners join
or leave. So a multicast flow in the flowtable continues to send to
the output that it resolved, also after the last listener left, and
the counters of the multicast routing stop.

IPv4 multicast stays out of the flowtable only because the reverse
route lookup in nft_flow_route() refuses a multicast source address.
The IPv6 route lookup has no such check, so IPv6 multicast that
ip6mr forwards goes into the flowtable.

Skip a multicast destination for both families, so that the result
no longer depends on the route lookup.

Fixes: a3c90f7a2323 ("netfilter: nf_tables: flow offload expression")
Signed-off-by: Koral Ilgun <koral.ilgun@adtran.com>
---
Tested on an arm64 router with Linux 6.18.54, with an inet flowtable
that offloads forwarded flows, and IPv4 and IPv6 multicast routed from
the WAN to one VLAN. Without this change, the IPv6 flow went into the
flowtable after its first packets: the ip6mr vif counters showed 25 of
1,500 packets, and the receiver still got all 400 packets after its
leave. With it, the vif counters showed 1,500 of 1,500, the flow stayed
out of the flowtable, and the stream stopped about 2.5 s after the
leave. IPv4 multicast stayed out of the flowtable before and after.
Unicast IPv4 and IPv6 flows still went into the flowtable.

 net/netfilter/nft_flow_offload.c | 11 +++++++++++
 1 file changed, 11 insertions(+)

diff --git a/net/netfilter/nft_flow_offload.c b/net/netfilter/nft_flow_offload.c
index d3c5651dd699..1fc755e594b0 100644
--- a/net/netfilter/nft_flow_offload.c
+++ b/net/netfilter/nft_flow_offload.c
@@ -10,6 +10,7 @@
 #include <linux/netfilter/nf_conntrack_common.h>
 #include <linux/netfilter/nf_tables.h>
 #include <net/ip.h>
+#include <net/ipv6.h>
 #include <net/flow.h>
 #include <net/netfilter/nf_tables.h>
 #include <net/netfilter/nf_tables_core.h>
@@ -35,6 +36,16 @@ static bool nft_flow_offload_skip(struct sk_buff *skb, int family)
 			return true;
 	}
 
+	/* A flow keeps the output it resolved at offload time, but the
+	 * outputs of a multicast route change when listeners join or leave.
+	 */
+	switch (family) {
+	case NFPROTO_IPV4:
+		return ipv4_is_multicast(ip_hdr(skb)->daddr);
+	case NFPROTO_IPV6:
+		return ipv6_addr_is_multicast(&ipv6_hdr(skb)->daddr);
+	}
+
 	return false;
 }
 

base-commit: 734bef548a96e8774d6eef5dfa2026c6edc4e80b
-- 
2.43.0