[PATCH net 08/14] netfilter: nf_socket: skip socket lookup for non-first fragments

Pablo Neira Ayuso
2026-05-01T12:22:54Z

From: Fernando Fernandez Mancera <fmancera@suse.de>

Both nft_socket and xt_socket relies on L4 headers to perform socket
lookup in the slow path. For fragmented packets, while the IP protocol
remains constant across all fragments, only the first fragment contains
the actual L4 header.

As the expression/match could be attached to a chain with a priority
lower than -400, it could bypass defragmentation.

Add a check for fragmentation in the lookup functions directly so the
problem is handled for both nft_socket and xt_socket at the same time.
In addition, future users of the functions would not need to care about
this.

Fixes: 902d6a4c2a4f ("netfilter: nf_defrag: Skip defrag if NOTRACK is set")
Fixes: 554ced0a6e29 ("netfilter: nf_tables: add support for native socket matching")
Signed-off-by: Fernando Fernandez Mancera <fmancera@suse.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
---
net/ipv4/netfilter/nf_socket_ipv4.c | 3 +++
net/ipv6/netfilter/nf_socket_ipv6.c | 5 +++--
2 files changed, 6 insertions(+), 2 deletions(-)

diff --git a/net/ipv4/netfilter/nf_socket_ipv4.c b/net/ipv4/netfilter/nf_socket_ipv4.c
index 5080fa5fbf6a..f9c6755f5ec5 100644
--- a/net/ipv4/netfilter/nf_socket_ipv4.c
+++ b/net/ipv4/netfilter/nf_socket_ipv4.c
@@ -94,6 +94,9 @@ struct sock *nf_sk_lookup_slow_v4(struct net *net, const struct sk_buff *skb,
#endif
int doff = 0;

+ if (ntohs(iph->frag_off) & IP_OFFSET)
+ return NULL;
+
if (iph->protocol == IPPROTO_UDP || iph->protocol == IPPROTO_TCP) {
struct tcphdr _hdr;
struct udphdr *hp;
diff --git a/net/ipv6/netfilter/nf_socket_ipv6.c b/net/ipv6/netfilter/nf_socket_ipv6.c
index ced8bd44828e..893f2aeb4711 100644
--- a/net/ipv6/netfilter/nf_socket_ipv6.c
+++ b/net/ipv6/netfilter/nf_socket_ipv6.c
@@ -100,6 +100,7 @@ struct sock *nf_sk_lookup_slow_v6(struct net *net, const struct sk_buff *skb,
const struct in6_addr *daddr = NULL, *saddr = NULL;
struct ipv6hdr *iph = ipv6_hdr(skb), ipv6_var;
struct sk_buff *data_skb = NULL;
+ unsigned short fragoff = 0;
int doff = 0;
int thoff = 0, tproto;
#if IS_ENABLED(CONFIG_NF_CONNTRACK)
@@ -107,8 +108,8 @@ struct sock *nf_sk_lookup_slow_v6(struct net *net, const struct sk_buff *skb,
struct nf_conn const *ct;
#endif

- tproto = ipv6_find_hdr(skb, &thoff, -1, NULL, NULL);
- if (tproto < 0) {
+ tproto = ipv6_find_hdr(skb, &thoff, -1, &fragoff, NULL);
+ if (tproto < 0 || fragoff) {
pr_debug("unable to find transport header in IPv6 packet, dropping\n");
return NULL;
}
--
2.47.3