net: psp: update the TCP MSS to reflect PSP packet overhead

PSP eats 40B of header space. Adjust MSS appropriately.

We can either modify tcp_mtu_to_mss() / tcp_mss_to_mtu()
or reuse icsk_ext_hdr_len. The former option is more TCP
specific and has runtime overhead. The latter is a bit
of a hack as PSP is not an ext_hdr. If one squints hard
enough, UDP encap is just a more practical version of
IPv6 exthdr, so go with the latter. Happy to change.

Reviewed-by: Willem de Bruijn <willemb@google.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Daniel Zahka <daniel.zahka@gmail.com>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Link: https://patch.msgid.link/20250917000954.859376-10-daniel.zahka@gmail.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>

6 files changed, 32 insertions, 6 deletions

diff --git a/include/net/psp/functions.h b/include/net/psp/functions.h
index 0d7141230f47..183a3c9216b7 100644
--- a/include/net/psp/functions.h
+++ b/include/net/psp/functions.h
@@ -5,6 +5,7 @@
 
 #include <linux/skbuff.h>
 #include <linux/rcupdate.h>
+#include <linux/udp.h>
 #include <net/sock.h>
 #include <net/tcp.h>
 #include <net/psp/types.h>
@@ -143,6 +144,14 @@ static inline struct psp_assoc *psp_skb_get_assoc_rcu(struct sk_buff *skb)
 
 	return psp_sk_get_assoc_rcu(skb->sk);
 }
+
+static inline unsigned int psp_sk_overhead(const struct sock *sk)
+{
+	int psp_encap = sizeof(struct udphdr) + PSP_HDR_SIZE + PSP_TRL_SIZE;
+	bool has_psp = rcu_access_pointer(sk->psp_assoc);
+
+	return has_psp ? psp_encap : 0;
+}
 #else
 static inline void psp_sk_assoc_free(struct sock *sk) { }
 static inline void
@@ -182,6 +191,11 @@ static inline struct psp_assoc *psp_skb_get_assoc_rcu(struct sk_buff *skb)
 {
 	return NULL;
 }
+
+static inline unsigned int psp_sk_overhead(const struct sock *sk)
+{
+	return 0;
+}
 #endif
 
 static inline unsigned long
diff --git a/include/net/psp/types.h b/include/net/psp/types.h
index b0e32e7165a3..f93ad0e6c04f 100644
--- a/include/net/psp/types.h
+++ b/include/net/psp/types.h
@@ -93,6 +93,9 @@ struct psp_dev_caps {
 
 #define PSP_MAX_KEY	32
 
+#define PSP_HDR_SIZE	16	/* We don't support optional fields, yet */
+#define PSP_TRL_SIZE	16	/* AES-GCM/GMAC trailer size */
+
 struct psp_skb_ext {
 	__be32 spi;
 	u16 dev_id;
diff --git a/net/ipv4/tcp_ipv4.c b/net/ipv4/tcp_ipv4.c
index f27f6f865a48..b1fcf3e4e1ce 100644
--- a/net/ipv4/tcp_ipv4.c
+++ b/net/ipv4/tcp_ipv4.c
@@ -294,9 +294,9 @@ int tcp_v4_connect(struct sock *sk, struct sockaddr *uaddr, int addr_len)
 	inet->inet_dport = usin->sin_port;
 	sk_daddr_set(sk, daddr);
 
-	inet_csk(sk)->icsk_ext_hdr_len = 0;
+	inet_csk(sk)->icsk_ext_hdr_len = psp_sk_overhead(sk);
 	if (inet_opt)
-		inet_csk(sk)->icsk_ext_hdr_len = inet_opt->opt.optlen;
+		inet_csk(sk)->icsk_ext_hdr_len += inet_opt->opt.optlen;
 
 	tp->rx_opt.mss_clamp = TCP_MSS_DEFAULT;
 
diff --git a/net/ipv6/ipv6_sockglue.c b/net/ipv6/ipv6_sockglue.c
index e66ec623972e..a61e742794f9 100644
--- a/net/ipv6/ipv6_sockglue.c
+++ b/net/ipv6/ipv6_sockglue.c
@@ -49,6 +49,7 @@
 #include <net/xfrm.h>
 #include <net/compat.h>
 #include <net/seg6.h>
+#include <net/psp.h>
 
 #include <linux/uaccess.h>
 
@@ -107,7 +108,10 @@ struct ipv6_txoptions *ipv6_update_options(struct sock *sk,
 		    !((1 << sk->sk_state) & (TCPF_LISTEN | TCPF_CLOSE)) &&
 		    inet_sk(sk)->inet_daddr != LOOPBACK4_IPV6) {
 			struct inet_connection_sock *icsk = inet_csk(sk);
-			icsk->icsk_ext_hdr_len = opt->opt_flen + opt->opt_nflen;
+
+			icsk->icsk_ext_hdr_len =
+				psp_sk_overhead(sk) +
+				opt->opt_flen + opt->opt_nflen;
 			icsk->icsk_sync_mss(sk, icsk->icsk_pmtu_cookie);
 		}
 	}
diff --git a/net/ipv6/tcp_ipv6.c b/net/ipv6/tcp_ipv6.c
index 4da8eb9183d7..43d1109e2180 100644
--- a/net/ipv6/tcp_ipv6.c
+++ b/net/ipv6/tcp_ipv6.c
@@ -302,10 +302,10 @@ static int tcp_v6_connect(struct sock *sk, struct sockaddr *uaddr,
 	sk->sk_gso_type = SKB_GSO_TCPV6;
 	ip6_dst_store(sk, dst, NULL, NULL);
 
-	icsk->icsk_ext_hdr_len = 0;
+	icsk->icsk_ext_hdr_len = psp_sk_overhead(sk);
 	if (opt)
-		icsk->icsk_ext_hdr_len = opt->opt_flen +
-					 opt->opt_nflen;
+		icsk->icsk_ext_hdr_len += opt->opt_flen +
+					  opt->opt_nflen;
 
 	tp->rx_opt.mss_clamp = IPV6_MIN_MTU - sizeof(struct tcphdr) - sizeof(struct ipv6hdr);
 
diff --git a/net/psp/psp_sock.c b/net/psp/psp_sock.c
index 8ebccee94593..10e1fda30aa0 100644
--- a/net/psp/psp_sock.c
+++ b/net/psp/psp_sock.c
@@ -180,6 +180,7 @@ int psp_sock_assoc_set_tx(struct sock *sk, struct psp_dev *psd,
 			  u32 version, struct psp_key_parsed *key,
 			  struct netlink_ext_ack *extack)
 {
+	struct inet_connection_sock *icsk;
 	struct psp_assoc *pas, *dummy;
 	int err;
 
@@ -236,6 +237,10 @@ int psp_sock_assoc_set_tx(struct sock *sk, struct psp_dev *psd,
 	tcp_write_collapse_fence(sk);
 	pas->upgrade_seq = tcp_sk(sk)->rcv_nxt;
 
+	icsk = inet_csk(sk);
+	icsk->icsk_ext_hdr_len += psp_sk_overhead(sk);
+	icsk->icsk_sync_mss(sk, icsk->icsk_pmtu_cookie);
+
 exit_free_dummy:
 	kfree(dummy);
 exit_unlock: