net: tcp: allow tcp_timewait_sock to validate skbs before handing to device

Provide a callback to validate skb's originating from tcp timewait
socks before passing to the device layer. Full socks have a
sk_validate_xmit_skb member for checking that a device is capable of
performing offloads required for transmitting an skb. With psp, tcp
timewait socks will inherit the crypto state from their corresponding
full socks. Any ACKs or RSTs that originate from a tcp timewait sock
carrying psp state should be psp encapsulated.

Reviewed-by: Willem de Bruijn <[e-mail]>
Signed-off-by: Daniel Zahka <[e-mail]>
Reviewed-by: Eric Dumazet <[e-mail]>
Link: https://patch.msgid.link/[e-mail]
Signed-off-by: Paolo Abeni <[e-mail]>

include/net/inet_timewait_sock.h

···
 #if IS_ENABLED(CONFIG_INET_PSP)
 	struct psp_assoc __rcu	  *psp_assoc;
 #endif
+#ifdef CONFIG_SOCK_VALIDATE_XMIT
+	struct sk_buff*		(*tw_validate_xmit_skb)(struct sock *sk,
+							struct net_device *dev,
+							struct sk_buff *skb);
+#endif
 };
 #define tw_tclass tw_tos
 

net/core/dev.c

···
 					    struct net_device *dev)
 {
 #ifdef CONFIG_SOCK_VALIDATE_XMIT
+	struct sk_buff *(*sk_validate)(struct sock *sk, struct net_device *dev,
+				       struct sk_buff *skb);
 	struct sock *sk = skb->sk;
 
-	if (sk && sk_fullsock(sk) && sk->sk_validate_xmit_skb) {
-		skb = sk->sk_validate_xmit_skb(sk, dev, skb);
+	sk_validate = NULL;
+	if (sk) {
+		if (sk_fullsock(sk))
+			sk_validate = sk->sk_validate_xmit_skb;
+		else if (sk_is_inet(sk) && sk->sk_state == TCP_TIME_WAIT)
+			sk_validate = inet_twsk(sk)->tw_validate_xmit_skb;
+	}
+
+	if (sk_validate) {
+		skb = sk_validate(sk, dev, skb);
 	} else if (unlikely(skb_is_decrypted(skb))) {
 		pr_warn_ratelimited("unencrypted skb with no associated socket - dropping\n");
 		kfree_skb(skb);

net/ipv4/inet_timewait_sock.c

···
 		atomic64_set(&tw->tw_cookie, atomic64_read(&sk->sk_cookie));
 		twsk_net_set(tw, sock_net(sk));
 		timer_setup(&tw->tw_timer, tw_timer_handler, 0);
+#ifdef CONFIG_SOCK_VALIDATE_XMIT
+		tw->tw_validate_xmit_skb = NULL;
+#endif
 		/*
 		 * Because we use RCU lookups, we should not set tw_refcnt
 		 * to a non null value before everything is setup for this