tcp: replace TCP_SKB_CB(skb)->tcp_tw_isn with a per-cpu field

TCP can transform a TIMEWAIT socket into a SYN_RECV one from
a SYN packet, and the ISN of the SYNACK packet is normally
generated using TIMEWAIT tw_snd_nxt :

tcp_timewait_state_process()
...
    u32 isn = tcptw->tw_snd_nxt + 65535 + 2;
    if (isn == 0)
        isn++;
    TCP_SKB_CB(skb)->tcp_tw_isn = isn;
    return TCP_TW_SYN;

This SYN packet also bypasses normal checks against listen queue
being full or not.

tcp_conn_request()
...
       __u32 isn = TCP_SKB_CB(skb)->tcp_tw_isn;
...
        /* TW buckets are converted to open requests without
         * limitations, they conserve resources and peer is
         * evidently real one.
         */
        if ((syncookies == 2 || inet_csk_reqsk_queue_is_full(sk)) && !isn) {
                want_cookie = tcp_syn_flood_action(sk, rsk_ops->slab_name);
                if (!want_cookie)
                        goto drop;
        }

This was using TCP_SKB_CB(skb)->tcp_tw_isn field in skb.

Unfortunately this field has been accidentally cleared
after the call to tcp_timewait_state_process() returning
TCP_TW_SYN.

Using a field in TCP_SKB_CB(skb) for a temporary state
is overkill.

Switch instead to a per-cpu variable.

As a bonus, we do not have to clear tcp_tw_isn in TCP receive
fast path.
It is temporarily set then cleared only in the TCP_TW_SYN dance.

Fixes: 4ad19de8774e ("net: tcp6: fix double call of tcp_v6_fill_cb()")
Fixes: eeea10b83a13 ("tcp: add tcp_v4_fill_cb()/tcp_v4_restore_cb()")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>

4 files changed, 24 insertions, 14 deletions

diff --git a/net/ipv4/tcp.c b/net/ipv4/tcp.c
index 664c8ecb076b..b07aa71b24ec 100644
--- a/net/ipv4/tcp.c
+++ b/net/ipv4/tcp.c
@@ -290,6 +290,9 @@ enum {
 DEFINE_PER_CPU(unsigned int, tcp_orphan_count);
 EXPORT_PER_CPU_SYMBOL_GPL(tcp_orphan_count);
 
+DEFINE_PER_CPU(u32, tcp_tw_isn);
+EXPORT_PER_CPU_SYMBOL_GPL(tcp_tw_isn);
+
 long sysctl_tcp_mem[3] __read_mostly;
 EXPORT_SYMBOL(sysctl_tcp_mem);
 
diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
index 48c275e6ef02..5a45a0923a1f 100644
--- a/net/ipv4/tcp_input.c
+++ b/net/ipv4/tcp_input.c
@@ -7097,7 +7097,6 @@ int tcp_conn_request(struct request_sock_ops *rsk_ops,
 		     struct sock *sk, struct sk_buff *skb)
 {
 	struct tcp_fastopen_cookie foc = { .len = -1 };
-	__u32 isn = TCP_SKB_CB(skb)->tcp_tw_isn;
 	struct tcp_options_received tmp_opt;
 	struct tcp_sock *tp = tcp_sk(sk);
 	struct net *net = sock_net(sk);
@@ -7107,21 +7106,28 @@ int tcp_conn_request(struct request_sock_ops *rsk_ops,
 	struct dst_entry *dst;
 	struct flowi fl;
 	u8 syncookies;
+	u32 isn;
 
 #ifdef CONFIG_TCP_AO
 	const struct tcp_ao_hdr *aoh;
 #endif
 
-	syncookies = READ_ONCE(net->ipv4.sysctl_tcp_syncookies);
+	isn = __this_cpu_read(tcp_tw_isn);
+	if (isn) {
+		/* TW buckets are converted to open requests without
+		 * limitations, they conserve resources and peer is
+		 * evidently real one.
+		 */
+		__this_cpu_write(tcp_tw_isn, 0);
+	} else {
+		syncookies = READ_ONCE(net->ipv4.sysctl_tcp_syncookies);
 
-	/* TW buckets are converted to open requests without
-	 * limitations, they conserve resources and peer is
-	 * evidently real one.
-	 */
-	if ((syncookies == 2 || inet_csk_reqsk_queue_is_full(sk)) && !isn) {
-		want_cookie = tcp_syn_flood_action(sk, rsk_ops->slab_name);
-		if (!want_cookie)
-			goto drop;
+		if (syncookies == 2 || inet_csk_reqsk_queue_is_full(sk)) {
+			want_cookie = tcp_syn_flood_action(sk,
+							   rsk_ops->slab_name);
+			if (!want_cookie)
+				goto drop;
+		}
 	}
 
 	if (sk_acceptq_is_full(sk)) {
diff --git a/net/ipv4/tcp_ipv4.c b/net/ipv4/tcp_ipv4.c
index 81e2f05c244d..1e650ec71d2f 100644
--- a/net/ipv4/tcp_ipv4.c
+++ b/net/ipv4/tcp_ipv4.c
@@ -2146,7 +2146,6 @@ static void tcp_v4_fill_cb(struct sk_buff *skb, const struct iphdr *iph,
 				    skb->len - th->doff * 4);
 	TCP_SKB_CB(skb)->ack_seq = ntohl(th->ack_seq);
 	TCP_SKB_CB(skb)->tcp_flags = tcp_flag_byte(th);
-	TCP_SKB_CB(skb)->tcp_tw_isn = 0;
 	TCP_SKB_CB(skb)->ip_dsfield = ipv4_get_dsfield(iph);
 	TCP_SKB_CB(skb)->sacked	 = 0;
 	TCP_SKB_CB(skb)->has_rxtstamp =
@@ -2168,6 +2167,7 @@ int tcp_v4_rcv(struct sk_buff *skb)
 	bool refcounted;
 	struct sock *sk;
 	int ret;
+	u32 isn;
 
 	drop_reason = SKB_DROP_REASON_NOT_SPECIFIED;
 	if (skb->pkt_type != PACKET_HOST)
@@ -2383,7 +2383,7 @@ do_time_wait:
 		inet_twsk_put(inet_twsk(sk));
 		goto csum_error;
 	}
-	switch (tcp_timewait_state_process(inet_twsk(sk), skb, th)) {
+	switch (tcp_timewait_state_process(inet_twsk(sk), skb, th, &isn)) {
 	case TCP_TW_SYN: {
 		struct sock *sk2 = inet_lookup_listener(net,
 							net->ipv4.tcp_death_row.hashinfo,
@@ -2397,6 +2397,7 @@ do_time_wait:
 			sk = sk2;
 			tcp_v4_restore_cb(skb);
 			refcounted = false;
+			__this_cpu_write(tcp_tw_isn, isn);
 			goto process;
 		}
 	}
diff --git a/net/ipv4/tcp_minisocks.c b/net/ipv4/tcp_minisocks.c
index 5b21a07ddf9a..f53c7ada2ace 100644
--- a/net/ipv4/tcp_minisocks.c
+++ b/net/ipv4/tcp_minisocks.c
@@ -95,7 +95,7 @@ static void twsk_rcv_nxt_update(struct tcp_timewait_sock *tcptw, u32 seq)
  */
 enum tcp_tw_status
 tcp_timewait_state_process(struct inet_timewait_sock *tw, struct sk_buff *skb,
-			   const struct tcphdr *th)
+			   const struct tcphdr *th, u32 *tw_isn)
 {
 	struct tcp_options_received tmp_opt;
 	struct tcp_timewait_sock *tcptw = tcp_twsk((struct sock *)tw);
@@ -228,7 +228,7 @@ kill:
 		u32 isn = tcptw->tw_snd_nxt + 65535 + 2;
 		if (isn == 0)
 			isn++;
-		TCP_SKB_CB(skb)->tcp_tw_isn = isn;
+		*tw_isn = isn;
 		return TCP_TW_SYN;
 	}