Subject: [PATCH] tcp: Reset tcp connections in SYN-SENT state

From: Manoj Basapathi <[email-masked]>
Date: Mon, 5 Apr 2021 22:32:42 +0530

Userspace sends tcp connection (sock) destroy on network switch
i.e switching the default network of the device between multiple
networks(Cellular/Wifi/Ethernet).

Kernel though doesn't send reset for the connections in SYN-SENT state
and these connections continue to remain.
Even as per RFC 793, there is no hard rule to not send RST on ABORT in
this state.

Modify tcp_abort and tcp_disconnect behavior to send RST for connections
in syn-sent state to avoid lingering connections on network switch.

Signed-off-by: Manoj Basapathi <[email-masked]>
Signed-off-by: Sauvik Saha <[email-masked]>
---
 net/ipv4/tcp.c | 5 ++---
 1 file changed, 2 insertions(+), 3 deletions(-)

diff --git a/net/ipv4/tcp.c b/net/ipv4/tcp.c
index e14fd0c50c10..627a472161fb 100644
--- a/net/ipv4/tcp.c
+++ b/net/ipv4/tcp.c
@@ -2888,7 +2888,7 @@ static inline bool tcp_need_reset(int state)
 {
 	return (1 << state) &
 	       (TCPF_ESTABLISHED | TCPF_CLOSE_WAIT | TCPF_FIN_WAIT1 |
-		TCPF_FIN_WAIT2 | TCPF_SYN_RECV);
+		TCPF_FIN_WAIT2 | TCPF_SYN_RECV | TCPF_SYN_SENT);
 }
 
 static void tcp_rtx_queue_purge(struct sock *sk)
@@ -2954,8 +2954,7 @@ int tcp_disconnect(struct sock *sk, int flags)
 		 */
 		tcp_send_active_reset(sk, gfp_any());
 		sk->sk_err = ECONNRESET;
-	} else if (old_state == TCP_SYN_SENT)
-		sk->sk_err = ECONNRESET;
+	}
 
 	tcp_clear_xmit_timers(sk);
 	__skb_queue_purge(&sk->sk_receive_queue);
-- 
2.29.0