tcp: rstreason: fully support in tcp_check_req()

We're going to send an RST due to invalid syn packet which is already
checked whether 1) it is in sequence, 2) it is a retransmitted skb.

As RFC 793 says, if the state of socket is not CLOSED/LISTEN/SYN-SENT,
then we should send an RST when receiving bad syn packet:
"fourth, check the SYN bit,...If the SYN is in the window it is an
error, send a reset"
Signed-off-by: Jason Xing <kernelxing@tencent.com>
Link: https://lore.kernel.org/r/20240510122502.27850-6-kerneljasonxing@gmail.comSigned-off-by: Jakub Kicinski <kuba@kernel.org>

include/net/rstreason.h

@@ -16,6 +16,7 @@
 	FN(TCP_OLD_ACK)			\
 	FN(TCP_ABORT_ON_DATA)		\
 	FN(TCP_TIMEWAIT_SOCKET)		\
+	FN(INVALID_SYN)			\
 	FN(MPTCP_RST_EUNSPEC)		\
 	FN(MPTCP_RST_EMPTCP)		\
 	FN(MPTCP_RST_ERESOURCE)		\
@@ -76,6 +77,13 @@ enum sk_rst_reason {
 	/* Here start with the independent reasons */
 	/** @SK_RST_REASON_TCP_TIMEWAIT_SOCKET: happen on the timewait socket */
 	SK_RST_REASON_TCP_TIMEWAIT_SOCKET,
+	/**
+	 * @SK_RST_REASON_INVALID_SYN: receive bad syn packet
+	 * RFC 793 says if the state is not CLOSED/LISTEN/SYN-SENT then
+	 * "fourth, check the SYN bit,...If the SYN is in the window it is
+	 * an error, send a reset"
+	 */
+	SK_RST_REASON_INVALID_SYN,
 
 	/* Copy from include/uapi/linux/mptcp.h.
 	 * These reset fields will not be changed since they adhere to

net/ipv4/tcp_minisocks.c

@@ -879,7 +879,7 @@ struct sock *tcp_check_req(struct sock *sk, struct sk_buff *skb,
 		 * avoid becoming vulnerable to outside attack aiming at
 		 * resetting legit local connections.
 		 */
-		req->rsk_ops->send_reset(sk, skb, SK_RST_REASON_NOT_SPECIFIED);
+		req->rsk_ops->send_reset(sk, skb, SK_RST_REASON_INVALID_SYN);
 	} else if (fastopen) { /* received a valid RST pkt */
 		reqsk_fastopen_remove(sk, req, true);
 		tcp_reset(sk, skb);