tcp: add TCP_RFC7323_TW_PAWS drop reason

Devices in the networking path, such as firewalls, NATs, or routers, which
can perform SNAT or DNAT, use addresses from their own limited address
pools to masquerade the source address during forwarding, causing PAWS
verification to fail more easily.

Currently, packet loss statistics for PAWS can only be viewed through MIB,
which is a global metric and cannot be precisely obtained through tracing
to get the specific 4-tuple of the dropped packet. In the past, we had to
use kprobe ret to retrieve relevant skb information from
tcp_timewait_state_process().

We add a drop_reason pointer, similar to what previous commit does:
commit e34100c2ecbb ("tcp: add a drop_reason pointer to tcp_check_req()")

This commit addresses the PAWSESTABREJECTED case and also sets the
corresponding drop reason.

We use 'pwru' to test.

Before this commit:
''''
./pwru 'port 9999'
2025/04/07 13:40:19 Listening for events..
TUPLE                                        FUNC
172.31.75.115:12345->172.31.75.114:9999(tcp) sk_skb_reason_drop(SKB_DROP_REASON_NOT_SPECIFIED)
'''

After this commit:
'''
./pwru 'port 9999'
2025/04/07 13:51:34 Listening for events..
TUPLE                                        FUNC
172.31.75.115:12345->172.31.75.114:9999(tcp) sk_skb_reason_drop(SKB_DROP_REASON_TCP_RFC7323_TW_PAWS)
'''

Suggested-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: Jiayuan Chen <jiayuan.chen@linux.dev>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Link: https://patch.msgid.link/20250409112614.16153-2-jiayuan.chen@linux.dev
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

5 files changed, 17 insertions, 5 deletions

diff --git a/include/net/dropreason-core.h b/include/net/dropreason-core.h
index e4fdc6b54cef..9701d7f936f6 100644
--- a/include/net/dropreason-core.h
+++ b/include/net/dropreason-core.h
@@ -40,6 +40,7 @@
 	FN(TCP_OFOMERGE)		\
 	FN(TCP_RFC7323_PAWS)		\
 	FN(TCP_RFC7323_PAWS_ACK)	\
+	FN(TCP_RFC7323_TW_PAWS)		\
 	FN(TCP_RFC7323_TSECR)		\
 	FN(TCP_LISTEN_OVERFLOW)		\
 	FN(TCP_OLD_SEQUENCE)		\
@@ -284,6 +285,11 @@ enum skb_drop_reason {
 	 */
 	SKB_DROP_REASON_TCP_RFC7323_PAWS_ACK,
 	/**
+	 * @SKB_DROP_REASON_TCP_RFC7323_TW_PAWS: PAWS check, socket is in
+	 * TIME_WAIT state.
+	 */
+	SKB_DROP_REASON_TCP_RFC7323_TW_PAWS,
+	/**
 	 * @SKB_DROP_REASON_TCP_RFC7323_TSECR: PAWS check, invalid TSEcr.
 	 * Corresponds to LINUX_MIB_TSECRREJECTED.
 	 */
diff --git a/include/net/tcp.h b/include/net/tcp.h
index 4450c384ef17..5078ad868fee 100644
--- a/include/net/tcp.h
+++ b/include/net/tcp.h
@@ -427,7 +427,8 @@ enum tcp_tw_status {
 enum tcp_tw_status tcp_timewait_state_process(struct inet_timewait_sock *tw,
 					      struct sk_buff *skb,
 					      const struct tcphdr *th,
-					      u32 *tw_isn);
+					      u32 *tw_isn,
+					      enum skb_drop_reason *drop_reason);
 struct sock *tcp_check_req(struct sock *sk, struct sk_buff *skb,
 			   struct request_sock *req, bool fastopen,
 			   bool *lost_race, enum skb_drop_reason *drop_reason);
diff --git a/net/ipv4/tcp_ipv4.c b/net/ipv4/tcp_ipv4.c
index 8cce0d5489da..d5b5c32115d2 100644
--- a/net/ipv4/tcp_ipv4.c
+++ b/net/ipv4/tcp_ipv4.c
@@ -2417,7 +2417,8 @@ do_time_wait:
 		goto csum_error;
 	}
 
-	tw_status = tcp_timewait_state_process(inet_twsk(sk), skb, th, &isn);
+	tw_status = tcp_timewait_state_process(inet_twsk(sk), skb, th, &isn,
+					       &drop_reason);
 	switch (tw_status) {
 	case TCP_TW_SYN: {
 		struct sock *sk2 = inet_lookup_listener(net,
diff --git a/net/ipv4/tcp_minisocks.c b/net/ipv4/tcp_minisocks.c
index fb9349be36b8..27511bf58c0f 100644
--- a/net/ipv4/tcp_minisocks.c
+++ b/net/ipv4/tcp_minisocks.c
@@ -97,7 +97,8 @@ 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, u32 *tw_isn)
+			   const struct tcphdr *th, u32 *tw_isn,
+			   enum skb_drop_reason *drop_reason)
 {
 	struct tcp_timewait_sock *tcptw = tcp_twsk((struct sock *)tw);
 	u32 rcv_nxt = READ_ONCE(tcptw->tw_rcv_nxt);
@@ -245,8 +246,10 @@ kill:
 		return TCP_TW_SYN;
 	}
 
-	if (paws_reject)
+	if (paws_reject) {
+		*drop_reason = SKB_DROP_REASON_TCP_RFC7323_TW_PAWS;
 		__NET_INC_STATS(twsk_net(tw), LINUX_MIB_PAWSESTABREJECTED);
+	}
 
 	if (!th->rst) {
 		/* In this case we must reset the TIMEWAIT timer.
diff --git a/net/ipv6/tcp_ipv6.c b/net/ipv6/tcp_ipv6.c
index b03c223eda4f..7dcb33f879ee 100644
--- a/net/ipv6/tcp_ipv6.c
+++ b/net/ipv6/tcp_ipv6.c
@@ -1970,7 +1970,8 @@ do_time_wait:
 		goto csum_error;
 	}
 
-	tw_status = tcp_timewait_state_process(inet_twsk(sk), skb, th, &isn);
+	tw_status = tcp_timewait_state_process(inet_twsk(sk), skb, th, &isn,
+					       &drop_reason);
 	switch (tw_status) {
 	case TCP_TW_SYN:
 	{