[PATCH net v3] tcp: reset late connection after listening socket close

["Asbjørn Sloth Tønnesen" <ast@fiberby.net>]

When __inet_inherit_port() returns -ENOENT, the new connection is
dropped silently.

In that case the client sees the connection as ESTABLISHED, however in
tcp_v{4,6}_syn_recv_sock() the call to __inet_inherit_port() returns
-ENOENT, and the new connection is dropped by put_and_exit.

A client may therefore hang indefinitely on a blocking read() if the
used data communication protocol is initiated by the server, like SMTP
and the reporter[1]'s MariaDB protocol both are.

Had the new connection been processed before the listening socket was
closed, it would either have been added to the accept queue, or
inet_csk_reqsk_queue_add() should have sent RST.

The call to __inet_inherit_port() returns -ENOENT because
inet_csk(sk)->icsk_bind_hash is NULL, after inet_put_port() has been
called by tcp_set_state(sk, TCP_CLOSE).

This patch adds -ENOENT handling to both __inet_inherit_port() call
sites, and ensures that RST is sent before the connection is dropped.

Reproducer:
  https://files.fiberby.net/ast/2026/kernel/socket_teardown_test.c

Reported-by: Kristian Nielsen <knielsen@knielsen-hq.org>
Link: https://lore.kernel.org/87sf0ldk41.fsf@urd.knielsen-hq.org # [1]
Fixes: c2f34a65a61c ("tcp/dccp: fix potential NULL deref in __inet_inherit_port()")
Cc: <stable@vger.kernel.org>
Signed-off-by: Asbjørn Sloth Tønnesen <ast@fiberby.net>
---

Changelog:
v3:
  - Rewrite commit message around fixing commit c2f34a65a61c.
  - Call tcp_v{4,6}_send_reset() directly again (but with sk, not newsk).
  - Nest the two return value checks, and wrap in unlikely().
  (Thanks again Kuniyuki)
v2: https://lore.kernel.org/20260810205642.1611338-1-ast@fiberby.net
  - Use return from __inet_inherit_port() to trigger send_reply()
  - Use req->rsk_ops->send_reset.
  - Clarity commit message, and update to reflect the changes.
  (Thanks Kuniyuki)
v1: https://lore.kernel.org/20260807194513.1263310-1-ast@fiberby.net

 net/ipv4/tcp_ipv4.c | 9 ++++++++-
 net/ipv6/tcp_ipv6.c | 9 ++++++++-
 2 files changed, 16 insertions(+), 2 deletions(-)

diff --git a/net/ipv4/tcp_ipv4.c b/net/ipv4/tcp_ipv4.c
index b8887cdd66c5..9a14c2e56ec3 100644
--- a/net/ipv4/tcp_ipv4.c
+++ b/net/ipv4/tcp_ipv4.c
@@ -1690,6 +1690,7 @@ struct sock *tcp_v4_syn_recv_sock(const struct sock *sk, struct sk_buff *skb,
 	int l3index;
 #endif
 	struct ip_options_rcu *inet_opt;
+	int ret;
 
 	if (sk_acceptq_is_full(sk))
 		goto exit_overflow;
@@ -1756,8 +1757,12 @@ struct sock *tcp_v4_syn_recv_sock(const struct sock *sk, struct sk_buff *skb,
 		goto put_and_exit; /* OOM, release back memory */
 #endif
 
-	if (__inet_inherit_port(sk, newsk) < 0)
+	ret = __inet_inherit_port(sk, newsk);
+	if (unlikely(ret < 0)) {
+		if (ret == -ENOENT)
+			goto send_reset_and_exit;
 		goto put_and_exit;
+	}
 	*own_req = inet_ehash_nolisten(newsk, req_to_sk(req_unhash),
 				       &found_dup_sk);
 	if (likely(*own_req)) {
@@ -1784,6 +1789,8 @@ struct sock *tcp_v4_syn_recv_sock(const struct sock *sk, struct sk_buff *skb,
 exit:
 	tcp_listendrop(sk);
 	return NULL;
+send_reset_and_exit:
+	tcp_v4_send_reset(sk, skb, SK_RST_REASON_TCP_STATE);
 put_and_exit:
 	newinet->inet_opt = NULL;
 	inet_csk_prepare_forced_close(newsk);
diff --git a/net/ipv6/tcp_ipv6.c b/net/ipv6/tcp_ipv6.c
index 9e9155b1b3aa..ecb0b405703c 100644
--- a/net/ipv6/tcp_ipv6.c
+++ b/net/ipv6/tcp_ipv6.c
@@ -1400,6 +1400,7 @@ static struct sock *tcp_v6_syn_recv_sock(const struct sock *sk, struct sk_buff *
 	int l3index;
 #endif
 	struct flowi6 fl6;
+	int ret;
 
 	if (skb->protocol == htons(ETH_P_IP))
 		return tcp_v4_syn_recv_sock(sk, skb, req, dst,
@@ -1512,8 +1513,12 @@ static struct sock *tcp_v6_syn_recv_sock(const struct sock *sk, struct sk_buff *
 		goto put_and_exit; /* OOM */
 #endif
 
-	if (__inet_inherit_port(sk, newsk) < 0)
+	ret = __inet_inherit_port(sk, newsk);
+	if (unlikely(ret < 0)) {
+		if (ret == -ENOENT)
+			goto send_reset_and_exit;
 		goto put_and_exit;
+	}
 	*own_req = inet_ehash_nolisten(newsk, req_to_sk(req_unhash),
 				       &found_dup_sk);
 	if (*own_req) {
@@ -1547,6 +1552,8 @@ static struct sock *tcp_v6_syn_recv_sock(const struct sock *sk, struct sk_buff *
 exit:
 	tcp_listendrop(sk);
 	return NULL;
+send_reset_and_exit:
+	tcp_v6_send_reset(sk, skb, SK_RST_REASON_TCP_STATE);
 put_and_exit:
 	inet_csk_prepare_forced_close(newsk);
 	tcp_done(newsk);

base-commit: cba9ccb47e9fa4cc77692fb896cc5ab57a667882
-- 
2.55.0