net/rds: fix recursive lock in rds_tcp_conn_slots_available

syzbot reported a recursive lock warning in rds_tcp_get_peer_sport() as
it calls inet6_getname() which acquires the socket lock that was already
held by __release_sock().

 kworker/u8:6/2985 is trying to acquire lock:
 ffff88807a07aa20 (k-sk_lock-AF_INET6){+.+.}-{0:0}, at: lock_sock include/net/sock.h:1709 [inline]
 ffff88807a07aa20 (k-sk_lock-AF_INET6){+.+.}-{0:0}, at: inet6_getname+0x15d/0x650 net/ipv6/af_inet6.c:533

 but task is already holding lock:
 ffff88807a07aa20 (k-sk_lock-AF_INET6){+.+.}-{0:0}, at: lock_sock include/net/sock.h:1709 [inline]
 ffff88807a07aa20 (k-sk_lock-AF_INET6){+.+.}-{0:0}, at: tcp_sock_set_cork+0x2c/0x2e0 net/ipv4/tcp.c:3694
   lock_sock_nested+0x48/0x100 net/core/sock.c:3780
   lock_sock include/net/sock.h:1709 [inline]
   inet6_getname+0x15d/0x650 net/ipv6/af_inet6.c:533
   rds_tcp_get_peer_sport net/rds/tcp_listen.c:70 [inline]
   rds_tcp_conn_slots_available+0x288/0x470 net/rds/tcp_listen.c:149
   rds_recv_hs_exthdrs+0x60f/0x7c0 net/rds/recv.c:265
   rds_recv_incoming+0x9f6/0x12d0 net/rds/recv.c:389
   rds_tcp_data_recv+0x7f1/0xa40 net/rds/tcp_recv.c:243
   __tcp_read_sock+0x196/0x970 net/ipv4/tcp.c:1702
   rds_tcp_read_sock net/rds/tcp_recv.c:277 [inline]
   rds_tcp_data_ready+0x369/0x950 net/rds/tcp_recv.c:331
   tcp_rcv_established+0x19e9/0x2670 net/ipv4/tcp_input.c:6675
   tcp_v6_do_rcv+0x8eb/0x1ba0 net/ipv6/tcp_ipv6.c:1609
   sk_backlog_rcv include/net/sock.h:1185 [inline]
   __release_sock+0x1b8/0x3a0 net/core/sock.c:3213

Reading from the socket struct directly is safe from possible paths. For
rds_tcp_accept_one(), the socket has just been accepted and is not yet
exposed to concurrent access. For rds_tcp_conn_slots_available(), direct
access avoids the recursive deadlock seen during backlog processing
where the socket lock is already held from the __release_sock().

However, rds_tcp_conn_slots_available() is also called from the normal
softirq path via tcp_data_ready() where the lock is not held. This is
also safe because inet_dport is a stable 16 bits field. A READ_ONCE()
annotation as the value might be accessed lockless in a concurrent
access context.

Note that it is also safe to call rds_tcp_conn_slots_available() from
rds_conn_shutdown() because the fan-out is disabled.

Fixes: 9d27a0f ("net/rds: Trigger rds_send_ping() more than once")
Reported-by: syzbot+5efae91f60932839f0a5@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=5efae91f60932839f0a5
Signed-off-by: Fernando Fernandez Mancera <fmancera@suse.de>
Reviewed-by: Allison Henderson <achender@kernel.org>
Link: https://patch.msgid.link/20260219075738.4403-1-fmancera@suse.de
Signed-off-by: Paolo Abeni <pabeni@redhat.com>

Author: ffmancera

net/rds/connection.c

@@ -455,6 +455,9 @@ void rds_conn_shutdown(struct rds_conn_path *cp)
 		rcu_read_unlock();
 	}
 
+	/* we do not hold the socket lock here but it is safe because
+	 * fan-out is disabled when calling conn_slots_available()
+	 */
 	if (conn->c_trans->conn_slots_available)
 		conn->c_trans->conn_slots_available(conn, false);
 }

net/rds/tcp_listen.c

@@ -59,30 +59,12 @@ void rds_tcp_keepalive(struct socket *sock)
 static int
 rds_tcp_get_peer_sport(struct socket *sock)
 {
-	union {
-		struct sockaddr_storage storage;
-		struct sockaddr addr;
-		struct sockaddr_in sin;
-		struct sockaddr_in6 sin6;
-	} saddr;
-	int sport;
-
-	if (kernel_getpeername(sock, &saddr.addr) >= 0) {
-		switch (saddr.addr.sa_family) {
-		case AF_INET:
-			sport = ntohs(saddr.sin.sin_port);
-			break;
-		case AF_INET6:
-			sport = ntohs(saddr.sin6.sin6_port);
-			break;
-		default:
-			sport = -1;
-		}
-	} else {
-		sport = -1;
-	}
+	struct sock *sk = sock->sk;
+
+	if (!sk)
+		return -1;
 
-	return sport;
+	return ntohs(READ_ONCE(inet_sk(sk)->inet_dport));
 }
 
 /* rds_tcp_accept_one_path(): if accepting on cp_index > 0, make sure the