netdev - [PATCH net 2/3] ovpn: explicitly reject netlink attr PEER_LOCAL_PORT in CMD_PEER

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Message-ID: <20250703114513.18071-3-antonio@openvpn.net>
Date: Thu,  3 Jul 2025 13:45:11 +0200
From: Antonio Quartulli <antonio@...nvpn.net>
To: netdev@...r.kernel.org
Cc: Antonio Quartulli <antonio@...nvpn.net>,
	Sabrina Dubroca <sd@...asysnail.net>,
	"David S . Miller" <davem@...emloft.net>,
	Eric Dumazet <edumazet@...gle.com>,
	Jakub Kicinski <kuba@...nel.org>,
	Paolo Abeni <pabeni@...hat.com>,
	Ralf Lici <ralf@...delbit.com>
Subject: [PATCH net 2/3] ovpn: explicitly reject netlink attr PEER_LOCAL_PORT in CMD_PEER_NEW/SET

The OVPN_A_PEER_LOCAL_PORT is designed to be a read-only attribute
that ovpn sends back to userspace to show the local port being used
to talk to that specific peer.

However, we forgot to reject it when parsing CMD_PEER_NEW/SET messages.
This is not a critical issue because the incoming value is just
ignored, but it may fool userspace which expects some change in
behaviour.

Explicitly error out and send back a message if OVPN_A_PEER_LOCAL_PORT
is specified in a CMD_PEER_NEW/SET message.

Reported-by: Ralf Lici <ralf@...delbit.com>
Closes: https://github.com/OpenVPN/ovpn-net-next/issues/19
Signed-off-by: Antonio Quartulli <antonio@...nvpn.net>
---
 drivers/net/ovpn/netlink.c | 11 +++++++++++
 1 file changed, 11 insertions(+)

diff --git a/drivers/net/ovpn/netlink.c b/drivers/net/ovpn/netlink.c
index a4ec53def46e..17d23d01c6d8 100644
--- a/drivers/net/ovpn/netlink.c
+++ b/drivers/net/ovpn/netlink.c
@@ -224,6 +224,17 @@ static int ovpn_nl_peer_precheck(struct ovpn_priv *ovpn,
 		return -EINVAL;
 	}
 
+	/* the local port cannot be set by userspace because the socket
+	 * being passed is already bound to one.
+	 * OVPN_A_PEER_LOCAL_PORT is for sending peer status only (check
+	 * ovpn_nl_send_peer())
+	 */
+	if (attrs[OVPN_A_PEER_LOCAL_PORT]) {
+		NL_SET_ERR_MSG_MOD(info->extack,
+				   "cannot specify local port (socket is bound already)");
+		return -EINVAL;
+	}
+
 	/* check that local and remote address families are the same even
 	 * after parsing v4mapped IPv6 addresses.
 	 * (if addresses are not provided, family will be AF_UNSPEC and
-- 
2.49.0