[PATCH net 2/2] ovpn: respect peer refcount in CMD_NEW_PEER error path

[David Carlier]

ovpn_nl_peer_new_doit()'s error path calls ovpn_peer_release() directly
rather than ovpn_peer_put(), bypassing the kref. The accompanying
comment ("peer was not yet hashed, thus it is not used in any context")
holds for UDP but not for TCP.

For UDP, the ovpn_socket union uses the .ovpn arm and never points back
at a peer; UDP encap_recv looks up peers via the not-yet-populated
hashtables, so the new peer is unreachable until ovpn_peer_add()
publishes it.

For TCP, ovpn_socket_new() sets ovpn_sock->peer and
ovpn_tcp_socket_attach() publishes ovpn_sock via rcu_assign_sk_user_data().
>From that moment until ovpn_socket_release() detaches in the error path,
the TCP fd is fully wired: userspace recvmsg / sendmsg / close / poll
on the fd, as well as the strparser-driven ovpn_tcp_rcv() path, can
reach the peer through sk_user_data -> ovpn_sock->peer and bump its
refcount via ovpn_peer_hold().

ovpn_tcp_socket_wait_finish() (called inside ovpn_socket_release())
drains strparser and the tx work, but does not synchronize with
userspace syscall callers that already hold a peer reference. If
ovpn_nl_peer_modify() or ovpn_peer_add() returns an error while such
a caller is in flight - notably an ovpn_tcp_recvmsg() blocked in
__skb_recv_datagram() on peer->tcp.user_queue - the direct
ovpn_peer_release() destroys the peer while the caller still holds
the reference, and the eventual ovpn_peer_put() from that caller
operates on freed memory.

Replace the direct destructor call with ovpn_peer_put() so the kref
correctly defers destruction until the last reference is dropped.
In the common case where no concurrent user is present, behaviour is
unchanged: the kref hits zero immediately and ovpn_peer_release_kref()
runs the same destructor.

Fixes: 11851cbd60ea ("ovpn: implement TCP transport")
Assisted-by: Claude:claude-opus-4-7
Signed-off-by: David Carlier <devnexen@xxxxxxxxx>
---
 drivers/net/ovpn/netlink.c | 8 +++++---
 1 file changed, 5 insertions(+), 3 deletions(-)

diff --git a/drivers/net/ovpn/netlink.c b/drivers/net/ovpn/netlink.c
index 291e2e5bb450..4c66c1ec497e 100644
--- a/drivers/net/ovpn/netlink.c
+++ b/drivers/net/ovpn/netlink.c
@@ -462,10 +462,12 @@ int ovpn_nl_peer_new_doit(struct sk_buff *skb, struct genl_info *info)
 sock_release:
 	ovpn_socket_release(peer);
 peer_release:
-	/* release right away because peer was not yet hashed, thus it is not
-	 * used in any context
+	/* For UDP, the peer is unreachable until added to the hashtables, so
+	 * dropping the initial reference is enough. For TCP, the peer may be
+	 * concurrently reachable via sk_user_data->peer until
+	 * ovpn_socket_release() detaches; rely on the refcount.
 	 */
-	ovpn_peer_release(peer);
+	ovpn_peer_put(peer);
 
 	return ret;
 }
-- 
2.53.0