[PATCH] ovpn: avoid putting unrelated P2P peer on socket release

[Qing Ming <a0yami@mailbox.org>]

ovpn_peer_release_p2p() is called when an OVPN UDP socket is being
destroyed. It checks the currently published P2P peer and releases it only
if that peer still uses the socket being destroyed.

A peer replacement can publish a new peer before the old UDP socket is
destroyed. When the old socket destruction path runs afterwards,
ovpn_peer_release_p2p() observes the new peer through ovpn->peer. Since the
new peer uses a different socket, the function takes the socket mismatch
branch.

That branch still calls ovpn_peer_put(peer). At this point, however, peer
is the currently published replacement peer, not the peer associated with
the socket being destroyed. Dropping its reference can free it while
ovpn->peer still points to it, leading to later use-after-free accesses
from the peer and socket cleanup paths.

KASAN reports this as a slab-use-after-free on the kmalloc-1k ovpn_peer
object. In the reproducer, the object is allocated from ovpn_peer_new() via
ovpn_nl_peer_new_doit(), and freed through ovpn_peer_release_rcu() from RCU
callback processing. Observed access sites include ovpn_peer_remove(),
ovpn_socket_release(), ovpn_nl_peer_del_notify(), and unlock_ovpn().

Fix this by returning from the socket mismatch branch without putting the
peer.

Fixes: f6226ae7a0cd ("ovpn: introduce the ovpn_socket object")
Signed-off-by: Qing Ming <a0yami@mailbox.org>
---
 drivers/net/ovpn/peer.c | 1 -
 1 file changed, 1 deletion(-)

diff --git a/drivers/net/ovpn/peer.c b/drivers/net/ovpn/peer.c
index a09d61296425..1844d97154ce 100644
--- a/drivers/net/ovpn/peer.c
+++ b/drivers/net/ovpn/peer.c
@@ -1167,7 +1167,6 @@ static void ovpn_peer_release_p2p(struct ovpn_priv *ovpn, struct sock *sk,
 		ovpn_sock = rcu_access_pointer(peer->sock);
 		if (!ovpn_sock || ovpn_sock->sk != sk) {
 			spin_unlock_bh(&ovpn->lock);
-			ovpn_peer_put(peer);
 			return;
 		}
 	}
-- 
2.53.0