l2tp: do not use sock_hold() in pppol2tp_session_get_sock()

pppol2tp_session_get_sock() is using RCU, it must be ready
for sk_refcnt being zero.

Commit ee40fb2 ("l2tp: protect sock pointer of
struct pppol2tp_session with RCU") was correct because it
had a call_rcu(..., pppol2tp_put_sk) which was later removed in blamed commit.

pppol2tp_recv() can use pppol2tp_session_get_sock() as well.

Fixes: c5cbaef ("l2tp: refactor ppp socket/session relationship")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: James Chapman <jchapman@katalix.com>
Reviewed-by: Guillaume Nault <gnault@redhat.com>
Link: https://patch.msgid.link/20250826134435.1683435-1-edumazet@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Author: edumazet

net/l2tp/l2tp_ppp.c

@@ -129,22 +129,12 @@ static const struct ppp_channel_ops pppol2tp_chan_ops = {
 
 static const struct proto_ops pppol2tp_ops;
 
-/* Retrieves the pppol2tp socket associated to a session.
- * A reference is held on the returned socket, so this function must be paired
- * with sock_put().
- */
+/* Retrieves the pppol2tp socket associated to a session. */
 static struct sock *pppol2tp_session_get_sock(struct l2tp_session *session)
 {
 	struct pppol2tp_session *ps = l2tp_session_priv(session);
-	struct sock *sk;
-
-	rcu_read_lock();
-	sk = rcu_dereference(ps->sk);
-	if (sk)
-		sock_hold(sk);
-	rcu_read_unlock();
 
-	return sk;
+	return rcu_dereference(ps->sk);
 }
 
 /* Helpers to obtain tunnel/session contexts from sockets.
@@ -206,14 +196,13 @@ static int pppol2tp_recvmsg(struct socket *sock, struct msghdr *msg,
 
 static void pppol2tp_recv(struct l2tp_session *session, struct sk_buff *skb, int data_len)
 {
-	struct pppol2tp_session *ps = l2tp_session_priv(session);
-	struct sock *sk = NULL;
+	struct sock *sk;
 
 	/* If the socket is bound, send it in to PPP's input queue. Otherwise
 	 * queue it on the session socket.
 	 */
 	rcu_read_lock();
-	sk = rcu_dereference(ps->sk);
+	sk = pppol2tp_session_get_sock(session);
 	if (!sk)
 		goto no_sock;
 
@@ -510,13 +499,14 @@ static void pppol2tp_show(struct seq_file *m, void *arg)
 	struct l2tp_session *session = arg;
 	struct sock *sk;
 
+	rcu_read_lock();
 	sk = pppol2tp_session_get_sock(session);
 	if (sk) {
 		struct pppox_sock *po = pppox_sk(sk);
 
 		seq_printf(m, "   interface %s\n", ppp_dev_name(&po->chan));
-		sock_put(sk);
 	}
+	rcu_read_unlock();
 }
 
 static void pppol2tp_session_init(struct l2tp_session *session)
@@ -1530,6 +1520,7 @@ static void pppol2tp_seq_session_show(struct seq_file *m, void *v)
 		port = ntohs(inet->inet_sport);
 	}
 
+	rcu_read_lock();
 	sk = pppol2tp_session_get_sock(session);
 	if (sk) {
 		state = sk->sk_state;
@@ -1565,8 +1556,8 @@ static void pppol2tp_seq_session_show(struct seq_file *m, void *v)
 		struct pppox_sock *po = pppox_sk(sk);
 
 		seq_printf(m, "   interface %s\n", ppp_dev_name(&po->chan));
-		sock_put(sk);
 	}
+	rcu_read_unlock();
 }
 
 static int pppol2tp_seq_show(struct seq_file *m, void *v)