[PATCH net-next] pppoe: pass bound packets directly to generic PPP

[Qingfang Deng <qingfang.deng@linux.dev>]

Bound PPPoE sockets pass received frames to the generic PPP layer. They
currently do so through __sk_receive_skb(), which takes the socket BH
lock and serializes calls to ppp_input().

The lock originally prevented ppp_input() from racing with
ppp_unregister_channel(). Commit ec4215683e47 ("ppp: defer channel free
to an RCU grace period to fix pppol2tp RX UAF") now keeps the generic
PPP channel alive until in-flight RCU readers have completed, so bound
packets can be passed directly to ppp_input() from pppoe_rcv().

That lifetime guarantee does not cover reuse of the ppp_channel embedded
in struct pppox_sock. An RX handler can find the old session before it
is unhashed, then resume after disconnect and reconnect have cleared and
re-registered po->chan. It could then race initialization of the new
channel or pass an old-session packet through it.

After unhashing an old session, call synchronize_net() before clearing
and reusing po->chan. This drains every receive path that could have
found the old binding while retaining concurrent delivery for the active
session.

Assisted-by: Codex:GPT-5.6
Signed-off-by: Qingfang Deng <qingfang.deng@linux.dev>
---
 drivers/net/ppp/pppoe.c | 19 +++++++++++++++----
 1 file changed, 15 insertions(+), 4 deletions(-)

diff --git a/drivers/net/ppp/pppoe.c b/drivers/net/ppp/pppoe.c
index 6874a1a8edaf..062411624182 100644
--- a/drivers/net/ppp/pppoe.c
+++ b/drivers/net/ppp/pppoe.c
@@ -345,10 +345,10 @@ static struct notifier_block pppoe_notifier = {
 
 /************************************************************************
  *
- * Do the real work of receiving a PPPoE Session frame.
+ * Backlog receive a PPPoE Session frame and deliver to userspace.
  *
  ***********************************************************************/
-static int pppoe_rcv_core(struct sock *sk, struct sk_buff *skb)
+static int pppoe_backlog_rcv(struct sock *sk, struct sk_buff *skb)
 {
 	struct pppox_sock *po = pppox_sk(sk);
 
@@ -373,7 +373,7 @@ static int pppoe_rcv_core(struct sock *sk, struct sk_buff *skb)
 
 /************************************************************************
  *
- * Receive wrapper called in BH context.
+ * Receive a PPPoE Session frame.
  *
  ***********************************************************************/
 static int pppoe_rcv(struct sk_buff *skb, struct net_device *dev,
@@ -420,6 +420,10 @@ static int pppoe_rcv(struct sk_buff *skb, struct net_device *dev,
 	if (!po)
 		goto drop;
 
+	if (likely(po->sk.sk_state & PPPOX_BOUND)) {
+		ppp_input(&po->chan, skb);
+		return NET_RX_SUCCESS;
+	}
 	return __sk_receive_skb(&po->sk, skb, 0, 1, false);
 
 drop:
@@ -524,7 +528,7 @@ static int pppoe_create(struct net *net, struct socket *sock, int kern)
 	sock->state	= SS_UNCONNECTED;
 	sock->ops	= &pppoe_ops;
 
-	sk->sk_backlog_rcv	= pppoe_rcv_core;
+	sk->sk_backlog_rcv	= pppoe_backlog_rcv;
 	sk->sk_destruct		= pppoe_destruct;
 	sk->sk_state		= PPPOX_NONE;
 	sk->sk_type		= SOCK_STREAM;
@@ -625,6 +629,13 @@ static int pppoe_connect(struct socket *sock, struct sockaddr_unsized *uservaddr
 		pn = pppoe_pernet(sock_net(sk));
 		delete_item(pn, po->pppoe_pa.sid,
 			    po->pppoe_pa.remote, po->pppoe_ifindex);
+
+		/* pppoe_rcv() can call ppp_input() without taking the socket
+		 * lock. Once the socket is unhashed, wait for any receive path
+		 * that found it earlier before clearing and reusing po->chan.
+		 */
+		synchronize_net();
+
 		if (po->pppoe_dev) {
 			dev_put(po->pppoe_dev);
 			po->pppoe_dev = NULL;
-- 
2.43.0