linux-kernel - [PATCH] can: j1939: implement NETDEV

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Message-ID: <a1e524f6-bf34-4ac1-b98d-26198c65253f@I-love.SAKURA.ne.jp>
Date: Mon, 25 Aug 2025 20:01:23 +0900
From: Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
To: syzbot+881d65229ca4f9ae8c84@...kaller.appspotmail.com,
        LKML <linux-kernel@...r.kernel.org>
Subject: [PATCH] can: j1939: implement NETDEV_UNREGISTER notification handler

syzbot is reporting

  unregister_netdevice: waiting for vcan0 to become free. Usage count = 2

problem, for j1939 protocol did not have NETDEV_UNREGISTER notification
handler for undoing changes made by j1939_sk_bind().

Commit 25fe97cb7620 ("can: j1939: move j1939_priv_put() into sk_destruct
callback") expects that a call to j1939_priv_put() can be unconditionally
delayed until j1939_sk_sock_destruct() is called. But we need to call
j1939_priv_put() against an extra ref held by j1939_sk_bind() call
(as a part of undoing changes made by j1939_sk_bind()) as soon as
NETDEV_UNREGISTER notification fires (i.e. before j1939_sk_sock_destruct()
is called via j1939_sk_release()). Otherwise, the extra ref on "struct
j1939_priv" held by j1939_sk_bind() call prevents "struct net_device" from
dropping the usage count to 1; making it impossible for
unregister_netdevice() to continue.

Reported-by: syzbot <syzbot+881d65229ca4f9ae8c84@...kaller.appspotmail.com>
Closes: https://syzkaller.appspot.com/bug?extid=881d65229ca4f9ae8c84
Fixes: 9d71dd0c7009 ("can: add support of SAE J1939 protocol")
Fixes: 25fe97cb7620 ("can: j1939: move j1939_priv_put() into sk_destruct callback")
Signed-off-by: Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
---
#syz test

 net/can/j1939/j1939-priv.h |  1 +
 net/can/j1939/main.c       |  3 +++
 net/can/j1939/socket.c     | 49 ++++++++++++++++++++++++++++++++++++++
 3 files changed, 53 insertions(+)

diff --git a/net/can/j1939/j1939-priv.h b/net/can/j1939/j1939-priv.h
index 31a93cae5111..81f58924b4ac 100644
--- a/net/can/j1939/j1939-priv.h
+++ b/net/can/j1939/j1939-priv.h
@@ -212,6 +212,7 @@ void j1939_priv_get(struct j1939_priv *priv);
 
 /* notify/alert all j1939 sockets bound to ifindex */
 void j1939_sk_netdev_event_netdown(struct j1939_priv *priv);
+void j1939_sk_netdev_event_unregister(struct j1939_priv *priv);
 int j1939_cancel_active_session(struct j1939_priv *priv, struct sock *sk);
 void j1939_tp_init(struct j1939_priv *priv);
 
diff --git a/net/can/j1939/main.c b/net/can/j1939/main.c
index 7e8a20f2fc42..3706a872ecaf 100644
--- a/net/can/j1939/main.c
+++ b/net/can/j1939/main.c
@@ -377,6 +377,9 @@ static int j1939_netdev_notify(struct notifier_block *nb,
 		j1939_sk_netdev_event_netdown(priv);
 		j1939_ecu_unmap_all(priv);
 		break;
+	case NETDEV_UNREGISTER:
+		j1939_sk_netdev_event_unregister(priv);
+		break;
 	}
 
 	j1939_priv_put(priv);
diff --git a/net/can/j1939/socket.c b/net/can/j1939/socket.c
index 493f49bfaf5d..72c649cec9e1 100644
--- a/net/can/j1939/socket.c
+++ b/net/can/j1939/socket.c
@@ -1303,6 +1303,55 @@ void j1939_sk_netdev_event_netdown(struct j1939_priv *priv)
 	read_unlock_bh(&priv->j1939_socks_lock);
 }
 
+void j1939_sk_netdev_event_unregister(struct j1939_priv *priv)
+{
+	struct sock *sk;
+	struct j1939_sock *jsk;
+	bool wait_rcu = false;
+
+ rescan: /* The caller is holding a ref on this "priv" via j1939_priv_get_by_ndev(). */
+	read_lock_bh(&priv->j1939_socks_lock);
+	list_for_each_entry(jsk, &priv->j1939_socks, list) {
+		/* Skip if j1939_jsk_add() is not called on this socket. */
+		if (!(jsk->state & J1939_SOCK_BOUND))
+			continue;
+		sk = &jsk->sk;
+		sock_hold(sk);
+		read_unlock_bh(&priv->j1939_socks_lock);
+		/* Check if j1939_jsk_del() is not yet called on this socket after holding
+		 * socket's lock, for both j1939_sk_bind() and j1939_sk_release() call
+		 * j1939_jsk_del() with socket's lock held.
+		 */
+		lock_sock(sk);
+		if (jsk->state & J1939_SOCK_BOUND) {
+			/* Neither j1939_sk_bind() nor j1939_sk_release() called j1939_jsk_del().
+			 * Make this socket no longer bound, by pretending as if j1939_sk_bind()
+			 * dropped old references but did not get new references.
+			 */
+			j1939_jsk_del(priv, jsk);
+			j1939_local_ecu_put(priv, jsk->addr.src_name, jsk->addr.sa);
+			j1939_netdev_stop(priv);
+			/* Call j1939_priv_put() now and prevent j1939_sk_sock_destruct() from
+			 * calling the corresponding j1939_priv_put().
+			 *
+			 * j1939_sk_sock_destruct() is supposed to call j1939_priv_put() after
+			 * an RCU grace period. But since the caller is holding a ref on this
+			 * "priv", we can defer synchronize_rcu() until immediately before
+			 * the caller calls j1939_priv_put().
+			 */
+			j1939_priv_put(priv);
+			jsk->priv = NULL;
+			wait_rcu = true;
+		}
+		release_sock(sk);
+		sock_put(sk);
+		goto rescan;
+	}
+	read_unlock_bh(&priv->j1939_socks_lock);
+	if (wait_rcu)
+		synchronize_rcu();
+}
+
 static int j1939_sk_no_ioctlcmd(struct socket *sock, unsigned int cmd,
 				unsigned long arg)
 {
-- 
2.51.0