can: j1939: make j1939_session_activate() fail if device is no longer registered

Tetsuo Handa · gregkh · commit ebb0dfd718dd · 2026-01-19T13:12:12.000+01:00
[ Upstream commit 5d56022 ] syzbot is still reporting unregister_netdevice: waiting for vcan0 to become free. Usage count = 2 even after commit 93a27b5 ("can: j1939: add missing calls in NETDEV_UNREGISTER notification handler") was added. A debug printk() patch found that j1939_session_activate() can succeed even after j1939_cancel_active_session() from j1939_netdev_notify(NETDEV_UNREGISTER) has completed. Since j1939_cancel_active_session() is processed with the session list lock held, checking ndev->reg_state in j1939_session_activate() with the session list lock held can reliably close the race window. Reported-by: syzbot <syzbot+881d65229ca4f9ae8c84@syzkaller.appspotmail.com> Closes: https://syzkaller.appspot.com/bug?extid=881d65229ca4f9ae8c84 Signed-off-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp> Acked-by: Oleksij Rempel <o.rempel@pengutronix.de> Link: https://patch.msgid.link/b9653191-d479-4c8b-8536-1326d028db5c@I-love.SAKURA.ne.jp Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de> Signed-off-by: Sasha Levin <sashal@kernel.org>
diff --git a/net/can/j1939/transport.c b/net/can/j1939/transport.c
@@ -1555,6 +1555,8 @@ int j1939_session_activate(struct j1939_session *session)
 	if (active) {
 		j1939_session_put(active);
 		ret = -EAGAIN;
+	} else if (priv->ndev->reg_state != NETREG_REGISTERED) {
+		ret = -ENODEV;
 	} else {
 		WARN_ON_ONCE(session->state != J1939_SESSION_NEW);
 		list_add_tail(&session->active_session_list_entry,
