From: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>

syzbot is still reporting

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

even after commit 93a27b5891b8 ("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 <redacted>
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>
---
 net/can/j1939/transport.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/net/can/j1939/transport.c b/net/can/j1939/transport.c
index fbf5c8001c9d..613a911dda10 100644
--- a/net/can/j1939/transport.c
+++ b/net/can/j1939/transport.c

@@ -1567,6 +1567,8 @@ 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,

base-commit: 885bebac9909994050bbbeed0829c727e42bd1b7
-- 
2.51.0