Re: [PATCH] can: j1939: implement NETDEV_UNREGISTER notification handler

[Oleksij Rempel <o.rempel@pengutronix.de>]

On Mon, Aug 25, 2025 at 11:07:24PM +0900, Tetsuo Handa wrote:
> 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@syzkaller.appspotmail.com>
> Closes: https://syzkaller.appspot.com/bug?extid=881d65229ca4f9ae8c84
> Tested-by: syzbot <syzbot+881d65229ca4f9ae8c84@syzkaller.appspotmail.com>
> 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@I-love.SAKURA.ne.jp>

Tested-by: Oleksij Rempel <o.rempel@pengutronix.de>
Acked-by: Oleksij Rempel <o.rempel@pengutronix.de>

Thank you!
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