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SubjectRe: [PATCH net] can: j1939: fix errant WARN_ON_ONCE in j1939_session_deactivate
On Mon, Sep 06, 2021 at 05:42:00PM +0800, Ziyang Xuan wrote:
> The conclusion "j1939_session_deactivate() should be called with a
> session ref-count of at least 2" is incorrect. In some concurrent
> scenarios, j1939_session_deactivate can be called with the session
> ref-count less than 2. But there is not any problem because it
> will check the session active state before session putting in
> j1939_session_deactivate_locked().
>
> Here is the concurrent scenario of the problem reported by syzbot
> and my reproduction log.
>
> cpu0 cpu1
> j1939_xtp_rx_eoma
> j1939_xtp_rx_abort_one
> j1939_session_get_by_addr [kref == 2]
> j1939_session_get_by_addr [kref == 3]
> j1939_session_deactivate [kref == 2]
> j1939_session_put [kref == 1]
> j1939_session_completed
> j1939_session_deactivate
> WARN_ON_ONCE(kref < 2)
>

Ok, I see, this warning makes sense only if session will actually be
deactivated.

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

Thank you!

> =====================================================
> WARNING: CPU: 1 PID: 21 at net/can/j1939/transport.c:1088 j1939_session_deactivate+0x5f/0x70
> CPU: 1 PID: 21 Comm: ksoftirqd/1 Not tainted 5.14.0-rc7+ #32
> Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.13.0-1ubuntu1 04/01/2014
> RIP: 0010:j1939_session_deactivate+0x5f/0x70
> Call Trace:
> j1939_session_deactivate_activate_next+0x11/0x28
> j1939_xtp_rx_eoma+0x12a/0x180
> j1939_tp_recv+0x4a2/0x510
> j1939_can_recv+0x226/0x380
> can_rcv_filter+0xf8/0x220
> can_receive+0x102/0x220
> ? process_backlog+0xf0/0x2c0
> can_rcv+0x53/0xf0
> __netif_receive_skb_one_core+0x67/0x90
> ? process_backlog+0x97/0x2c0
> __netif_receive_skb+0x22/0x80
>
> Fixes: 0c71437dd50d ("can: j1939: j1939_session_deactivate(): clarify lifetime of session object")
> Reported-by: syzbot+9981a614060dcee6eeca@syzkaller.appspotmail.com
> Signed-off-by: Ziyang Xuan <william.xuanziyang@huawei.com>
> ---
> net/can/j1939/transport.c | 4 ----
> 1 file changed, 4 deletions(-)
>
> diff --git a/net/can/j1939/transport.c b/net/can/j1939/transport.c
> index bdc95bd7a851..0f8309314075 100644
> --- a/net/can/j1939/transport.c
> +++ b/net/can/j1939/transport.c
> @@ -1079,10 +1079,6 @@ static bool j1939_session_deactivate(struct j1939_session *session)
> bool active;
>
> j1939_session_list_lock(priv);
> - /* This function should be called with a session ref-count of at
> - * least 2.
> - */
> - WARN_ON_ONCE(kref_read(&session->kref) < 2);
> active = j1939_session_deactivate_locked(session);
> j1939_session_list_unlock(priv);
>
> --
> 2.25.1
>
>

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