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Date:   Fri, 10 Sep 2021 14:40:05 +0200
From:   Oleksij Rempel <o.rempel@pengutronix.de>
To:     Ziyang Xuan <william.xuanziyang@huawei.com>
Cc:     robin@protonic.nl, linux@rempel-privat.de, socketcan@hartkopp.net,
        mkl@pengutronix.de, davem@davemloft.net, kuba@kernel.org,
        linux-can@vger.kernel.org, netdev@vger.kernel.org,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH net] can: j1939: fix errant WARN_ON_ONCE in
 j1939_session_deactivate
Message-ID: <20210910124005.GJ26100@pengutronix.de>
References: <20210906094200.95868-1-william.xuanziyang@huawei.com>
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In-Reply-To: <20210906094200.95868-1-william.xuanziyang@huawei.com>
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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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