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Subject: Re: [PATCH net] can: j1939: fix errant WARN_ON_ONCE in
 j1939_session_deactivate
To:     Oleksij Rempel <o.rempel@pengutronix.de>,
        Marc Kleine-Budde <mkl@pengutronix.de>
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>
References: <20210906094200.95868-1-william.xuanziyang@huawei.com>
 <20210910124005.GJ26100@pengutronix.de>
From:   "Ziyang Xuan (William)" <william.xuanziyang@huawei.com>
Message-ID: <17aeff56-716d-3ba5-40cd-b4e1da3f6909@huawei.com>
Date:   Wed, 10 Nov 2021 10:00:06 +0800
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Hello,

I notice that the patch is not applied in upstream. Is it missed
or any other problems?

Thank you!

> 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
>>
>>
>