netdev - Re: [PATCH net] net/smc: cancel event worker during device removal

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Date:   Mon, 9 Mar 2020 10:40:16 +0100
From:   Karsten Graul <kgraul@...ux.ibm.com>
To:     Leon Romanovsky <leon@...nel.org>
Cc:     davem@...emloft.net, netdev@...r.kernel.org,
        linux-s390@...r.kernel.org, heiko.carstens@...ibm.com,
        raspl@...ux.ibm.com, ubraun@...ux.ibm.com
Subject: Re: [PATCH net] net/smc: cancel event worker during device removal

On 09/03/2020 09:04, Leon Romanovsky wrote:
> On Sun, Mar 08, 2020 at 08:59:33PM +0100, Karsten Graul wrote:
>> On 08/03/2020 16:01, Leon Romanovsky wrote:
>>> On Fri, Mar 06, 2020 at 02:45:18PM +0100, Karsten Graul wrote:
>>>> During IB device removal, cancel the event worker before the device
>>>> structure is freed. In the worker, check if the device is being
>>>> terminated and do not proceed with the event work in that case.
>>>>
>>>> Fixes: a4cf0443c414 ("smc: introduce SMC as an IB-client")
>>>> Reported-by: syzbot+b297c6825752e7a07272@...kaller.appspotmail.com
>>>> Signed-off-by: Karsten Graul <kgraul@...ux.ibm.com>
>>>> Reviewed-by: Ursula Braun <ubraun@...ux.ibm.com>
>>>> ---
>>>>  net/smc/smc_ib.c | 4 ++++
>>>>  1 file changed, 4 insertions(+)
>>>>
>>>> diff --git a/net/smc/smc_ib.c b/net/smc/smc_ib.c
>>>> index d6ba186f67e2..5e4e64a9aa4b 100644
>>>> --- a/net/smc/smc_ib.c
>>>> +++ b/net/smc/smc_ib.c
>>>> @@ -240,6 +240,9 @@ static void smc_ib_port_event_work(struct work_struct *work)
>>>>  		work, struct smc_ib_device, port_event_work);
>>>>  	u8 port_idx;
>>>>
>>>> +	if (list_empty(&smcibdev->list))
>>>> +		return;
>>>> +
>>>
>>> How can it be true if you are not holding "smc_ib_devices.lock" during
>>> execution of smc_ib_port_event_work()?
>>>
>>
>> It is true when smc_ib_remove_dev() runs before the work actually started.
>> Other than that its only a shortcut to return earlier, when the item is
>> removed from the list after the check then the processing just takes a
>> little bit longer...its still save.
> 
> The check itself maybe safe, but it can't fix syzkaller bug reported above.
> As you said, the smc_ib_remove_dev() can be called immediately after
> your list_empty() check and we return to original behavior.
> 
> The correct design will be to ensure that smc_ib_port_event_work() is
> executed only smcibdev->list is not empty.
> 
> Thanks
> 

The fix I had in mind was the

	cancel_work_sync(&smcibdev->port_event_work);

to wait for a running port_event_work to finish before smcibdev is freed.
I can remove the list_empty() check if that is too confusing.

>>
>>>>  	for_each_set_bit(port_idx, &smcibdev->port_event_mask, SMC_MAX_PORTS) {
>>>>  		smc_ib_remember_port_attr(smcibdev, port_idx + 1);
>>>>  		clear_bit(port_idx, &smcibdev->port_event_mask);
>>>> @@ -582,6 +585,7 @@ static void smc_ib_remove_dev(struct ib_device *ibdev, void *client_data)
>>>>  	smc_smcr_terminate_all(smcibdev);
>>>>  	smc_ib_cleanup_per_ibdev(smcibdev);
>>>>  	ib_unregister_event_handler(&smcibdev->event_handler);
>>>> +	cancel_work_sync(&smcibdev->port_event_work);
>>>>  	kfree(smcibdev);
>>>>  }
>>>>
>>>> --
>>>> 2.17.1
>>>>
>>
>> --
>> Karsten
>>
>> (I'm a dude)
>>

-- 
Karsten

(I'm a dude)