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Date:   Mon, 9 Mar 2020 15:19:56 +0200
From:   Leon Romanovsky <leon@...nel.org>
To:     Karsten Graul <kgraul@...ux.ibm.com>
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 Mon, Mar 09, 2020 at 10:40:16AM +0100, Karsten Graul wrote:
> 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.

Yes, please.

Thanks

>
> >>
> >>>>  	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)
>

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