netdev - Re: [PATCH net v2] ravb: Fix races between ravb_tx_timeout

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Message-ID: <29603244-d65a-5ca0-90d4-fdd9f410e180@omp.ru>
Date: Thu, 16 Nov 2023 21:11:31 +0300
From: Sergey Shtylyov <s.shtylyov@....ru>
To: Yoshihiro Shimoda <yoshihiro.shimoda.uh@...esas.com>,
	"davem@...emloft.net" <davem@...emloft.net>, "edumazet@...gle.com"
	<edumazet@...gle.com>, "kuba@...nel.org" <kuba@...nel.org>,
	"pabeni@...hat.com" <pabeni@...hat.com>
CC: "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	"linux-renesas-soc@...r.kernel.org" <linux-renesas-soc@...r.kernel.org>
Subject: Re: [PATCH net v2] ravb: Fix races between ravb_tx_timeout_work() and
 net related ops

On 11/16/23 5:15 AM, Yoshihiro Shimoda wrote:
[...]

>>>>> Fix races between ravb_tx_timeout_work() and functions of net_device_ops
>>>>> and ethtool_ops by using rtnl_trylock() and rtnl_unlock(). Note that
>>>>> since ravb_close() is under the rtnl lock and calls cancel_work_sync(),
>>>>> ravb_tx_timeout_work() should calls rtnl_trylock(). Otherwise, a deadlock
>>>>> may happen in ravb_tx_timeout_work() like below:
>>>>>
>>>>> CPU0			CPU1
>>>>> 			ravb_tx_timeout()
>>>>> 			schedule_work()
>>>>> ...
>>>>> __dev_close_many()
>>>>> // Under rtnl lock
>>>>> ravb_close()
>>>>> cancel_work_sync()
>>>>> // Waiting
>>>>> 			ravb_tx_timeout_work()
>>>>> 			rtnl_lock()
>>>>> 			// This is possible to cause a deadlock
>>>>>
>>>>> Fixes: c156633f1353 ("Renesas Ethernet AVB driver proper")
>>>>> Signed-off-by: Yoshihiro Shimoda <yoshihiro.shimoda.uh@...esas.com>
>>>>
>>>> Reviewed-by: Sergey Shtylyov <s.shtylyov@....ru>
>>
>> [...]
>>
>>>>> diff --git a/drivers/net/ethernet/renesas/ravb_main.c b/drivers/net/ethernet/renesas/ravb_main.c
>>>>> index 0ef0b88b7145..300c1885e1e1 100644
>>>>> --- a/drivers/net/ethernet/renesas/ravb_main.c
>>>>> +++ b/drivers/net/ethernet/renesas/ravb_main.c
>>>>> @@ -1874,6 +1874,9 @@ static void ravb_tx_timeout_work(struct work_struct *work)
>>>>>  	struct net_device *ndev = priv->ndev;
>>>>>  	int error;
>>>>>
>>>>> +	if (!rtnl_trylock())
>>>>> +		return;
>>>>
>>>>    I wonder if we should reschedule the work here...
>>>
>>> I think so. But, it should reschedule the work if the netif is still running because
>>> Use-after-free issue happens again when cancel_work_sync() is calling. Also, I also think
>>> we should use schedule_delayed_work() instead. So, I'll submit such a patch as v3.
>>
>>    I'm not really sure about that one. Note that cancel_work_sync() should
>> work with the works requeueing themselves, the comments say...
> 
> I'm sorry, I completely mistook to explain this... I should have said:

   Don't worry, my uncertainty was about using the "delayed" flavor of
the works. :-)

> It should not reschedule the work if the netif is not running because
>           ~~~                                     ~~~
> use-after-free issue happens again when cancel_work_sync() is called from ravb_remove().
>                                                            ~~~~~~~~~~~~~~~~~~~~~~~~~~~~

   Well, it's called from ravb_close() -- which is called by the networking
core when unregister_netdev() is called bt ravb_remove()...

> Also, I completely misunderstood the behavior of cancel_{schedule_}work_sync().

   cancel_{delayed_}work_sync(), you meant...

> In the function(s), since WORK_STRUCT_PENDING_BIT is set, schedule_{delayed_}work()
> will not schedule the work anymore. So, I'll drop a condition netif_running()
> from the ravb_tx_timeout_work().

  Hm, this caused me to rummage in the work queue code for more time than
I could afford... still not sure what you meant... :-/

> Best regards,
> Yoshihiro Shimoda
[...]

MBR, Sergey