netdev - Re: [PATCH net-next v3] net: phy: Fix suspicious rcu

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Message-ID: <d512e107-68ac-4594-a7cb-8c26be4b3280@bp.renesas.com>
Date: Tue, 21 Jan 2025 15:44:34 +0000
From: Paul Barker <paul.barker.ct@...renesas.com>
To: Kory Maincent <kory.maincent@...tlin.com>
Cc: Jakub Kicinski <kuba@...nel.org>, "David S. Miller"
 <davem@...emloft.net>, netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
 Claudiu Beznea <claudiu.beznea.uj@...renesas.com>,
 thomas.petazzoni@...tlin.com, Andrew Lunn <andrew@...n.ch>,
 Heiner Kallweit <hkallweit1@...il.com>, Russell King
 <linux@...linux.org.uk>, Eric Dumazet <edumazet@...gle.com>,
 Paolo Abeni <pabeni@...hat.com>,
 Niklas Söderlund <niklas.soderlund@...natech.se>,
 Sergey Shtylyov <s.shtylyov@....ru>
Subject: Re: [PATCH net-next v3] net: phy: Fix suspicious rcu_dereference
 usage

On 21/01/2025 13:01, Kory Maincent wrote:
> On Tue, 21 Jan 2025 11:34:48 +0000
> Paul Barker <paul.barker.ct@...renesas.com> wrote:
> 
>> On 21/01/2025 09:38, Kory Maincent wrote:
>>> On Mon, 20 Jan 2025 11:12:28 -0800
>>> Jakub Kicinski <kuba@...nel.org> wrote:
>>>   
>>>> On Mon, 20 Jan 2025 15:19:25 +0100 Kory Maincent wrote:  
>>  [...]  
>>>>
>>>> I maintain that ravb is buggy, plenty of drivers take rtnl_lock 
>>>> from the .suspend callback. We need _some_ write protection here,
>>>> the patch as is only silences a legitimate warning.  
>>>
>>> Indeed if the suspend path is buggy we should fix it. Still there is lots of
>>> ethernet drivers calling phy_disconnect without rtnl (IIUC) if probe return
>>> an error or in the remove path. What should we do about it?
>>>
>>> About ravb suspend, I don't have the board, Claudiu could you try this
>>> instead of the current fix:
>>>
>>> diff --git a/drivers/net/ethernet/renesas/ravb_main.c
>>> b/drivers/net/ethernet/renesas/ravb_main.c index bc395294a32d..c9a0d2d6f371
>>> 100644 --- a/drivers/net/ethernet/renesas/ravb_main.c
>>> +++ b/drivers/net/ethernet/renesas/ravb_main.c
>>> @@ -3215,15 +3215,22 @@ static int ravb_suspend(struct device *dev)
>>>         if (!netif_running(ndev))
>>>                 goto reset_assert;
>>>  
>>> +       rtnl_lock();
>>>         netif_device_detach(ndev);
>>>  
>>> -       if (priv->wol_enabled)
>>> -               return ravb_wol_setup(ndev);
>>> +       if (priv->wol_enabled) {
>>> +               ret = ravb_wol_setup(ndev);
>>> +               rtnl_unlock();
>>> +               return ret;
>>> +       }
>>>  
>>>         ret = ravb_close(ndev);
>>> -       if (ret)
>>> +       if (ret) {
>>> +               rtnl_unlock();
>>>                 return ret;
>>> +       }
>>>  
>>> +       rtnl_unlock();
>>>         ret = pm_runtime_force_suspend(&priv->pdev->dev);
>>>         if (ret)
>>>                 return ret;
>>>
>>> Regards,  
>>
>> (Cc'ing Niklas and Sergey as this relates to the ravb driver)
> 
> Yes, thanks.
> 
>> Why do we need to hold the rtnl mutex across the calls to
>> netif_device_detach() and ravb_wol_setup()?
>>
>> My reading of Documentation/networking/netdevices.rst is that the rtnl
>> mutex is held when the net subsystem calls the driver's ndo_stop method,
>> which in our case is ravb_close(). So, we should take the rtnl mutex
>> when we call ravb_close() directly, in both ravb_suspend() and
>> ravb_wol_restore(). That would ensure that we do not call
>> phy_disconnect() without holding the rtnl mutex and should fix this
>> issue.
> 
> Not sure about it. For example ravb_ptp_stop() called in ravb_wol_setup() won't
> be protected by the rtnl lock.

ravb_ptp_stop() modifies a couple of device registers and calls
ptp_clock_unregister(). I don't see anything to suggest that this
requires the rtnl lock to be held, unless I am missing something.

> I don't know about netif_device_detach(). It doesn't seems to be the case as
> there is lots of driver using it without holding rtnl lock.

netif_device_detach() clears the present flag from the link state and
stops all TX queues, so I don't think the rtnl lock needs to be held to
call this.

> 
> Indeed we should add the rtnl lock also in the resume path. 
> 
>> Commit 35f7cad1743e ("net: Add the possibility to support a selected
>> hwtstamp in netdevice") may have unearthed the issue, but the fixes tag
>> should point to the commits adding those unlocked calls to ravb_close().
> 
> The current patch was on phy_device.c that's why the fixes tag does not point to
> a ravb commit, it will change.

Ack. Thanks for digging in to this!

-- 
Paul Barker
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