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

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Message-ID: <49d39ae2-fbe1-4054-bb78-7e0c54626a23@tuxon.dev>
Date: Thu, 23 Jan 2025 13:25:57 +0200
From: Claudiu Beznea <claudiu.beznea@...on.dev>
To: Kory Maincent <kory.maincent@...tlin.com>,
 Paul Barker <paul.barker.ct@...renesas.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

Hi, Kory,

On 21.01.2025 18:11, Kory Maincent wrote:
> On Tue, 21 Jan 2025 15:44:34 +0000
> Paul Barker <paul.barker.ct@...renesas.com> wrote:
> 
>> 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:  
>>  [...]  
>>  [...]  
>>>>  [...]    
>>  [...]  
>>  [...]  
>>>>
>>>> (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.
> 
> What happens if two ptp_clock_unregister() with the same ptp_clock pointer are 
> called simultaneously? From ravb_suspend and ravb_set_ringparam for example. It
> may cause some errors.

Can this happen? I see ethtool_ops::set_ringparam() references only in
ethtool or ioctl files:

net/ethtool/ioctl.c:2066
net/ethtool/ioctl.c:2081
net/ethtool/rings.c:212
net/ethtool/rings.c:304

At the time the suspend/resume APIs are called the user space threads are
frozen.

Thank you,
Claudiu

> For example the ptp->kworker pointer could be used after a kfree.
> https://elixir.bootlin.com/linux/v6.12.6/source/drivers/ptp/ptp_clock.c#L416
> 
> Regards,