Re: [PATCH net-next v3] net: phy: Fix suspicious rcu_dereference usage

Claudiu Beznea
2025-01-23T11:26:01Z

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@bp.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@bp.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,