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

On Tue, Jan 21, 2025 at 05:11:56PM +0100, Kory Maincent wrote:

On Tue, 21 Jan 2025 15:44:34 +0000
Paul Barker [off-list ref] wrote:

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On 21/01/2025 13:01, Kory Maincent wrote:

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On Tue, 21 Jan 2025 11:34:48 +0000
Paul Barker [off-list ref] wrote:
  

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On 21/01/2025 09:38, Kory Maincent wrote:  

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(Cc'ing Niklas and Sergey as this relates to the ravb driver)  

Yes, thanks.
  

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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.
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

Taking a look at where ravb_ptp_stop() is called from:

1. ravb_set_ringparam(). ethtool operation. RTNL will be held for this.
2. ravb_open() error-cleanup. RTNL will be held for this.
3. ravb_tx_timeout_work(). rtnl_trylock() is called and we will only
   call through to the above function if we grabbed the RTNL.
4. ravb_close(), again RTNL will be held here.
5. ravb_wol_setup(). Another ethtool operation. (1) applies.

Hence, it is not possible for two threads to execute ravb_ptp_stop()
symultaneously. However, if ptp_clock_register() in ravb_ptp_init()
fails, then priv->ptp.clock will be set to an error-pointer, and
subsequently passed to ptp_clock_unregister() which would cause a
kernel oops. No one seems to have thought about that... and that
definitely needs fixing.

However, one wonders why it's necessary to unregister a _user_
_interface_ when responding to a change in WoL, ring parameters, or
merely handling a transmit timeout. It doesn't seem particularly
nice to userspace for a device that its using to suddenly go away
for these reasons. I wonder whether anyone has tested anything
that uses the PTP clock interfaces while changing e.g. the WoL
settings.

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