netdev - Re: [PATCH net] net: ftgmac100: fix potential NULL pointer access in ftgmac100_phy

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Message-ID: <3050f970-5e37-4d93-8a15-f852d4cc8f78@linux.intel.com>
Date: Fri, 1 Aug 2025 08:19:45 +0200
From: Dawid Osuchowski <dawid.osuchowski@...ux.intel.com>
To: Heiner Kallweit <hkallweit1@...il.com>,
 Andrew Lunn <andrew+netdev@...n.ch>, Simon Horman <horms@...nel.org>,
 Jakub Kicinski <kuba@...nel.org>, David Miller <davem@...emloft.net>,
 Eric Dumazet <edumazet@...gle.com>, Paolo Abeni <pabeni@...hat.com>
Cc: Jacky Chou <jacky_chou@...eedtech.com>,
 Jacob Keller <jacob.e.keller@...el.com>,
 "netdev@...r.kernel.org" <netdev@...r.kernel.org>
Subject: Re: [PATCH net] net: ftgmac100: fix potential NULL pointer access in
 ftgmac100_phy_disconnect

On 2025-07-31 9:22 PM, Heiner Kallweit wrote:
> On 7/31/2025 10:58 AM, Dawid Osuchowski wrote:
>> On 2025-07-30 10:23 PM, Heiner Kallweit wrote:
>>> After the call to phy_disconnect() netdev->phydev is reset to NULL.
>>
>> phy_disconnect() in its flow does not set phydev to NULL, if anywhere it happens in of_phy_deregister_fixed_link(), which already calls fixed_phy_unregister() before setting phydev to NULL
> 
> I can't follow this argumentation. Look at phy_detach(), there we have:
> if (phydev->attached_dev)
> 	phydev->attached_dev->phydev = NULL;
> So netdev->phydev is NULL after the call to phy_disconnect, provided that the PHY was attached to the netdev before.
> If use_ncsi is true, then fixed_phy_unregister() will be called with a NULL argument.

You are absolutely correct. Somehow I missed that and I apologize for 
the confusion on my side.

With that cleared up:

Reviewed-by: Dawid Osuchowski <dawid.osuchowski@...ux.intel.com>

Thanks,
Dawid
> 
> This is independent of whether of_phy_is_fixed_link() is true or not.
> Very likely it's false in the NCSI case.
> 
>>
>>  From my understanding (which very much could be wrong) of ftgmac100_probe(), these two cases are mutually exclusive. The device either uses NCSI or will use a phy based on the DT "fixed-link" property
> 
>>> So fixed_phy_unregister() would be called with a NULL pointer as argument.
>>
>> Given my analysis above, I don't think this case is possible.
>>
>> Best regards,
>> Dawid
>>> Therefore cache the phy_device before this call.
>>>
>>> Fixes: e24a6c874601 ("net: ftgmac100: Get link speed and duplex for NC-SI")
>>> Cc: stable@...r.kernel.org
>>> Signed-off-by: Heiner Kallweit <hkallweit1@...il.com>
>>> ---
>>>    drivers/net/ethernet/faraday/ftgmac100.c | 7 ++++---
>>>    1 file changed, 4 insertions(+), 3 deletions(-)
>>>
>>> diff --git a/drivers/net/ethernet/faraday/ftgmac100.c b/drivers/net/ethernet/faraday/ftgmac100.c
>>> index 5d0c09068..a863f7841 100644
>>> --- a/drivers/net/ethernet/faraday/ftgmac100.c
>>> +++ b/drivers/net/ethernet/faraday/ftgmac100.c
>>> @@ -1750,16 +1750,17 @@ static int ftgmac100_setup_mdio(struct net_device *netdev)
>>>    static void ftgmac100_phy_disconnect(struct net_device *netdev)
>>>    {
>>>        struct ftgmac100 *priv = netdev_priv(netdev);
>>> +    struct phy_device *phydev = netdev->phydev;
>>>    -    if (!netdev->phydev)
>>> +    if (!phydev)
>>>            return;
>>>    -    phy_disconnect(netdev->phydev);
>>> +    phy_disconnect(phydev);
>>>        if (of_phy_is_fixed_link(priv->dev->of_node))
>>>            of_phy_deregister_fixed_link(priv->dev->of_node);
>>>          if (priv->use_ncsi)
>>> -        fixed_phy_unregister(netdev->phydev);
>>> +        fixed_phy_unregister(phydev);
>>>    }
>>>      static void ftgmac100_destroy_mdio(struct net_device *netdev)
>>
>