netdev - Re: [PATCH net-next v2] net: phy: Fix suspicious rcu

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Message-ID: <CANn89i+PM5JLdN1meKH_moPe88F_=Nsb3in+g-ZK5tiH4PH5GA@mail.gmail.com>
Date: Fri, 17 Jan 2025 20:06:28 +0100
From: Eric Dumazet <edumazet@...gle.com>
To: Kory Maincent <kory.maincent@...tlin.com>
Cc: "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>, Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>
Subject: Re: [PATCH net-next v2] net: phy: Fix suspicious rcu_dereference usage

On Fri, Jan 17, 2025 at 6:36 PM Kory Maincent <kory.maincent@...tlin.com> wrote:
>
> The phy_detach function can be called with or without the rtnl lock held.
> When the rtnl lock is not held, using rtnl_dereference() triggers a
> warning due to the lack of lock context.
>
> Add an rcu_read_lock() to ensure the lock is acquired and to maintain
> synchronization.
>
> Tested-by: Claudiu Beznea <claudiu.beznea.uj@...renesas.com>
> Reported-by: Claudiu Beznea <claudiu.beznea.uj@...renesas.com>
> Closes: https://lore.kernel.org/netdev/4c6419d8-c06b-495c-b987-d66c2e1ff848@tuxon.dev/
> Fixes: 35f7cad1743e ("net: Add the possibility to support a selected hwtstamp in netdevice")
> Signed-off-by: Kory Maincent <kory.maincent@...tlin.com>
> ---
>
> Changes in v2:
> - Add a missing ;
> ---
>  drivers/net/phy/phy_device.c | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/net/phy/phy_device.c b/drivers/net/phy/phy_device.c
> index 5b34d39d1d52..3eeee7cba923 100644
> --- a/drivers/net/phy/phy_device.c
> +++ b/drivers/net/phy/phy_device.c
> @@ -2001,12 +2001,14 @@ void phy_detach(struct phy_device *phydev)
>         if (dev) {
>                 struct hwtstamp_provider *hwprov;
>
> -               hwprov = rtnl_dereference(dev->hwprov);
> +               rcu_read_lock();
> +               hwprov = rcu_dereference(dev->hwprov);
>                 /* Disable timestamp if it is the one selected */
>                 if (hwprov && hwprov->phydev == phydev) {
>                         rcu_assign_pointer(dev->hwprov, NULL);
>                         kfree_rcu(hwprov, rcu_head);
>                 }
> +               rcu_read_unlock();
>
>                 phydev->attached_dev->phydev = NULL;
>                 phydev->attached_dev = NULL;
> --
> 2.34.1
>

If not protected by RTNL, what prevents two threads from calling this
function at the same time,
thus attempting to kfree_rcu() the same pointer twice ?