linux-kernel - Re: [RESEND] net: phy: fix NULL pointer dereference in phy_polling

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Message-ID: <aJSSNg4aZNfoqqZh@shell.armlinux.org.uk>
Date: Thu, 7 Aug 2025 12:47:02 +0100
From: "Russell King (Oracle)" <linux@...linux.org.uk>
To: Xu Yang <xu.yang_2@....com>
Cc: Andrew Lunn <andrew@...n.ch>, hkallweit1@...il.com,
	o.rempel@...gutronix.de, pabeni@...hat.com, netdev@...r.kernel.org,
	imx@...ts.linux.dev, linux-kernel@...r.kernel.org
Subject: Re: [RESEND] net: phy: fix NULL pointer dereference in
 phy_polling_mode()

On Thu, Aug 07, 2025 at 07:21:46PM +0800, Xu Yang wrote:
> Hi Russell and Andrew,
> 
> With more debug on why asix_devices.c driver is creating so many mdio devices,
> I found the mdio->phy_mask setting may be missing.

mdio->phy_mask is really only a workaround/optimisation to prevent
the automatic scanning of the MDIO bus.

If we know for certain that we're only interested in a PHY at a
certain set of addresses, then it's appropriate to tell the MDIO/phylib
layer not to bother scanning the other addresses, but this will mean
if the driver uses e.g. phy_find_first(), it will find the first PHY
amongst those that phy_mask allows to be scanned, rather than the first
on the bus.

In other words... it's dependent on the driver.

> diff --git a/drivers/net/usb/asix_devices.c b/drivers/net/usb/asix_devices.c
> index 9b0318fb50b5..9fba1cb17134 100644
> --- a/drivers/net/usb/asix_devices.c
> +++ b/drivers/net/usb/asix_devices.c
> @@ -676,6 +676,7 @@ static int ax88772_init_mdio(struct usbnet *dev)
>         priv->mdio->read = &asix_mdio_bus_read;
>         priv->mdio->write = &asix_mdio_bus_write;
>         priv->mdio->name = "Asix MDIO Bus";
> +       priv->mdio->phy_mask = ~BIT(priv->phy_addr);
>         /* mii bus name is usb-<usb bus number>-<usb device number> */
>         snprintf(priv->mdio->id, MII_BUS_ID_SIZE, "usb-%03d:%03d",
>                  dev->udev->bus->busnum, dev->udev->devnum);
> 
> Is this the right thing to do?

If we're only expecting a MDIO device at priv->phy_addr, then I
guess it's fine. Looking at the driver, I don't understand the
mixture of dev->mii.* and priv->mdio->*, and sadly I don't have
time to look in depth at this driver to work that out.

-- 
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