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

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Message-ID: <aJSf0JaBl4cKphFi@pengutronix.de>
Date: Thu, 7 Aug 2025 14:45:04 +0200
From: Oleksij Rempel <o.rempel@...gutronix.de>
To: "Russell King (Oracle)" <linux@...linux.org.uk>
Cc: Xu Yang <xu.yang_2@....com>, Andrew Lunn <andrew@...n.ch>,
	hkallweit1@...il.com, 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 12:47:02PM +0100, Russell King (Oracle) wrote:
> 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.

Hm, I guess, with this change there will be a subtile regression.
In case of an external PHYs the ax88772_init_phy() is using PHYlib to
suspend the internal PHY.

May be:
  priv->mdio->phy_mask = ~(BIT(priv->phy_addr) | BIT(AX_EMBD_PHY_ADDR));

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