Subject: Re: Lockup in phy_probe() for MDIO device (Broadcom's switch)

From: "Russell King (Oracle)" <[email-masked]>
Date: Thu, 30 Sep 2021 14:07:44 +0100

On Thu, Sep 30, 2021 at 02:51:40PM +0200, Rafał Miłecki wrote:
> On 30.09.2021 14:30, Russell King (Oracle) wrote:
> > > It's actually OpenWrt's downstream swconfig-based b53 driver that
> > > matches this device.
> > > 
> > > I'm confused as downstream b53_mdio.c calls phy_driver_register(). Why
> > > does it match MDIO device then? I thought MDIO devices should be
> > > matches only with drivers using mdio_driver_register().
> > 
> > Note that I've no idea what he swconfig-based b53 driver looks like,
> > I don't have the source for that to hand.
> > 
> > If it calls phy_driver_register(), then it is registering a driver for
> > a MDIO device wrapped in a struct phy_device. If this driver has a
> > .of_match_table member set, then this is wrong - the basic rule is
> > 
> > 	PHY drivers must never match using DT compatibles.
> > 
> > because this is exactly what will occur - it bypasses the check that
> > the mdio_device being matched is in fact wrapped by a struct phy_device,
> > and we will access members of the non-existent phy_device, including
> > the "uninitialised" mutex.
> > 
> > If the swconfig-based b53 driver does want to bind to a phy_device based
> > DT node, then it needs to match using either a custom .match_phy_device
> > method in the PHY driver, or it needs to match using the PHY IDs.
> 
> Sorry, I should have linked downstream b53_mdio.c . It's:
> https://git.openwrt.org/?p=openwrt/openwrt.git;a=blob;f=target/linux/generic/files/drivers/net/phy/b53/b53_mdio.c;h=98cdbffe73c7354f4401389dfcc96014bff62588;hb=HEAD

Yes, I just found a version of the driver

> You can see that is *uses* of_match_table.
> 
> What about refusing bugged drivers like above b53 with something like:

That will break all the MDIO based DSA and other non-PHY drivers,
sorry.

I suppose we could detect if the driver has the MDIO_DEVICE_IS_PHY flag
set, and reject any device that does not have MDIO_DEVICE_IS_PHY set:

diff --git a/drivers/net/phy/mdio_bus.c b/drivers/net/phy/mdio_bus.c
index 53f034fc2ef7..7bc06126ce10 100644
--- a/drivers/net/phy/mdio_bus.c
+++ b/drivers/net/phy/mdio_bus.c
@@ -939,6 +939,12 @@ EXPORT_SYMBOL_GPL(mdiobus_modify);
 static int mdio_bus_match(struct device *dev, struct device_driver *drv)
 {
 	struct mdio_device *mdio = to_mdio_device(dev);
+	struct mdio_driver *mdiodrv = to_mdio_driver(drv);
+
+	/* Both the driver and device must type-match */
+	if (!(mdiodrv->mdiodrv.flags & MDIO_DEVICE_IS_PHY) ==
+	    !(mdio->flags & MDIO_DEVICE_FLAG_PHY))
+		return 0;
 
 	if (of_driver_match_device(dev, drv))
 		return 1;

In other words, the driver's state of the MDIO_DEVICE_IS_PHY flag
must match the device's MDIO_DEVICE_FLAG_PHY flag before we attempt
any matches.

If that's not possible, then we need to prevent phylib drivers from
using .of_match_table:

diff --git a/drivers/net/phy/phy_device.c b/drivers/net/phy/phy_device.c
index e62f7a232307..dacf0b31b167 100644
--- a/drivers/net/phy/phy_device.c
+++ b/drivers/net/phy/phy_device.c
@@ -2501,6 +2501,16 @@ int phy_driver_register(struct phy_driver *new_driver, struct module *owner)
 		return -EINVAL;
 	}
 
+	/* PHYLIB device drivers must not match using a DT compatible table
+	 * as this bypasses our checks that the mdiodev that is being matched
+	 * is backed by a struct phy_device. If such a case happens, we will
+	 * make out-of-bounds accesses and lockup in phydev->lock.
+	 */
+	if (WARN(new_driver->mdiodrv.driver.of_match_table,
+		 "%s: driver must not provide a DT match table\n",
+		 new_driver->name))
+		return -EINVAL;
+
 	new_driver->mdiodrv.flags |= MDIO_DEVICE_IS_PHY;
 	new_driver->mdiodrv.driver.name = new_driver->name;
 	new_driver->mdiodrv.driver.bus = &mdio_bus_type;

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