[PATCH net v2 2/2] net: phy: xilinx-gmii2rgmii: Restore PHY driver on remove

[Vineeth Karumanchi]

The GMII-to-RGMII converter replaces phydev->drv with a modified copy
of the attached PHY driver. This copied driver is embedded in the
converter's private data and is released when the converter is
removed.

Without a remove callback, phydev->drv continues to point to the freed
copy after the converter is unbound. A subsequent PHY operation can
dereference this stale pointer and result in a use-after-free.

With Generic KASAN enabled, unbinding only the converter while the
external PHY remains active produces the following report (abridged):

  BUG: KASAN: slab-use-after-free in phy_check_link_status+0x2d8/0x338
  Read of size 8 at addr ffff000006c359b0 by task kworker/2:0/27
  Workqueue: events_power_efficient phy_state_machine
  Call trace:
   phy_check_link_status+0x2d8/0x338
   _phy_state_machine+0xdc/0xa4c
   phy_state_machine+0x2c/0x70
   process_one_work+0x554/0xe44
   worker_thread+0x6d0/0x1180
   kthread+0x2e8/0x5d4
   ret_from_fork+0x10/0x20

  Allocated by task 55:
   ...
   devm_kmalloc+0xac/0x2ac
   xgmiitorgmii_probe+0xa0/0x37c
   mdio_probe+0x68/0xb4
   ...

  Freed by task 642:
   ...
   kfree+0x14c/0x38c
   release_nodes+0xb4/0x1e0
   devres_release_all+0x140/0x1f4
   device_unbind_cleanup+0x20/0x190
   device_release_driver_internal+0x344/0x460
   device_driver_detach+0x3c/0x54
   unbind_store+0xe0/0xf8
   ...

Store the converter private data in its own MDIO device and add a
remove callback. Restore the attached PHY's original driver only if
phydev->drv still points to the converter's copy. Hold phydev->lock to
serialize the restore against PHY callbacks that take that mutex.

Also release the device reference acquired by of_phy_find_device().

Fixes: f411a6160bd4 ("net: phy: Add gmiitorgmii converter support")
Signed-off-by: Vineeth Karumanchi <vineeth.karumanchi at amd.com>
---
Changes in v2:
- Restore the original PHY driver only if phydev->drv still points to
  the converter's copy.
- Update the comment and commit message to describe serialization against
  PHY callbacks that hold phydev->lock.

Link to v1: https://lore.kernel.org/netdev/20261001074718.3944521-1-vineeth.karumanchi@amd.com/

 drivers/net/phy/xilinx_gmii2rgmii.c | 18 ++++++++++++++++++
 1 file changed, 18 insertions(+)

diff --git a/drivers/net/phy/xilinx_gmii2rgmii.c b/drivers/net/phy/xilinx_gmii2rgmii.c
index 61f71e977a57..9dcdb91cd441 100644
--- a/drivers/net/phy/xilinx_gmii2rgmii.c
+++ b/drivers/net/phy/xilinx_gmii2rgmii.c
@@ -128,10 +128,27 @@ static int xgmiitorgmii_probe(struct mdio_device *mdiodev)
 	priv->conv_phy_drv.read_status = xgmiitorgmii_read_status;
 	priv->conv_phy_drv.set_loopback = xgmiitorgmii_set_loopback;
 	priv->phy_dev->drv = &priv->conv_phy_drv;
+	mdiodev_set_drvdata(mdiodev, priv);
 
 	return 0;
 }
 
+static void xgmiitorgmii_remove(struct mdio_device *mdiodev)
+{
+	struct gmii2rgmii *priv = mdiodev_get_drvdata(mdiodev);
+
+	/*
+	 * Restore the original driver only if the converter's copy is still
+	 * installed. Serialize against PHY callbacks that hold phydev->lock.
+	 */
+	mutex_lock(&priv->phy_dev->lock);
+	if (priv->phy_dev->drv == &priv->conv_phy_drv)
+		priv->phy_dev->drv = priv->phy_drv;
+	mutex_unlock(&priv->phy_dev->lock);
+
+	put_device(&priv->phy_dev->mdio.dev);
+}
+
 static const struct of_device_id xgmiitorgmii_of_match[] = {
 	{ .compatible = "xlnx,gmii-to-rgmii-1.0" },
 	{},
@@ -140,6 +157,7 @@ MODULE_DEVICE_TABLE(of, xgmiitorgmii_of_match);
 
 static struct mdio_driver xgmiitorgmii_driver = {
 	.probe	= xgmiitorgmii_probe,
+	.remove	= xgmiitorgmii_remove,
 	.mdiodrv.driver = {
 		.name = "xgmiitorgmii",
 		.of_match_table = xgmiitorgmii_of_match,
-- 
2.43.0