[PATCH net] net: macb: fix NULL pointer dereference on unbind with fixed-link

[Vineeth Karumanchi <vineeth.karumanchi@amd.com>]

When the device tree describes a fixed-link and has no "mdio" child
node, macb_mii_init() returns early without allocating the MDIO bus,
leaving bp->mii_bus as NULL.

Two cleanup paths then dereference this NULL bus:

1. On driver unbind, macb_remove() unconditionally calls
   mdiobus_unregister(bp->mii_bus), which oopses:

  Unable to handle kernel NULL pointer dereference at virtual address 00000000000004a8
  pc : mdiobus_unregister+0x14/0xa4
  lr : macb_remove+0x38/0xa4
  Call trace:
   mdiobus_unregister+0x14/0xa4 (P)
   macb_remove+0x38/0xa4
   platform_remove+0x20/0x30
   device_release_driver_internal+0x1c8/0x224
   unbind_store+0xb4/0xbc

2. On the probe error path in macb_probe(), reached when
   macb_mii_init() has succeeded but a subsequent step fails, the
   err_out_unregister_mdio label runs the same unconditional cleanup.

mdiobus_unregister() and mdiobus_free() do not guard against a NULL
bus, so guard the calls in both macb_remove() and the probe error
path.

Fixes: d0c3601f2c4e ("net: macb: Avoid 20s boot delay by skipping MDIO bus registration for fixed-link PHY")
Signed-off-by: Vineeth Karumanchi <vineeth.karumanchi@amd.com>
---
 drivers/net/ethernet/cadence/macb_main.c | 12 ++++++++----
 1 file changed, 8 insertions(+), 4 deletions(-)

diff --git a/drivers/net/ethernet/cadence/macb_main.c b/drivers/net/ethernet/cadence/macb_main.c
index 76ee4f506033..88eebe187eac 100644
--- a/drivers/net/ethernet/cadence/macb_main.c
+++ b/drivers/net/ethernet/cadence/macb_main.c
@@ -5971,8 +5971,10 @@ static int macb_probe(struct platform_device *pdev)
 	macb_free_tieoff(bp);
 
 err_out_unregister_mdio:
-	mdiobus_unregister(bp->mii_bus);
-	mdiobus_free(bp->mii_bus);
+	if (bp->mii_bus) {
+		mdiobus_unregister(bp->mii_bus);
+		mdiobus_free(bp->mii_bus);
+	}
 
 err_out_phy_exit:
 	phy_exit(bp->phy);
@@ -6001,8 +6003,10 @@ static void macb_remove(struct platform_device *pdev)
 		unregister_netdev(netdev);
 		macb_free_tieoff(bp);
 		phy_exit(bp->phy);
-		mdiobus_unregister(bp->mii_bus);
-		mdiobus_free(bp->mii_bus);
+		if (bp->mii_bus) {
+			mdiobus_unregister(bp->mii_bus);
+			mdiobus_free(bp->mii_bus);
+		}
 
 		device_set_wakeup_enable(&bp->pdev->dev, 0);
 		cancel_delayed_work_sync(&bp->tx_lpi_work);
-- 
2.43.0