[PATCH net-next v13 04/11] net: mdio: realtek-rtl9300: suppress sysfs bind/unbind attributes

[Markus Stockhausen <markus.stockhausen@gmx.de>]

The Realtek MDIO driver relies on devm managed resources for lifetime
management and allocates its internal central controller structure
otto_emdio_priv via devm_kzalloc(). This can lead to a use-after-free
in the following sitauation:

- If the driver is removed or unbound via sysfs the controller devm
  cleanup is triggered immediately. A corrupt devicetree might produce
  a load failure and a cleanup too.
- However the MDIO buses are allocated with devm_mdiobus_alloc_size()
  and are reference counted. So if a MAC driver still holds a reference
  to a PHY device on the bus, the mii_bus will remain alive.
- Any action that is run for the buses afterwards still relies on the
  availability of the central priv structure. This will access already
  freed memory.

There are multiple mitigations to choose from

- Clone all controller->priv data into the bus->priv structure.
- disable sysfs bind/unbind attributes
- Check for "bus->state != MDIOBUS_REGISTERED" in all callbacks

This SoC-integrated MDIO controller cannot be physically hot-plugged
and manual unbinding gives no benefit. The driver is usually
integrated into the kernel and not loaded as a module. Avoid a
complex driver rewrite (data duplication) for now and set
suppress_bind_attrs to true to prevent unbinding via sysfs. This
mitigates issues for the known use cases.

Signed-off-by: Markus Stockhausen <markus.stockhausen@gmx.de>
---
 drivers/net/mdio/mdio-realtek-rtl9300.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/drivers/net/mdio/mdio-realtek-rtl9300.c b/drivers/net/mdio/mdio-realtek-rtl9300.c
index 73ac5fdcd267..1873928b362d 100644
--- a/drivers/net/mdio/mdio-realtek-rtl9300.c
+++ b/drivers/net/mdio/mdio-realtek-rtl9300.c
@@ -830,6 +830,7 @@ static struct platform_driver otto_emdio_driver = {
 	.driver = {
 		.name = "mdio-rtl9300",
 		.of_match_table = otto_emdio_ids,
+		.suppress_bind_attrs = true,
 	},
 };
 
-- 
2.55.0