If bus->state is equal to MDIOBUS_ALLOCATED, mdiobus_free(bus) will free
the "bus". But bus->name is still used in the next line, which will lead
to a use after free.

We can fix it by assigning dev_err_probe() to dev_err before the bus is
freed to avoid the uaf.

Signed-off-by: Jianglei Nie <niejianglei2021@163.com>
---
 drivers/net/ethernet/arc/emac_mdio.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/drivers/net/ethernet/arc/emac_mdio.c b/drivers/net/ethernet/arc/emac_mdio.c
index 9acf589b1178..795a25c5848a 100644
--- a/drivers/net/ethernet/arc/emac_mdio.c
+++ b/drivers/net/ethernet/arc/emac_mdio.c
@@ -165,9 +165,10 @@ int arc_mdio_probe(struct arc_emac_priv *priv)
 
 	error = of_mdiobus_register(bus, priv->dev->of_node);
 	if (error) {
-		mdiobus_free(bus);
-		return dev_err_probe(priv->dev, error,
+		int dev_err = dev_err_probe(priv->dev, error,
 				     "cannot register MDIO bus %s\n", bus->name);
+		mdiobus_free(bus);
+		return dev_err;
 	}
 
 	return 0;
-- 
2.25.1

Hello,

> -----Original Message-----
> From: Jianglei Nie <niejianglei2021@163.com>
> Sent: 2022年3月3日 9:30
> To: davem@davemloft.net; kuba@kernel.org; Cai,Huoqing
> Cc: netdev@vger.kernel.org; linux-kernel@vger.kernel.org; Jianglei Nie
> Subject: [PATCH] net: arc_emac: Fix use after free in arc_mdio_probe()
> 
> If bus->state is equal to MDIOBUS_ALLOCATED, mdiobus_free(bus) will free
> the "bus". But bus->name is still used in the next line, which will lead
> to a use after free.
> 
> We can fix it by assigning dev_err_probe() to dev_err before the bus is
> freed to avoid the uaf.
Add fix log here:)
Fixes: 95b5fc03c189 ("net: arc_emac: Make use of the helper function dev_err_probe()")
> 
> Signed-off-by: Jianglei Nie <niejianglei2021@163.com>
> ---
>  drivers/net/ethernet/arc/emac_mdio.c | 5 +++--
>  1 file changed, 3 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/net/ethernet/arc/emac_mdio.c
> b/drivers/net/ethernet/arc/emac_mdio.c
> index 9acf589b1178..795a25c5848a 100644
> --- a/drivers/net/ethernet/arc/emac_mdio.c
> +++ b/drivers/net/ethernet/arc/emac_mdio.c
> @@ -165,9 +165,10 @@ int arc_mdio_probe(struct arc_emac_priv *priv)
> 
>  	error = of_mdiobus_register(bus, priv->dev->of_node);
>  	if (error) {
> -		mdiobus_free(bus);
> -		return dev_err_probe(priv->dev, error,
> +		int dev_err = dev_err_probe(priv->dev, error,
>  				     "cannot register MDIO bus %s\n", bus-
> >name);
> +		mdiobus_free(bus);
> +		return dev_err;
>  	}
> 
>  	return 0;
> --
> 2.25.1

On Thu,  3 Mar 2022 09:30:22 +0800 Jianglei Nie wrote:
> If bus->state is equal to MDIOBUS_ALLOCATED, mdiobus_free(bus) will free
> the "bus". But bus->name is still used in the next line, which will lead
> to a use after free.
> 
> We can fix it by assigning dev_err_probe() to dev_err before the bus is
> freed to avoid the uaf.
> 
> Signed-off-by: Jianglei Nie <niejianglei2021@163.com>
> ---
>  drivers/net/ethernet/arc/emac_mdio.c | 5 +++--
>  1 file changed, 3 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/net/ethernet/arc/emac_mdio.c b/drivers/net/ethernet/arc/emac_mdio.c
> index 9acf589b1178..795a25c5848a 100644
> --- a/drivers/net/ethernet/arc/emac_mdio.c
> +++ b/drivers/net/ethernet/arc/emac_mdio.c
> @@ -165,9 +165,10 @@ int arc_mdio_probe(struct arc_emac_priv *priv)
>  
>  	error = of_mdiobus_register(bus, priv->dev->of_node);
>  	if (error) {
> -		mdiobus_free(bus);
> -		return dev_err_probe(priv->dev, error,
> +		int dev_err = dev_err_probe(priv->dev, error,
>  				     "cannot register MDIO bus %s\n", bus->name);

Bus name is a constant please put it in a local variable:

	const char *name = "Synopsys MII Bus";

	...
	bus->name = name;

and then you can use name in the error message without referring to bus.

> +		mdiobus_free(bus);
> +		return dev_err;
>  	}
>  
>  	return 0;