netdev - Re: [PATCH] net: arc_emac: Fix use after free in arc_mdio

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Date:   Thu, 3 Mar 2022 08:04:46 -0800
From:   Jakub Kicinski <kuba@...nel.org>
To:     Jianglei Nie <niejianglei2021@....com>
Cc:     davem@...emloft.net, caihuoqing@...du.com, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] net: arc_emac: Fix use after free in arc_mdio_probe()

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@....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;