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Date:   Tue, 8 Mar 2022 14:42:44 +0100
From:   Andrew Lunn <andrew@lunn.ch>
To:     Jianglei Nie <niejianglei2021@163.com>
Cc:     davem@davemloft.net, kuba@kernel.org, caihuoqing@baidu.com,
        netdev@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH] net: arc_emac: Fix use after free in arc_mdio_probe()
Message-ID: <YiddVEBJvM81u1jJ@lunn.ch>
References: <20220308111005.4953-1-niejianglei2021@163.com>
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On Tue, Mar 08, 2022 at 07:10:05PM +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 putting the bus->name in a local variable and then use
> the name in the error message without referring to bus 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..33fd63d227ef 100644
> --- a/drivers/net/ethernet/arc/emac_mdio.c
> +++ b/drivers/net/ethernet/arc/emac_mdio.c
> @@ -134,6 +134,7 @@ int arc_mdio_probe(struct arc_emac_priv *priv)
>  	struct device_node *np = priv->dev->of_node;
>  	struct mii_bus *bus;
>  	int error;
> +	const char *name = "Synopsys MII Bus";

Netdev uses reverse christmass tree, meaning you need to sort
variables longest to shortest.

I'm also wondering about the lifetime of name. name itself is a stack
variable, so it will disappear as soon as the function exits. The
string itself is in the rodata section. But is a copy made onto the
stack, or does bus->name point to the rodata?

       Andrew

>  	bus = mdiobus_alloc();
>  	if (!bus)
> @@ -142,7 +143,7 @@ int arc_mdio_probe(struct arc_emac_priv *priv)
>  	priv->bus = bus;
>  	bus->priv = priv;
>  	bus->parent = priv->dev;
> -	bus->name = "Synopsys MII Bus";
> +	bus->name = name;
>  	bus->read = &arc_mdio_read;
>  	bus->write = &arc_mdio_write;
>  	bus->reset = &arc_mdio_reset;
> @@ -167,7 +168,7 @@ int arc_mdio_probe(struct arc_emac_priv *priv)
>  	if (error) {
>  		mdiobus_free(bus);
>  		return dev_err_probe(priv->dev, error,
> -				     "cannot register MDIO bus %s\n", bus->name);
> +				     "cannot register MDIO bus %s\n", name);
>  	}
>  
>  	return 0;
> -- 
> 2.25.1
>