linux-hardening - Re: [PATCH] net: mdio: replace deprecated strncpy with strscpy

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Date: Mon, 16 Oct 2023 12:40:34 -0700
From: Kees Cook <keescook@...omium.org>
To: Justin Stitt <justinstitt@...gle.com>
Cc: Andrew Lunn <andrew@...n.ch>, Heiner Kallweit <hkallweit1@...il.com>,
	Russell King <linux@...linux.org.uk>,
	"David S. Miller" <davem@...emloft.net>,
	Eric Dumazet <edumazet@...gle.com>,
	Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>,
	netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-hardening@...r.kernel.org
Subject: Re: [PATCH] net: mdio: replace deprecated strncpy with strscpy

On Thu, Oct 12, 2023 at 09:43:02PM +0000, Justin Stitt wrote:
> strncpy() is deprecated for use on NUL-terminated destination strings
> [1] and as such we should prefer more robust and less ambiguous string
> interfaces.
> 
> We expect new_bus->id to be NUL-terminated but not NUL-padded based on
> its prior assignment through snprintf:
> |       snprintf(new_bus->id, MII_BUS_ID_SIZE, "gpio-%x", bus_id);
> 
> Due to this, a suitable replacement is `strscpy` [2] due to the fact
> that it guarantees NUL-termination on the destination buffer without
> unnecessarily NUL-padding.
> 
> Link: https://www.kernel.org/doc/html/latest/process/deprecated.html#strncpy-on-nul-terminated-strings [1]
> Link: https://manpages.debian.org/testing/linux-manual-4.8/strscpy.9.en.html [2]
> Link: https://github.com/KSPP/linux/issues/90
> Cc: linux-hardening@...r.kernel.org
> Signed-off-by: Justin Stitt <justinstitt@...gle.com>
> ---
> Note: build-tested only.
> 
> Found with: $ rg "strncpy\("
> ---
>  drivers/net/mdio/mdio-gpio.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/net/mdio/mdio-gpio.c b/drivers/net/mdio/mdio-gpio.c
> index 0fb3c2de0845..a1718d646504 100644
> --- a/drivers/net/mdio/mdio-gpio.c
> +++ b/drivers/net/mdio/mdio-gpio.c
> @@ -125,7 +125,7 @@ static struct mii_bus *mdio_gpio_bus_init(struct device *dev,
>  	if (bus_id != -1)
>  		snprintf(new_bus->id, MII_BUS_ID_SIZE, "gpio-%x", bus_id);
>  	else
> -		strncpy(new_bus->id, "gpio", MII_BUS_ID_SIZE);
> +		strscpy(new_bus->id, "gpio", sizeof(new_bus->id));

struct mii_bus {
	...
        char id[MII_BUS_ID_SIZE];

Yup, looks good. (I wonder about changing to sizeof() in the snprintf()
above it, but for a strscpy() refactor, I think this is fine.)

Reviewed-by: Kees Cook <keescook@...omium.org>

-- 
Kees Cook