linux-hardening - Re: [PATCH] hwmon: refactor deprecated strncpy

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Date:   Thu, 14 Sep 2023 22:17:31 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Justin Stitt <justinstitt@...gle.com>
Cc:     Jean Delvare <jdelvare@...e.com>,
        Guenter Roeck <linux@...ck-us.net>,
        linux-hwmon@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-hardening@...r.kernel.org
Subject: Re: [PATCH] hwmon: refactor deprecated strncpy

On Thu, Sep 14, 2023 at 10:50:15PM +0000, Justin Stitt wrote:
> `strncpy` is deprecated for use on NUL-terminated destination strings [1].
> 
> We should prefer more robust and less ambiguous string interfaces.
> 
> 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>
> ---
>  drivers/hwmon/acpi_power_meter.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/hwmon/acpi_power_meter.c b/drivers/hwmon/acpi_power_meter.c
> index fa28d447f0df..9bafe7c85ff7 100644
> --- a/drivers/hwmon/acpi_power_meter.c
> +++ b/drivers/hwmon/acpi_power_meter.c
> @@ -803,7 +803,7 @@ static int read_capabilities(struct acpi_power_meter_resource *resource)
>  			goto error;
>  		}
>  
> -		strncpy(*str, element->string.pointer, element->string.length);
> +		strscpy(*str, element->string.pointer, element->string.length);

This will truncate the destination by 1 training character. This is better
replaced with just memcpy() (all the size information has already been
calculated and %NUL termination space made in the kcalloc() above it.

-Kees

>  		str++;
>  	}
>  
> 
> ---
> base-commit: 3669558bdf354cd352be955ef2764cde6a9bf5ec
> change-id: 20230914-strncpy-drivers-hwmon-acpi_power_meter-c-c9f2d8053bef
> 
> Best regards,
> --
> Justin Stitt <justinstitt@...gle.com>
> 

-- 
Kees Cook