linux-hardening - Re: [PATCH v3] hwmon: refactor deprecated strncpy

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Date: Sat, 23 Sep 2023 20:18:29 -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 v3] hwmon: refactor deprecated strncpy

On Thu, Sep 21, 2023 at 05:41:46AM +0000, Justin Stitt wrote:
> `strncpy` is deprecated for use on NUL-terminated destination strings [1].
> 
> Let's refactor this kcalloc() + strncpy() into a kmemdup_nul() which has
> more obvious behavior and is less error prone.
> 
> To avoid truncating the last byte supply `...length + 1` to
> kmemdup_nul() as `element->string.length` does not account for the
> trailing null as made obvious from it's definition (and associated
> comment):
> |       u32 length;	/* # of bytes in string, excluding trailing null */
> 
> ... this is precisely what the original kcalloc invocation did as well.
> 
> Link: https://www.kernel.org/doc/html/latest/process/deprecated.html#strncpy-on-nul-terminated-strings [1]
> Link: https://github.com/KSPP/linux/issues/90
> Cc: linux-hardening@...r.kernel.org
> Signed-off-by: Justin Stitt <justinstitt@...gle.com>
> ---
> Changes in v3:
> - refactor to use kmemdup_nul() (thanks Thomas and Kees)
> - change commit msg to reflect ^
> - rebase onto 2cf0f71562387282
> - Link to v2: https://lore.kernel.org/r/20230919-strncpy-drivers-hwmon-acpi_power_meter-c-v2-1-8348432d6442@google.com
> 
> Changes in v2:
> - use memcpy over strscpy (thanks Kees)
> - Link to v1: https://lore.kernel.org/r/20230914-strncpy-drivers-hwmon-acpi_power_meter-c-v1-1-905297479fe8@google.com
> ---
>  drivers/hwmon/acpi_power_meter.c | 5 ++---
>  1 file changed, 2 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/hwmon/acpi_power_meter.c b/drivers/hwmon/acpi_power_meter.c
> index fa28d447f0df..146c4f09c897 100644
> --- a/drivers/hwmon/acpi_power_meter.c
> +++ b/drivers/hwmon/acpi_power_meter.c
> @@ -796,14 +796,13 @@ static int read_capabilities(struct acpi_power_meter_resource *resource)
>  			goto error;
>  		}
>  
> -		*str = kcalloc(element->string.length + 1, sizeof(u8),
> -			       GFP_KERNEL);
> +		*str = kmemdup_nul(element->string.pointer, element->string.length + 1,
> +						 GFP_KERNEL);

kmemdup_nul() already adds the "+ 1", so it's not needed here.

For v4, please improve the Subject along the lines of "...: Replace
open-coded kmemdup_nul"

-Kees

>  		if (!*str) {
>  			res = -ENOMEM;
>  			goto error;
>  		}
>  
> -		strncpy(*str, element->string.pointer, element->string.length);
>  		str++;
>  	}
>  
> 
> ---
> base-commit: 2cf0f715623872823a72e451243bbf555d10d032
> change-id: 20230914-strncpy-drivers-hwmon-acpi_power_meter-c-c9f2d8053bef
> 
> Best regards,
> --
> Justin Stitt <justinstitt@...gle.com>
> 

-- 
Kees Cook