linux-hardening - Re: [PATCH] drivers: misc: ti-st: replace deprecated strncpy with strscpy

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Date: Mon, 2 Oct 2023 11:05:14 -0700
From: Kees Cook <keescook@...omium.org>
To: Justin Stitt <justinstitt@...gle.com>
Cc: Arnd Bergmann <arnd@...db.de>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	linux-kernel@...r.kernel.org, linux-hardening@...r.kernel.org
Subject: Re: [PATCH] drivers: misc: ti-st: replace deprecated strncpy with
 strscpy

On Wed, Sep 27, 2023 at 06:17:34AM +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 both `kim_data->dev_name` and `kim_gdata->dev_name` to be
> NUL-terminated.
> 
> `kim_data->dev_name` seems to not require NUL-padding.
> 
> `kim_gdata` is already zero-allocated and as such does not require
> NUL-padding:
> |       kim_gdata = kzalloc(sizeof(struct kim_data_s), GFP_KERNEL);
> 
> Considering the above, 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.
> ---
>  drivers/misc/ti-st/st_kim.c | 5 +++--
>  1 file changed, 3 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/misc/ti-st/st_kim.c b/drivers/misc/ti-st/st_kim.c
> index fe682e0553b2..a034b5d8b585 100644
> --- a/drivers/misc/ti-st/st_kim.c
> +++ b/drivers/misc/ti-st/st_kim.c
> @@ -590,7 +590,7 @@ static ssize_t store_dev_name(struct device *dev,
>  {
>  	struct kim_data_s *kim_data = dev_get_drvdata(dev);
>  	pr_debug("storing dev name >%s<", buf);
> -	strncpy(kim_data->dev_name, buf, count);
> +	strscpy(kim_data->dev_name, buf, count);

This is again the early truncation issue. This should use
sizeof(kim_data->dev_name). "buf" will be %NUL terminated already.

>  	pr_debug("stored dev name >%s<", kim_data->dev_name);
>  	return count;
>  }
> @@ -751,7 +751,8 @@ static int kim_probe(struct platform_device *pdev)
>  	}
>  
>  	/* copying platform data */
> -	strncpy(kim_gdata->dev_name, pdata->dev_name, UART_DEV_NAME_LEN);
> +	strscpy(kim_gdata->dev_name, pdata->dev_name,
> +		sizeof(kim_gdata->dev_name));

This looks correct.

>  	kim_gdata->flow_cntrl = pdata->flow_cntrl;
>  	kim_gdata->baud_rate = pdata->baud_rate;
>  	pr_info("sysfs entries created\n");
> 
> ---
> base-commit: 6465e260f48790807eef06b583b38ca9789b6072
> change-id: 20230927-strncpy-drivers-misc-ti-st-st_kim-c-e1663a211a0c
> 
> Best regards,
> --
> Justin Stitt <justinstitt@...gle.com>
> 

-- 
Kees Cook