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

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Date:   Thu, 14 Sep 2023 22:24:48 -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>,
        Michael Ellerman <mpe@...erman.id.au>,
        Nicholas Piggin <npiggin@...il.com>,
        Christophe Leroy <christophe.leroy@...roup.eu>,
        linux-hwmon@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org,
        linux-kernel@...r.kernel.org, linux-hardening@...r.kernel.org
Subject: Re: [PATCH] hwmon: (ibmpowernv) refactor deprecated strncpy

On Thu, Sep 14, 2023 at 11:21:06PM +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 since `buf` is already zero-initialized.
> 
> 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/ibmpowernv.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/hwmon/ibmpowernv.c b/drivers/hwmon/ibmpowernv.c
> index 594254d6a72d..57d829dbcda6 100644
> --- a/drivers/hwmon/ibmpowernv.c
> +++ b/drivers/hwmon/ibmpowernv.c
> @@ -234,7 +234,7 @@ static int get_sensor_index_attr(const char *name, u32 *index, char *attr)
>  	if (copy_len >= sizeof(buf))
>  		return -EINVAL;
>  
> -	strncpy(buf, hash_pos + 1, copy_len);
> +	strscpy(buf, hash_pos + 1, copy_len);

This is another case of precise byte copying -- this just needs to be
memcpy. Otherwise this truncates the trailing character. Imagine a name
input of "fan#2-data". "buf" wants to get "2". copy_len is 1, and
strscpy would eat it. :)

-Kees

>  
>  	err = kstrtou32(buf, 10, index);
>  	if (err)
> 
> ---
> base-commit: 3669558bdf354cd352be955ef2764cde6a9bf5ec
> change-id: 20230914-strncpy-drivers-hwmon-ibmpowernv-c-80a03f16d93a
> 
> Best regards,
> --
> Justin Stitt <justinstitt@...gle.com>
> 

-- 
Kees Cook