Re: [PATCH] ASoC: Intel: Skylake: replace deprecated strncpy with strscpy

[Kees Cook <keescook@chromium.org>]

On Wed, Jul 26, 2023 at 09:12:18PM +0000, justinstitt@google.com wrote:
> `strncpy` is deprecated for use on NUL-terminated destination strings [1].
> 
> A suitable replacement is `strscpy` [2] due to the fact that it
> guarantees NUL-termination on its destination buffer argument which is
> _not_ the case for `strncpy`!
> 
> It was pretty difficult, in this case, to try and figure out whether or
> not the destination buffer was zero-initialized. If it is and this
> behavior is relied on then perhaps `strscpy_pad` is the preferred
> option here.
> 
> Kees was able to help me out and identify the following code snippet
> which seems to show that the destination buffer is zero-initialized.
> 
> |       skl = devm_kzalloc(&pci->dev, sizeof(*skl), GFP_KERNEL);
> 
> With this information, I opted for `strscpy` since padding is seemingly
> not required.

We did notice that str_elem->string is 44 bytes, but
skl->lib_info[ref_count].name is 128 bytes. If str_elem->string isn't
NUL-terminated, this can still hit an over-read condition (though
CONFIG_FORTIFY_SOURCE would have caught it both before with strncpy()
and now with strscpy()). So I assume it is expected to be
NUL-terminated?

> Also within this patch is a change to an instance of  `x > y - 1` to `x >= y`
> which tends to be more robust and readable. Consider, for instance, if `y` was
> somehow `INT_MIN`.

I'd split this change into a separate patch -- it's logically unrelated
(but seems a reasonable cleanup).

> 
> [1]: www.kernel.org/doc/html/latest/process/deprecated.html#strncpy-on-nul-terminated-strings
> [2]: manpages.debian.org/testing/linux-manual-4.8/strscpy.9.en.html
> 
> Link: https://github.com/KSPP/linux/issues/90
> Suggested-by: Kees Cook <keescook@chromium.org>
> Signed-off-by: Justin Stitt <justinstitt@google.com>
> ---
>  sound/soc/intel/skylake/skl-topology.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/sound/soc/intel/skylake/skl-topology.c b/sound/soc/intel/skylake/skl-topology.c
> index 96cfebded072..67f08ec3a2ea 100644
> --- a/sound/soc/intel/skylake/skl-topology.c
> +++ b/sound/soc/intel/skylake/skl-topology.c
> @@ -3154,12 +3154,12 @@ static int skl_tplg_fill_str_mfest_tkn(struct device *dev,
>  
>  	switch (str_elem->token) {
>  	case SKL_TKN_STR_LIB_NAME:
> -		if (ref_count > skl->lib_count - 1) {
> +		if (ref_count >= skl->lib_count) {
>  			ref_count = 0;
>  			return -EINVAL;
>  		}
>  
> -		strncpy(skl->lib_info[ref_count].name,
> +		strscpy(skl->lib_info[ref_count].name,
>  			str_elem->string,
>  			ARRAY_SIZE(skl->lib_info[ref_count].name));
>  		ref_count++;
> 
> ---
> base-commit: 0b4a9fdc9317440a71d4d4c264a5650bf4a90f3c
> change-id: 20230726-asoc-intel-skylake-remove-deprecated-strncpy-9dbcfc26040c
> 
> Best regards,
> -- 
> Justin Stitt <justinstitt@google.com>
> 

-- 
Kees Cook