Re: [PATCH v2 3/6] ALSA: control: Apply sanity check of input values for user elements

[Takashi Sakamoto <o-takashi@xxxxxxxxxxxxx>]

Hi,

On Fri, Jun 14, 2024 at 05:37:12PM +0200, Takashi Iwai wrote:
> Although we have already a mechanism for sanity checks of input values
> for control writes, it's not applied unless the kconfig
> CONFIG_SND_CTL_INPUT_VALIDATION is set due to the performance reason.
> Nevertheless, it still makes sense to apply the check for user
> elements despite of its cost, as that's the only way to filter out the
> invalid values; the user controls are handled solely in ALSA core
> code, and there is no corresponding driver, after all.
> 
> This patch enables the input value validation for user control
> elements no matter whether CONFIG_SND_CTL_INPUT_VALIDATION is set or
> not.  The kselftest will be happier with this change, as the incorrect
> values will be bailed out now with errors.
> 
> For other normal controls, the check is applied still only when
> CONFIG_SND_CTL_INPUT_VALIDATION is set.
> 
> Reported-by: Paul Menzel <pmenzel@xxxxxxxxxxxxx>
> Closes: https://lore.kernel.org/r/1d44be36-9bb9-4d82-8953-5ae2a4f09405@xxxxxxxxxxxxx
> Reviewed-by: Jaroslav Kysela <perex@xxxxxxxx>
> Reviewed-by: Mark Brown <broonie@xxxxxxxxxx>
> Signed-off-by: Takashi Iwai <tiwai@xxxxxxx>
> ---
>  sound/core/control.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/sound/core/control.c b/sound/core/control.c
> index fb0c60044f7b..50890983d7e2 100644
> --- a/sound/core/control.c
> +++ b/sound/core/control.c
> @@ -1317,7 +1317,8 @@ static int snd_ctl_elem_write(struct snd_card *card, struct snd_ctl_file *file,
>  	snd_ctl_build_ioff(&control->id, kctl, index_offset);
>  	result = snd_power_ref_and_wait(card);
>  	/* validate input values */
> -	if (IS_ENABLED(CONFIG_SND_CTL_INPUT_VALIDATION) && !result) {
> +	if ((IS_ENABLED(CONFIG_SND_CTL_INPUT_VALIDATION) ||
> +	     (vd->access & SNDRV_CTL_ELEM_ACCESS_USER)) && !result) {
>  		struct snd_ctl_elem_info info;
>  
>  		memset(&info, 0, sizeof(info));

In my opinion, the validation in 'snd_ctl_elem_user_put()' is preferable
instead. In the function, it is free to access to 'struct
user_element.info' for the validation.

In the commit coment, I can see "that's the only way to filter out the
invalid values", however it not so good idea, since the ALSA control core
function loses transparency against control elements somehow.
Furthermore, I can see "there is no corresponding driver", however it is
suspicious somehow. It would be smart to charge the validation
implementation for user-defined control element set if forcing it.


Regards

Takashi Sakamoto