[alsa-devel] [PATCH 1/2] ALSA: control: add dimension validator for userspace element

[Takashi Iwai]

At Fri, 29 May 2015 17:58:56 +0900,
Takashi Sakamoto wrote:
> 
> The 'dimen' field in struct snd_ctl_elem_info is used to compose
> all of channels in the control element as multi-dimensional matrix.
> The field has four members. Each member represents the number in
> each dimension level. For example, if the channels consist of typical
> two dimensional matrix, the dimen[0] represents the number of rows
> and dimen[1] represents the number of columns, or vise-versa.
> 
> The total elements in the matrix should be within the number of
> channels in the control element, while current implementation has
> no validator of this information. In a view of userspace applications,
> the information must be valid so that it cannot cause any bugs such as
> buffer-over-run.
> 
> This commit adds a validator of dimen information for userspace
> applications which add new control elements. When they add the elements
> with wrong dimen information, they receive EINVAL.
> 
> Signed-off-by: Takashi Sakamoto <o-takashi at sakamocchi.jp>
> ---
>  sound/core/control.c | 21 +++++++++++++++++++++
>  1 file changed, 21 insertions(+)
> 
> diff --git a/sound/core/control.c b/sound/core/control.c
> index 196a6fe..9b77afd 100644
> --- a/sound/core/control.c
> +++ b/sound/core/control.c
> @@ -805,6 +805,25 @@ static int snd_ctl_elem_list(struct snd_card *card,
>  	return 0;
>  }
>  
> +static bool validate_dimen(struct snd_ctl_elem_info *info)
> +{
> +	unsigned int elements;
> +	unsigned int i;
> +
> +	/*
> +	 * When drivers don't use dimen field, this value is zero and pass the
> +	 * validation. Else, calculated number of elements is validated.
> +	 */
> +	elements = info->dimen.d[0];
> +	for (i = 1; i < ARRAY_SIZE(info->dimen.d); ++i) {
> +		if (info->dimen.d[i] == 0)
> +			break;

It'd be better to have a value sanity check, also for avoiding an
overflow.  A simple one like below should suffice:

	if (info->dimens.d[i] > info->count)
		return false;

Takashi


> +		elements *= info->dimen.d[i];
> +	}
> +
> +	return elements <= info->count;
> +}
> +
>  static int snd_ctl_elem_info(struct snd_ctl_file *ctl,
>  			     struct snd_ctl_elem_info *info)
>  {
> @@ -1272,6 +1291,8 @@ static int snd_ctl_elem_add(struct snd_ctl_file *file,
>  	if (info->count < 1 ||
>  	    info->count > max_value_counts[info->type])
>  		return -EINVAL;
> +	if (!validate_dimen(info))
> +		return -EINVAL;
>  	private_size = value_sizes[info->type] * info->count;
>  
>  	/*
> -- 
> 2.1.4
>