[alsa-devel] [PATCH 2/3] ALSA: control: add dimension validator for userspace element

[Takashi Iwai]

On Fri, 01 Jul 2016 06:15:10 +0200,
Takashi Sakamoto wrote:
> 
> The 'dimen' field in struct snd_ctl_elem_info is used to compose all of
> members in the element as multi-dimensional matrix. The field has four
> members. Each member represents the width in each dimension level by
> element member unit. For example, if the members 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 members in the matrix should be within the number of members in
> the 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 dimension information for userspace
> applications which add new element sets. When they add the element sets
> with wrong dimension information, they receive -EINVAL.
> 
> Signed-off-by: Takashi Sakamoto <o-takashi at sakamocchi.jp>
> ---
>  sound/core/control.c | 39 +++++++++++++++++++++++++++++++++++++++
>  1 file changed, 39 insertions(+)
> 
> diff --git a/sound/core/control.c b/sound/core/control.c
> index a85d455..eeb080f 100644
> --- a/sound/core/control.c
> +++ b/sound/core/control.c
> @@ -805,6 +805,43 @@ static int snd_ctl_elem_list(struct snd_card *card,
>  	return 0;
>  }
>  
> +static bool validate_element_member_dimension(struct snd_ctl_elem_info *info)
> +{
> +	unsigned int members;
> +	unsigned int i = 0;
> +
> +	/* The value for each level should be zero or positive. */
> +	if (info->dimen.d[0] < 0)
> +		return false;
> +	members = info->dimen.d[0];
> +
> +	if (members > 0) {
> +		for (++i; i < ARRAY_SIZE(info->dimen.d); ++i) {
> +			if (info->dimen.d[i] < 0)
> +				return false;
> +			if (info->dimen.d[i] == 0)
> +				break;
> +
> +			/* Prevention of division by zero, for safe. */
> +			if (members == 0)
> +				return false;
> +			/* Prevent arithmetic overflow. */
> +			if (info->dimen.d[i] > UINT_MAX / members)
> +				return false;
> +
> +			members *= info->dimen.d[i];
> +		}
> +	}
> +
> +	/* The rest of level should be zero. */
> +	for (++i; i < ARRAY_SIZE(info->dimen.d); ++i) {
> +		if (info->dimen.d[i] != 0)
> +			return false;
> +	}
> +
> +	return members <= info->count;

Well, since the dimen is 16bit short, it's easier to check the
overflow just by comparing the value with info->count at each time
(supposing info->count being the right value).

That is, something like this:

	/* If no dimension is given, it's OK */
	if (!info->dimen.d[0])
		return true;

	elements = 1;
	for (i = 0; i < ARRAY_SIZE(info->dimen.d); i++) {
		if (info->dimen.d[i] < 0)
			return false;
		if (!info->dimen.d[i])
			break;
		elements *= info->dimen.d[i];
		/* overflow? */
		if (elements > info->count)
			return false;
	}

	/* The rest should be all zero */
	for (i++; i < ARRAY_SIZE(info->dimen.d); i++)
		if (info->dimen.d[i])
			return false;

	return true;


thanks,

Takashi