Re: [PATCH 05/10] backlight: qcom-wled: Fix off-by-one maximum with default num_strings

[Daniel Thompson <daniel.thompson@linaro.org>]

On Mon, Oct 04, 2021 at 09:27:36PM +0200, Marijn Suijten wrote:
> When not specifying num-strings in the DT the default is used, but +1 is
> added to it which turns wled3 into 4 and wled4/5 into 5 strings instead
> of 3 and 4 respectively, causing out of bounds reads and register
> read/writes.  This +1 exists for a deficiency in the DT parsing code,
> and is simply omitted entirely - solving this oob issue - by allowing
> one extra iteration of the wled_var_cfg function parsing this particular
> property.
> 
> Fixes: 93c64f1ea1e8 ("leds: add Qualcomm PM8941 WLED driver")
> Signed-off-by: Marijn Suijten <marijn.suijten@somainline.org>
> Reviewed-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@somainline.org>
> ---
>  drivers/video/backlight/qcom-wled.c | 8 +++-----
>  1 file changed, 3 insertions(+), 5 deletions(-)
> 
> diff --git a/drivers/video/backlight/qcom-wled.c b/drivers/video/backlight/qcom-wled.c
> index 27e8949c7922..66ce77ee3099 100644
> --- a/drivers/video/backlight/qcom-wled.c
> +++ b/drivers/video/backlight/qcom-wled.c
> @@ -1255,17 +1255,17 @@ static const struct wled_var_cfg wled5_ovp_cfg = {
>  
>  static u32 wled3_num_strings_values_fn(u32 idx)
>  {
> -	return idx + 1;
> +	return idx;
>  }
>  
>  static const struct wled_var_cfg wled3_num_strings_cfg = {
>  	.fn = wled3_num_strings_values_fn,
> -	.size = 3,
> +	.size = 4, /* [0, 3] */

0 is not a valid value for this property.


>  };
>  
>  static const struct wled_var_cfg wled4_num_strings_cfg = {
>  	.fn = wled3_num_strings_values_fn,
> -	.size = 4,
> +	.size = 5, /* [0, 4] */

Ditto.


>  };
>  
>  static u32 wled3_switch_freq_values_fn(u32 idx)
> @@ -1520,8 +1520,6 @@ static int wled_configure(struct wled *wled)
>  			*bool_opts[i].val_ptr = true;
>  	}
>  
> -	cfg->num_strings = cfg->num_strings + 1;
> -
>  	string_len = of_property_count_elems_of_size(dev->of_node,
>  						     "qcom,enabled-strings",
>  						     sizeof(u32));
> -- 
> 2.33.0
>