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

[Marijn Suijten]

On 2021-10-05 10:19:47, Daniel Thompson wrote:
> 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 at somainline.org>
> > Reviewed-by: AngeloGioacchino Del Regno <angelogioacchino.delregno at 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.

These comments represent the possible loop iterations the DT "cfg
parser" runs through, starting at j=0 and running up until and including
j=3.  Should I make that more clear or omit these comments entirely?

- Marijn

> >  };
> >  
> >  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
> >