Re: [PATCH v13 2/2] leds: Add driver for Qualcomm LPG

[Doug Anderson <dianders@chromium.org>]

Hi,

On Wed, Mar 2, 2022 at 4:03 PM Doug Anderson <dianders@chromium.org> wrote:
>
> Hi,
>
> On Fri, Feb 18, 2022 at 10:29 AM Bjorn Andersson
> <bjorn.andersson@linaro.org> wrote:
> >
> > +static void lpg_pwm_get_state(struct pwm_chip *chip, struct pwm_device *pwm,
> > +                             struct pwm_state *state)
> > +{
> > +       struct lpg *lpg = container_of(chip, struct lpg, pwm);
> > +       struct lpg_channel *chan = &lpg->channels[pwm->hwpwm];
> > +       unsigned int pre_div;
> > +       unsigned int refclk;
> > +       unsigned int val;
> > +       unsigned int m;
> > +       u16 pwm_value;
> > +       int ret;
> > +
> > +       ret = regmap_read(lpg->map, chan->base + LPG_SIZE_CLK_REG, &val);
> > +       if (ret)
> > +               return;
> > +
> > +       refclk = lpg_clk_rates[(val & PWM_CLK_SELECT_MASK) - 1];
>
> I don't know why I didn't notice it before (maybe I was accidentally
> not building with KASAN?), but in my recent boots I'm getting a KASAN
> error pointing at the line above.
>
> Sure enough, the above looks a bit on the unsafe side. If (val & 0x3)
> is 0 then the "-1" will not be so wonderful. I put some printouts and,
> indeed, it's not so great.
>
> [    7.201635] DOUG: val is 0x00000004
>
> Amazingly my `refclk` ends up as 0 and I guess somehow this doesn't
> cause a divide by 0.

I dug a little more and found a document that talks about this
register. I guess the answer here is that at boot time on my device
the PWM is disabled and has never been enabled. That explains why, at
boot time, the "clk_select" is 0 AKA "no clock". So we do an invalid
memory access here and that's not so great, but it doesn't _truly_
cause any harm. All we need is something like this right before the
array dereference:

if ((val & PWM_CLK_SELECT_MASK) == 0)
  return;

I'm still pretty interested in seeing this patch series land and, if
it helps it land sooner, I wouldn't object to the above getting fixed
in a followup patch.

-Doug