Re: [PATCH] hwmon: (adt7470) Prevent divide by zero in adt7470_fan_write()

[Dan Carpenter <dan.carpenter@oracle.com>]

On Thu, Jan 20, 2022 at 10:10:45PM -0800, Guenter Roeck wrote:
> On 1/20/22 9:39 PM, Dan Carpenter wrote:
> > The "val" variable is controlled by the user and comes from
> > hwmon_attr_store().  The FAN_RPM_TO_PERIOD() macro divides by "val"
> > so a zero will crash the system.  Check for that and return -EINVAL.
> > 
> > Fixes: fc958a61ff6d ("hwmon: (adt7470) Convert to devm_hwmon_device_register_with_info API")
> > Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
> > ---
> >   drivers/hwmon/adt7470.c | 3 +++
> >   1 file changed, 3 insertions(+)
> > 
> > diff --git a/drivers/hwmon/adt7470.c b/drivers/hwmon/adt7470.c
> > index d519aca4a9d6..cd474584dc0b 100644
> > --- a/drivers/hwmon/adt7470.c
> > +++ b/drivers/hwmon/adt7470.c
> > @@ -662,6 +662,9 @@ static int adt7470_fan_write(struct device *dev, u32 attr, int channel, long val
> >   	struct adt7470_data *data = dev_get_drvdata(dev);
> >   	int err;
> > +	if (!val)
> > +		return -EINVAL;
> > +
> 
> Technically that restores old (pre-fc958a61ff6d) behavior, but it is still bad:
> Userspace can provide a value of -1 (or any other negative number), and it will
> translate to 5400000 RPM. So it should either be
> 
> 	if (val <= 0)
> 		return -EINVAL;
> 
> or
> 	if (val <= 0)
> 		val = 1;

There is a clamp() which does already turn invalid values into something
valid.

	val = FAN_RPM_TO_PERIOD(val);
	val = clamp_val(val, 1, 65534);

But I will make the <= 0 return -EINVAL change and resend.

regards,
dan carpenter