Re: [PATCH 2/2] iio: tsl2772: Use scnprintf() for avoiding potential buffer overflow

[Brian Masney <masneyb@onstation.org>]

On Sun, Mar 15, 2020 at 09:58:34AM +0000, Jonathan Cameron wrote:
> On Wed, 11 Mar 2020 08:43:25 +0100
> Takashi Iwai <tiwai@suse.de> wrote:
> 
> > Since snprintf() returns the would-be-output size instead of the
> > actual output size, the succeeding calls may go beyond the given
> > buffer limit.  Fix it by replacing with scnprintf().
> > 
> > Signed-off-by: Takashi Iwai <tiwai@suse.de>
> 
> This one is printing a short well defined list of values.  No way they go
> anywhere near the smallest possible PAGE_SIZE buffer that it's printing
> into.
> 
> Which is handy given the remaining space isn't adjusted as we add items
> to the string.  Hence even with scnprintf it would overflow.
> 
> Brian, can you take a look at this when you get a moment?

I also agree that this won't overflow in practice, however we should fix
this up. Maybe the scnprintf() calls should be this:

    offset += scnprintf(buf + offset, PAGE_SIZE - offset, ...);

Brian


> 
> Thanks,
> 
> Jonathan
> 
> > ---
> >  drivers/iio/light/tsl2772.c | 4 ++--
> >  1 file changed, 2 insertions(+), 2 deletions(-)
> > 
> > diff --git a/drivers/iio/light/tsl2772.c b/drivers/iio/light/tsl2772.c
> > index be37fcbd4654..44a0b56a558c 100644
> > --- a/drivers/iio/light/tsl2772.c
> > +++ b/drivers/iio/light/tsl2772.c
> > @@ -986,7 +986,7 @@ static ssize_t in_illuminance0_lux_table_show(struct device *dev,
> >  	int offset = 0;
> >  
> >  	while (i < TSL2772_MAX_LUX_TABLE_SIZE) {
> > -		offset += snprintf(buf + offset, PAGE_SIZE, "%u,%u,",
> > +		offset += scnprintf(buf + offset, PAGE_SIZE, "%u,%u,",
> >  			chip->tsl2772_device_lux[i].ch0,
> >  			chip->tsl2772_device_lux[i].ch1);
> >  		if (chip->tsl2772_device_lux[i].ch0 == 0) {
> > @@ -1000,7 +1000,7 @@ static ssize_t in_illuminance0_lux_table_show(struct device *dev,
> >  		i++;
> >  	}
> >  
> > -	offset += snprintf(buf + offset, PAGE_SIZE, "\n");
> > +	offset += scnprintf(buf + offset, PAGE_SIZE, "\n");
> >  	return offset;
> >  }
> >