Re: [PATCH] iio staging: tsl2x7x: clean up limit checks

[Brian Masney <masneyb@onstation.org>]

On Tue, Sep 05, 2017 at 05:02:55PM -0400, Brian Masney wrote:
> On Tue, Sep 05, 2017 at 05:58:54PM +0300, Dan Carpenter wrote:
> > On Sun, Sep 03, 2017 at 10:12:46PM -0400, Brian Masney wrote:
> > > On Sun, Sep 03, 2017 at 12:35:05PM +0100, Jonathan Cameron wrote:
> > > > On Mon, 21 Aug 2017 13:11:03 +0300
> > > > Dan Carpenter <dan.carpenter@oracle.com> wrote:
> > > > 
> > > > > The second part of this patch is probably the most interesting.  We
> > > > > use "TSL2X7X_MAX_LUX_TABLE_SIZE * 3" as the limit instead of just
> > > > > "TSL2X7X_MAX_LUX_TABLE_SIZE".  It creates a static checker warning that
> > > > > we are going of of bounds, but in real life we always hit the break
> > > > > statement on the last element so it's fine.
> > > > > 
> > > > > The situation is that we normally have arrays with 3 elements of struct
> > > > > tsl2x7x_lux which has 3 unsigned integers.  If we load the table with
> > > > > sysfs then we're allow to have 9 elements instead.
> > > > > 
> > > > > So the size of the default table in bytes is sizeof(int) times 3 struct
> > > > > members times 3 elements.  The original code wrote it as sizeof(int)
> > > > > times the number of elements in the bigger table (9).  It happens that
> > > > > 9 is the same thing as 3 * 3 but expressing it that way is misleading.
> > > > > 
> > > > > For the second part of the patch, the original code just had an extra
> > > > > "multiply by three" and now that is removed.  The last element in the
> > > > > array is always zeroed memory whether this uses the default tables or it
> > > > > gets loaded with sysfs so we always hit the break statement anyway.
> > > > > 
> > > > > Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
> > > > 
> > > > Looks sensible to me.
> > > > 
> > > > Cc'd Brian who has been working extensively on this driver recently as I'd
> > > > like his input.
> > > > 
> > > > Jonathan
> > > > > 
> > > > > diff --git a/drivers/staging/iio/light/tsl2x7x.h b/drivers/staging/iio/light/tsl2x7x.h
> > > > > index ecae92211216..1beb8d2eb848 100644
> > > > > --- a/drivers/staging/iio/light/tsl2x7x.h
> > > > > +++ b/drivers/staging/iio/light/tsl2x7x.h
> > > > > @@ -23,10 +23,6 @@
> > > > >  #define __TSL2X7X_H
> > > > >  #include <linux/pm.h>
> > > > >  
> > > > > -/* Max number of segments allowable in LUX table */
> > > > > -#define TSL2X7X_MAX_LUX_TABLE_SIZE		9
> > > > > -#define MAX_DEFAULT_TABLE_BYTES (sizeof(int) * TSL2X7X_MAX_LUX_TABLE_SIZE)
> > > > > -
> > > > >  struct iio_dev;
> > > > >  
> > > > >  struct tsl2x7x_lux {
> > > > > @@ -35,6 +31,11 @@ struct tsl2x7x_lux {
> > > > >  	unsigned int ch1;
> > > > >  };
> > > > >  
> > > > > +/* Max number of segments allowable in LUX table */
> > > > > +#define TSL2X7X_MAX_LUX_TABLE_SIZE		9
> > > > > +/* The default tables are all 3 elements */
> > > > > +#define MAX_DEFAULT_TABLE_BYTES (sizeof(struct tsl2x7x_lux) * 3)
> > > > > +
> > > > >  /**
> > > > >   * struct tsl2x7x_default_settings - power on defaults unless
> > > > >   *                                   overridden by platform data.
> > > > > diff --git a/drivers/staging/iio/light/tsl2x7x.c b/drivers/staging/iio/light/tsl2x7x.c
> > > > > index 786e93f16ce9..2db1715ff659 100644
> > > > > --- a/drivers/staging/iio/light/tsl2x7x.c
> > > > > +++ b/drivers/staging/iio/light/tsl2x7x.c
> > > > > @@ -1113,7 +1113,7 @@ static ssize_t in_illuminance0_lux_table_show(struct device *dev,
> > > > >  	int i = 0;
> > > > >  	int offset = 0;
> > > > >  
> > > > > -	while (i < (TSL2X7X_MAX_LUX_TABLE_SIZE * 3)) {
> > > > > +	while (i < TSL2X7X_MAX_LUX_TABLE_SIZE) {
> > > > >  		offset += snprintf(buf + offset, PAGE_SIZE, "%u,%u,%u,",
> > > > >  			chip->tsl2x7x_device_lux[i].ratio,
> > > > >  			chip->tsl2x7x_device_lux[i].ch0,
> > > 
> > > There is a minor issue regarding the structure sizes in with both this
> > > patch and the in-tree code. The following two structures define nine
> > > rows in the lux table:
> > > 
> > > tsl2x7x.h:
> > > #define TSL2X7X_MAX_LUX_TABLE_SIZE         9
> > > 
> > > struct tsl2X7X_platform_data {
> > >   ...
> > >   struct tsl2x7x_lux platform_lux_table[TSL2X7X_MAX_LUX_TABLE_SIZE];
> > > }
> > > 
> > > tsl2x7x.c:
> > > struct tsl2X7X_chip {
> > >   ...
> > >   struct tsl2x7x_lux tsl2x7x_device_lux[TSL2X7X_MAX_LUX_TABLE_SIZE];
> > > }
> > > 
> > > tsl2x7x_defaults() has this code snippet:
> > > 
> > >   memcpy(chip->tsl2x7x_device_lux,
> > >          (struct tsl2x7x_lux *)tsl2x7x_default_lux_table_group[chip->id],
> > >                          MAX_DEFAULT_TABLE_BYTES);
> > > 
> > > With the old and new code, memcpy will only copy the first three rows of
> > > the lux table. There is no security issue though with the actual
> > > implementation since the four *_lux_table structures that are defined in
> > > code only have three rows defined.
> > 
> > Agreed.
> > 
> > > 
> > > I believe that the correct fix is to define MAX_DEFAULT_TABLE_BYTES in
> > > Dan's patch as follows (and keeping his other changes):
> > > 
> > > #define MAX_DEFAULT_TABLE_BYTES (sizeof(struct tsl2x7x_lux) *
> > > 					TSL2X7X_MAX_LUX_TABLE_SIZE)
> > > 
> > > We may also want to shorten those #defines to keep it under 80
> > > characters.
> > > 
> > 
> > 
> > That's not a good idea because we would be filling chip->tsl2x7x_device_lux
> > with garbage from beyond the end of the tsl2x7x_default_lux_table_group[]
> > element.  It would be harmless but ugly.  We could just add a:
> > 
> > 	memset(chip->tsl2x7x_device_lux, 0, sizeof(chip->tsl2x7x_device_lux));
> > 
> > to the start of the tsl2x7x_defaults() and the in_illuminance0_lux_table_store()
> > functions.  But I don't really see a need...  This code will need to be
> > restructured a bit if we start using different sized default tables,
> > yes, but we can't really "future proof" this code without seeing what
> > the future change is.
> 
> Agreed. Looking through this again, this improves the existing code and
> can be applied. Jonathan, you can add my:
> 
> Acked-by: Brian Masney <masneyb@onstation.org>
> 
> I think we need a followup patch to improve this further before the
> driver can be moved out of staging. Namely, if a new entry is added
> to tsl2x7x_default_lux_table_group with more than 3 elements, then the
> user will possibly see erroneous readings from the sensor. To make it
> easier to review future changes, what do you think about removing the
> MAX_DEFAULT_TABLE_BYTES #define and replacing it with a new array
> below tsl2x7x_default_lux_table_group that has the size of each default
> lux table? The new array will only be used by the memcpy() call above.
> If it sounds acceptable, then I can add that to my queue. I'm going to
> start working on this driver again this week after a few months of being
> away from it.

Alternatively, we can keep this simple and just add a comment above the
definition of tsl2x7x_default_lux_table_group saying that the number of
rows in all of the various defaults need to be the same. If additional
rows are added, then the definition of MAX_DEFAULT_TABLE_BYTES needs to
be changed as well.

Brian