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SubjectRe: [PATCH] iio staging: tsl2x7x: clean up limit checks
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.

regards,
dan carpenter

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