RE: [PATCH v2] iio: addac: ad74413r: Do not reference negative array offsets

["Tanislav, Cosmin" <Cosmin.Tanislav@analog.com>]

Reviewed-by: Cosmin Tanislav <cosmin.tanislav@analog.com>

> -----Original Message-----
> From: Kees Cook <keescook@chromium.org>
> Sent: Wednesday, January 12, 2022 10:35 PM
> To: Lars-Peter Clausen <lars@metafoo.de>
> Cc: Kees Cook <keescook@chromium.org>; Hennerich, Michael
> <Michael.Hennerich@analog.com>; Jonathan Cameron <jic23@kernel.org>;
> linux-iio@vger.kernel.org; Tanislav, Cosmin <Cosmin.Tanislav@analog.com>;
> Jonathan Cameron <Jonathan.Cameron@huawei.com>; Linus Walleij
> <linus.walleij@linaro.org>; linux-kernel@vger.kernel.org; linux-
> hardening@vger.kernel.org
> Subject: [PATCH v2] iio: addac: ad74413r: Do not reference negative array
> offsets
> 
> [External]
> 
> Instead of aiming rx_buf at an invalid array-boundary-crossing location,
> just skip the first increment. Seen when building with -Warray-bounds:
> 
> drivers/iio/addac/ad74413r.c: In function 'ad74413r_update_scan_mode':
> drivers/iio/addac/ad74413r.c:843:22: warning: array subscript -4 is below
> array bounds of 'u8[16]' { aka 'unsigned char[16]'} [-Warray-bounds]
>   843 |         u8 *rx_buf = &st->adc_samples_buf.rx_buf[-1 *
> AD74413R_FRAME_SIZE];
>       |
> ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> drivers/iio/addac/ad74413r.c:84:20: note: while referencing 'rx_buf'
>    84 |                 u8 rx_buf[AD74413R_FRAME_SIZE *
> AD74413R_CHANNEL_MAX];
>       |                    ^~~~~~
> 
> Cc: Lars-Peter Clausen <lars@metafoo.de>
> Cc: Michael Hennerich <Michael.Hennerich@analog.com>
> Cc: Jonathan Cameron <jic23@kernel.org>
> Cc: linux-iio@vger.kernel.org
> Fixes: fea251b6a5db ("iio: addac: add AD74413R driver")
> Reviewed-by: Cosmin Tanislav <cosmin.tanislav@analog.com>
> Signed-off-by: Kees Cook <keescook@chromium.org>
> ---
> v1: https://urldefense.com/v3/__https://lore.kernel.org/linux-
> hardening/20220105180214.2435001-1-
> keescook@chromium.org/__;!!A3Ni8CS0y2Y!oWs0KcGPANFn-
> L0qJPZgP47AQIYpBXJxg5LHiLDFGa_-SI2DwmSMzjgl3ehyu-8JYPgq$
> v2:
>  - Update commit Subject prefix
>  - add Reviewed-by
> ---
>  drivers/iio/addac/ad74413r.c | 7 ++++---
>  1 file changed, 4 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/iio/addac/ad74413r.c b/drivers/iio/addac/ad74413r.c
> index 5271073bb74e..aba9a643a4ca 100644
> --- a/drivers/iio/addac/ad74413r.c
> +++ b/drivers/iio/addac/ad74413r.c
> @@ -840,7 +840,7 @@ static int ad74413r_update_scan_mode(struct iio_dev
> *indio_dev,
>  {
>  	struct ad74413r_state *st = iio_priv(indio_dev);
>  	struct spi_transfer *xfer = st->adc_samples_xfer;
> -	u8 *rx_buf = &st->adc_samples_buf.rx_buf[-1 *
> AD74413R_FRAME_SIZE];
> +	u8 *rx_buf = st->adc_samples_buf.rx_buf;
>  	u8 *tx_buf = st->adc_samples_tx_buf;
>  	unsigned int channel;
>  	int ret = -EINVAL;
> @@ -894,9 +894,10 @@ static int ad74413r_update_scan_mode(struct
> iio_dev *indio_dev,
> 
>  		spi_message_add_tail(xfer, &st->adc_samples_msg);
> 
> -		xfer++;
>  		tx_buf += AD74413R_FRAME_SIZE;
> -		rx_buf += AD74413R_FRAME_SIZE;
> +		if (xfer != st->adc_samples_xfer)
> +			rx_buf += AD74413R_FRAME_SIZE;
> +		xfer++;
>  	}
> 
>  	xfer->rx_buf = rx_buf;
> --
> 2.30.2