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

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

> -----Original Message-----
> From: Kees Cook <keescook@chromium.org>
> Sent: Thursday, December 16, 2021 1:23 AM
> To: Lars-Peter Clausen <lars@metafoo.de>
> Cc: Kees Cook <keescook@chromium.org>; Hennerich, Michael
> <Michael.Hennerich@analog.com>; Tanislav, Cosmin
> <Cosmin.Tanislav@analog.com>; Jonathan Cameron <jic23@kernel.org>;
> Linus Walleij <linus.walleij@linaro.org>; linux-kernel@vger.kernel.org; linux-
> iio@vger.kernel.org; linux-hardening@vger.kernel.org
> Subject: [PATCH] iio: addac: Do not reference negative array offsets
> 
> [External]
> 
> Instead of aiming rx_buf at an invalid array-boundary-crossing location,
> just skip the first assignment. 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];
>       |                    ^~~~~~
> 
> Fixes: fea251b6a5db ("iio: addac: add AD74413R driver")
> Signed-off-by: Kees Cook <keescook@chromium.org>
> ---
>  drivers/iio/addac/ad74413r.c | 10 ++++++----
>  1 file changed, 6 insertions(+), 4 deletions(-)
> 
> diff --git a/drivers/iio/addac/ad74413r.c b/drivers/iio/addac/ad74413r.c
> index cbd9aa9b399a..b0a6d8ee5133 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;
> @@ -877,9 +877,8 @@ static int ad74413r_update_scan_mode(struct iio_dev
> *indio_dev,
>  		if (ret)
>  			goto out;
> 
> -		st->adc_active_channels++;
> 
> -		if (xfer == st->adc_samples_xfer)
> +		if (xfer == st->adc_samples_xfer || st->adc_active_channels
> == 0)

You can probably keep only one of the checks. Both xfer and adc_active_channels
will be incremented after your changes anyway.

>  			xfer->rx_buf = NULL;
>  		else
>  			xfer->rx_buf = rx_buf;
> @@ -896,7 +895,10 @@ static int ad74413r_update_scan_mode(struct
> iio_dev *indio_dev,
> 
>  		xfer++;
>  		tx_buf += AD74413R_FRAME_SIZE;
> -		rx_buf += AD74413R_FRAME_SIZE;
> +		if (st->adc_active_channels)
> +			rx_buf += AD74413R_FRAME_SIZE;
> +
> +		st->adc_active_channels++;
>  	}
> 
>  	xfer->rx_buf = rx_buf;
> --
> 2.30.2