iio:adc:ti-adc0832 Fix alignment issue with timestamp

One of a class of bugs pointed out by Lars in a recent review.
iio_push_to_buffers_with_timestamp assumes the buffer used is aligned
to the size of the timestamp (8 bytes).  This is not guaranteed in
this driver which uses an array of smaller elements on the stack.

We fix this issues by moving to a suitable structure in the iio_priv()
data with alignment explicitly requested.  This data is allocated
with kzalloc so no data can leak apart from previous readings.
Note that previously no data could leak 'including' previous readings
but I don't think it is an issue to potentially leak them like
this now does.

In this case the postioning of the timestamp is depends on what
other channels are enabled. As such we cannot use a structure to
make the alignment explicit as it would be missleading by suggesting
only one possible location for the timestamp.

Fixes: 815bbc87 ("iio: ti-adc0832: add triggered buffer support")
Reported-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Cc: Akinobu Mita <akinobu.mita@gmail.com>
Cc: <Stable@vger.kernel.org>
Link: https://lore.kernel.org/r/20200722155103.979802-25-jic23@kernel.org

drivers/iio/adc/ti-adc0832.c

@@ -29,6 +29,12 @@ struct adc0832 {
 	struct regulator *reg;
 	struct mutex lock;
 	u8 mux_bits;
+	/*
+	 * Max size needed: 16x 1 byte ADC data + 8 bytes timestamp
+	 * May be shorter if not all channels are enabled subject
+	 * to the timestamp remaining 8 byte aligned.
+	 */
+	u8 data[24] __aligned(8);
 
 	u8 tx_buf[2] ____cacheline_aligned;
 	u8 rx_buf[2];
@@ -200,7 +206,6 @@ static irqreturn_t adc0832_trigger_handler(int irq, void *p)
 	struct iio_poll_func *pf = p;
 	struct iio_dev *indio_dev = pf->indio_dev;
 	struct adc0832 *adc = iio_priv(indio_dev);
-	u8 data[24] = { }; /* 16x 1 byte ADC data + 8 bytes timestamp */
 	int scan_index;
 	int i = 0;
 
@@ -218,10 +223,10 @@ static irqreturn_t adc0832_trigger_handler(int irq, void *p)
 			goto out;
 		}
 
-		data[i] = ret;
+		adc->data[i] = ret;
 		i++;
 	}
-	iio_push_to_buffers_with_timestamp(indio_dev, data,
+	iio_push_to_buffers_with_timestamp(indio_dev, adc->data,
 					   iio_get_time_ns(indio_dev));
 out:
 	mutex_unlock(&adc->lock);