iio:health:afe4404 Fix timestamp alignment and prevent data leak.

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 a 40 byte array of smaller elements on the stack.
As Lars also noted this anti pattern can involve a leak of data to
userspace and that indeed can happen here.  We close both 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 appart from previous readings.

Fixes: 87aec56e ("iio: health: Add driver for the TI AFE4404 heart monitor")
Reported-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Acked-by: Andrew F. Davis <afd@ti.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

drivers/iio/health/afe4404.c

@@ -83,6 +83,7 @@ static const struct reg_field afe4404_reg_fields[] = {
  * @regulator: Pointer to the regulator for the IC
  * @trig: IIO trigger for this device
  * @irq: ADC_RDY line interrupt number
+ * @buffer: Used to construct a scan to push to the iio buffer.
  */
 struct afe4404_data {
 	struct device *dev;
@@ -91,6 +92,7 @@ struct afe4404_data {
 	struct regulator *regulator;
 	struct iio_trigger *trig;
 	int irq;
+	s32 buffer[10] __aligned(8);
 };
 
 enum afe4404_chan_id {
@@ -328,17 +330,17 @@ static irqreturn_t afe4404_trigger_handler(int irq, void *private)
 	struct iio_dev *indio_dev = pf->indio_dev;
 	struct afe4404_data *afe = iio_priv(indio_dev);
 	int ret, bit, i = 0;
-	s32 buffer[10];
 
 	for_each_set_bit(bit, indio_dev->active_scan_mask,
 			 indio_dev->masklength) {
 		ret = regmap_read(afe->regmap, afe4404_channel_values[bit],
-				  &buffer[i++]);
+				  &afe->buffer[i++]);
 		if (ret)
 			goto err;
 	}
 
-	iio_push_to_buffers_with_timestamp(indio_dev, buffer, pf->timestamp);
+	iio_push_to_buffers_with_timestamp(indio_dev, afe->buffer,
+					   pf->timestamp);
 err:
 	iio_trigger_notify_done(indio_dev->trig);