iio:health:afe4403 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 32 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: eec96d1e ("iio: health: Add driver for the TI AFE4403 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>
Cc: <Stable@vger.kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

drivers/iio/health/afe4403.c

@@ -65,6 +65,7 @@ static const struct reg_field afe4403_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 data layout to push into IIO buffer.
  */
 struct afe4403_data {
 	struct device *dev;
@@ -74,6 +75,8 @@ struct afe4403_data {
 	struct regulator *regulator;
 	struct iio_trigger *trig;
 	int irq;
+	/* Ensure suitable alignment for timestamp */
+	s32 buffer[8] __aligned(8);
 };
 
 enum afe4403_chan_id {
@@ -309,7 +312,6 @@ static irqreturn_t afe4403_trigger_handler(int irq, void *private)
 	struct iio_dev *indio_dev = pf->indio_dev;
 	struct afe4403_data *afe = iio_priv(indio_dev);
 	int ret, bit, i = 0;
-	s32 buffer[8];
 	u8 tx[4] = {AFE440X_CONTROL0, 0x0, 0x0, AFE440X_CONTROL0_READ};
 	u8 rx[3];
 
@@ -326,7 +328,7 @@ static irqreturn_t afe4403_trigger_handler(int irq, void *private)
 		if (ret)
 			goto err;
 
-		buffer[i++] = get_unaligned_be24(&rx[0]);
+		afe->buffer[i++] = get_unaligned_be24(&rx[0]);
 	}
 
 	/* Disable reading from the device */
@@ -335,7 +337,8 @@ static irqreturn_t afe4403_trigger_handler(int irq, void *private)
 	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);