iio: adis_buffer: Fix out-of-bounds memory access

commit d590faf9 upstream.

The SPI tx and rx buffers are both supposed to be scan_bytes amount of
bytes large and a common allocation is used to allocate both buffers. This
puts the beginning of the tx buffer scan_bytes bytes after the rx buffer.
The initialization of the tx buffer pointer is done adding scan_bytes to
the beginning of the rx buffer, but since the rx buffer is of type __be16
this will actually add two times as much and the tx buffer ends up pointing
after the allocated buffer.

Fix this by using scan_count, which is scan_bytes / 2, instead of
scan_bytes when initializing the tx buffer pointer.

Fixes: aacff892 ("staging:iio:adis: Preallocate transfer message")
Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>
Signed-off-by: Luis Henriques <luis.henriques@canonical.com>

drivers/iio/imu/adis_buffer.c

@@ -43,7 +43,7 @@ int adis_update_scan_mode(struct iio_dev *indio_dev,
 		return -ENOMEM;
 
 	rx = adis->buffer;
-	tx = rx + indio_dev->scan_bytes;
+	tx = rx + scan_count;
 
 	spi_message_init(&adis->msg);