media: si470x: fix potential Spectre variant 1

band->index can be controlled by user-space, hence leading to a potential exploitation of the Spectre variant 1 vulnerability.

This issue was detected with the help of Smatch:
drivers/media/radio/si470x/radio-si470x-common.c:758 si470x_vidioc_enum_freq_bands() warn: potential spectre issue 'bands'

Fix this by sanitizing band->index before using it to index `bands'

Notice that given that speculation windows are large, the policy is to kill the speculation on the first load and not worry if it can be completed with a dependent load/store [1].

[1] https://marc.info/?l=linux-kernel&m=152449131114778&w=2

---
 drivers/media/radio/si470x/radio-si470x-common.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

index c40e175..e81f9aa 100644
@@ -110,6 +110,10 @@
 /* kernel includes */
 #include "radio-si470x.h"
 
+/* Hardening for Spectre-v1 */
+#include <linux/nospec.h>
+
+
 /**************************************************************************
  * Module Parameters
  **************************************************************************/

@@ -755,7 +759,7 @@ static int si470x_vidioc_enum_freq_bands(struct file *file, void *priv,
 		return -EINVAL;
 	if (band->index >= ARRAY_SIZE(bands))
 		return -EINVAL;
-	*band = bands[band->index];
+	*band = bands[array_index_nospec(band->index, ARRAY_SIZE(bands))];
 	return 0;
 }
 

2.7.4