[PATCH 3.16 162/328] hwmon: (nct6775) Fix potential Spectre v1

[Ben Hutchings]

3.16.62-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: "Gustavo A. R. Silva" <gustavo@xxxxxxxxxxxxxx>

commit d49dbfade96d5b0863ca8a90122a805edd5ef50a upstream.

val can be indirectly 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:

vers/hwmon/nct6775.c:2698 store_pwm_weight_temp_sel() warn: potential
spectre issue 'data->temp_src' [r]

Fix this by sanitizing val before using it to index data->temp_src

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

Signed-off-by: Gustavo A. R. Silva <gustavo@xxxxxxxxxxxxxx>
Signed-off-by: Guenter Roeck <linux@xxxxxxxxxxxx>
[bwh: Backported to 3.16: adjust context]
Signed-off-by: Ben Hutchings <ben@xxxxxxxxxxxxxxx>
---
 drivers/hwmon/nct6775.c | 2 ++
 1 file changed, 2 insertions(+)

--- a/drivers/hwmon/nct6775.c
+++ b/drivers/hwmon/nct6775.c
@@ -57,6 +57,7 @@
 #include <linux/mutex.h>
 #include <linux/acpi.h>
 #include <linux/io.h>
+#include <linux/nospec.h>
 #include "lm75.h"
 
 #define USE_ALTERNATE
@@ -2438,6 +2439,7 @@ store_pwm_weight_temp_sel(struct device
 		return err;
 	if (val > NUM_TEMP)
 		return -EINVAL;
+	val = array_index_nospec(val, NUM_TEMP + 1);
 	if (val && (!(data->have_temp & (1 << (val - 1))) ||
 		    !data->temp_src[val - 1]))
 		return -EINVAL;