nl80211: Fix possible Spectre-v1 for CQM RSSI thresholds

Use array_index_nospec() to sanitize i with respect to speculation.

Note that the user doesn't control i directly, but can make it out
of bounds by not finding a threshold in the array.
Signed-off-by: Masashi Honma <masashi.honma@gmail.com>
[add note about user control, as explained by Masashi]
Signed-off-by: Johannes Berg <johannes.berg@intel.com>

net/wireless/nl80211.c

@@ -10231,7 +10231,7 @@ static int cfg80211_cqm_rssi_update(struct cfg80211_registered_device *rdev,
 	struct wireless_dev *wdev = dev->ieee80211_ptr;
 	s32 last, low, high;
 	u32 hyst;
-	int i, n;
+	int i, n, low_index;
 	int err;
 
 	/* RSSI reporting disabled? */
@@ -10268,10 +10268,19 @@ static int cfg80211_cqm_rssi_update(struct cfg80211_registered_device *rdev,
 		if (last < wdev->cqm_config->rssi_thresholds[i])
 			break;
 
-	low = i > 0 ?
-		(wdev->cqm_config->rssi_thresholds[i - 1] - hyst) : S32_MIN;
-	high = i < n ?
-		(wdev->cqm_config->rssi_thresholds[i] + hyst - 1) : S32_MAX;
+	low_index = i - 1;
+	if (low_index >= 0) {
+		low_index = array_index_nospec(low_index, n);
+		low = wdev->cqm_config->rssi_thresholds[low_index] - hyst;
+	} else {
+		low = S32_MIN;
+	}
+	if (i < n) {
+		i = array_index_nospec(i, n);
+		high = wdev->cqm_config->rssi_thresholds[i] + hyst - 1;
+	} else {
+		high = S32_MAX;
+	}
 
 	return rdev_set_cqm_rssi_range_config(rdev, dev, low, high);
 }