nl80211: Fix possible Spectre-v1 for CQM RSSI thresholds

commit 1222a16014 ("nl80211: Fix possible Spectre-v1 for CQM
RSSI thresholds") was incomplete and requires one more fix to
prevent accessing to rssi_thresholds[n] because user can control
rssi_thresholds[i] values to make i reach to n. For example,
rssi_thresholds = {-400, -300, -200, -100} when last is -34.

Cc: stable@vger.kernel.org
Fixes: 1222a16014 ("nl80211: Fix possible Spectre-v1 for CQM RSSI thresholds")
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Masashi Honma <masashi.honma@gmail.com>
Link: https://lore.kernel.org/r/20190908005653.17433-1-masashi.honma@gmail.com
Signed-off-by: Johannes Berg <johannes.berg@intel.com>

net/wireless/nl80211.c

@@ -10805,9 +10805,11 @@ static int cfg80211_cqm_rssi_update(struct cfg80211_registered_device *rdev,
 	hyst = wdev->cqm_config->rssi_hyst;
 	n = wdev->cqm_config->n_rssi_thresholds;
 
-	for (i = 0; i < n; i++)
+	for (i = 0; i < n; i++) {
+		i = array_index_nospec(i, n);
 		if (last < wdev->cqm_config->rssi_thresholds[i])
 			break;
+	}
 
 	low_index = i - 1;
 	if (low_index >= 0) {