nl80211: Fix possible Spectre-v1 for NL80211_TXRATE_HT

Use array_index_nospec() to sanitize ridx with respect to speculation.
Signed-off-by: Masashi Honma <masashi.honma@gmail.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>

CVE-2017-5753

(backported from commit 30fe6d50)
[juergh: Adjusted context.]
Signed-off-by: Juerg Haefliger <juergh@canonical.com>
Acked-by: Stefan Bader <stefan.bader@canonical.com>
Acked-by: Kleber Sacilotto de Souza <kleber.souza@canonical.com>
Signed-off-by: Kleber Sacilotto de Souza <kleber.souza@canonical.com>

net/wireless/nl80211.c

@@ -8259,6 +8259,7 @@ static bool ht_rateset_to_mask(struct ieee80211_supported_band *sband,
 			return false;
 
 		/* check availability */
+		ridx = array_index_nospec(ridx, IEEE80211_HT_MCS_MASK_LEN);
 		if (sband->ht_cap.mcs.rx_mask[ridx] & rbit)
 			mcs[ridx] |= rbit;
 		else