mac80211: fix a crash in minstrel_ht in HT mode with no supported MCS rates

When a client connects in HT mode but does not provide any valid MCS
rates, the function that finds the next sample rate gets stuck in an
infinite loop.
Fix this by falling back to legacy rates if no usable MCS rates are found.
Signed-off-by: Felix Fietkau <nbd@openwrt.org>
Cc: stable@kernel.org
Signed-off-by: John W. Linville <linville@tuxdriver.com>

net/mac80211/rc80211_minstrel_ht.c

@@ -659,18 +659,14 @@ minstrel_ht_update_caps(void *priv, struct ieee80211_supported_band *sband,
 	struct ieee80211_mcs_info *mcs = &sta->ht_cap.mcs;
 	struct ieee80211_local *local = hw_to_local(mp->hw);
 	u16 sta_cap = sta->ht_cap.cap;
+	int n_supported = 0;
 	int ack_dur;
 	int stbc;
 	int i;
 
 	/* fall back to the old minstrel for legacy stations */
-	if (!sta->ht_cap.ht_supported) {
-		msp->is_ht = false;
-		memset(&msp->legacy, 0, sizeof(msp->legacy));
-		msp->legacy.r = msp->ratelist;
-		msp->legacy.sample_table = msp->sample_table;
-		return mac80211_minstrel.rate_init(priv, sband, sta, &msp->legacy);
-	}
+	if (!sta->ht_cap.ht_supported)
+		goto use_legacy;
 
 	BUILD_BUG_ON(ARRAY_SIZE(minstrel_mcs_groups) !=
 		MINSTREL_MAX_STREAMS * MINSTREL_STREAM_GROUPS);
@@ -725,7 +721,22 @@ minstrel_ht_update_caps(void *priv, struct ieee80211_supported_band *sband,
 
 		mi->groups[i].supported =
 			mcs->rx_mask[minstrel_mcs_groups[i].streams - 1];
+
+		if (mi->groups[i].supported)
+			n_supported++;
 	}
+
+	if (!n_supported)
+		goto use_legacy;
+
+	return;
+
+use_legacy:
+	msp->is_ht = false;
+	memset(&msp->legacy, 0, sizeof(msp->legacy));
+	msp->legacy.r = msp->ratelist;
+	msp->legacy.sample_table = msp->sample_table;
+	return mac80211_minstrel.rate_init(priv, sband, sta, &msp->legacy);
 }
 
 static void