mac80211: fix invalid band deref building preq IEs

The function building probe-request IEs does not validate the band is
supported before dereferencing it. This can result in a panic when
all bands are traversed, as done during sched-scan start.

Warn when this happens and return an empty probe request. Also fix
sched-scan to not waste memory on unsupported bands.
Signed-off-by: Arik Nemtsov <arik@wizery.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>

net/mac80211/scan.c

@@ -928,6 +928,9 @@ int ieee80211_request_sched_scan_start(struct ieee80211_sub_if_data *sdata,
 	}
 
 	for (i = 0; i < IEEE80211_NUM_BANDS; i++) {
+		if (!local->hw.wiphy->bands[i])
+			continue;
+
 		local->sched_scan_ies.ie[i] = kzalloc(2 +
 						      IEEE80211_MAX_SSID_LEN +
 						      local->scan_ies_len +

net/mac80211/util.c

@@ -999,6 +999,8 @@ int ieee80211_build_preq_ies(struct ieee80211_local *local, u8 *buffer,
 	int ext_rates_len;
 
 	sband = local->hw.wiphy->bands[band];
+	if (WARN_ON_ONCE(!sband))
+		return 0;
 
 	pos = buffer;