mac80211: add check on hw->max_signal value on ieee80211_register_hw

When IEEE80211_HW_SIGNAL_UNSPEC is set, mac80211 will perform a
division by max_signal in ieee80211_bss_info_update. If max_signal
is not properly set by the driver (for example it is zero) this
leads to a divide error and crash.
Thanks to Larry Finger, who pointed me to this.
This patch adds in ieee80211_register_hw one more check to detect
this condition and eventually returns -EINVAL, as already done for
other checks already performed there.
Signed-off-by: Nandrea merello <andrea.merello@gmail.com>
[move to an already existing SIGNAL_UNSPEC check]
Signed-off-by: NJohannes Berg <johannes.berg@intel.com>

net/mac80211/main.c

@@ -893,10 +893,15 @@ int ieee80211_register_hw(struct ieee80211_hw *hw)
 	/* mac80211 supports control port protocol changing */
 	local->hw.wiphy->flags |= WIPHY_FLAG_CONTROL_PORT_PROTOCOL;
 
-	if (local->hw.flags & IEEE80211_HW_SIGNAL_DBM)
+	if (local->hw.flags & IEEE80211_HW_SIGNAL_DBM) {
 		local->hw.wiphy->signal_type = CFG80211_SIGNAL_TYPE_MBM;
-	else if (local->hw.flags & IEEE80211_HW_SIGNAL_UNSPEC)
+	} else if (local->hw.flags & IEEE80211_HW_SIGNAL_UNSPEC) {
 		local->hw.wiphy->signal_type = CFG80211_SIGNAL_TYPE_UNSPEC;
+		if (hw->max_signal <= 0) {
+			result = -EINVAL;
+			goto fail_wiphy_register;
+		}
+	}
 
 	WARN((local->hw.flags & IEEE80211_HW_SUPPORTS_UAPSD)
 	     && (local->hw.flags & IEEE80211_HW_PS_NULLFUNC_STACK),