mac80211: add missing length check for confirm frames

Although mesh_rx_plink_frame() already checks that frames have enough
bytes for the action code plus another two bytes for capability/reason
code, it doesn't take into account that confirm frames also have an
additional two-byte aid.  As a result, a corrupt frame could cause a
subsequent subtraction to wrap around to ill effect.  Add another
check for this case.
Signed-off-by: Bob Copeland <me@bobcopeland.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>

net/mac80211/mesh_plink.c

@@ -1122,6 +1122,9 @@ void mesh_rx_plink_frame(struct ieee80211_sub_if_data *sdata,
 						WLAN_SP_MESH_PEERING_CONFIRM) {
 		baseaddr += 4;
 		baselen += 4;
+
+		if (baselen > len)
+			return;
 	}
 	ieee802_11_parse_elems(baseaddr, len - baselen, true, &elems);
 	mesh_process_plink_frame(sdata, mgmt, &elems);