rtl8139: skip offload on short TCP header (CVE-2015-5165)

TCP Large Segment Offload accesses the TCP header in the packet.  If the
packet is too short we must not attempt to access header fields:

  tcp_header *p_tcp_hdr = (tcp_header*)(eth_payload_data + hlen);
  int tcp_hlen = TCP_HEADER_DATA_OFFSET(p_tcp_hdr);
Reported-by: N朱东海(启路) <donghai.zdh@alibaba-inc.com>
Reviewed-by: NJason Wang <jasowang@redhat.com>
Signed-off-by: NStefan Hajnoczi <stefanha@redhat.com>

hw/net/rtl8139.c

@@ -2210,6 +2210,11 @@ static int rtl8139_cplus_transmit_one(RTL8139State *s)
 
             if ((txdw0 & CP_TX_LGSEN) && ip_protocol == IP_PROTO_TCP)
             {
+                /* Large enough for the TCP header? */
+                if (ip_data_len < sizeof(tcp_header)) {
+                    goto skip_offload;
+                }
+
                 int large_send_mss = (txdw0 >> 16) & CP_TC_LGSEN_MSS_MASK;
 
                 DPRINTF("+++ C+ mode offloaded task TSO MTU=%d IP data %d "