Fix decompression of 6LowPan src (#4792)

Author: gpotter2

scapy/layers/sixlowpan.py

@@ -518,13 +518,18 @@ def _extract_upperaddress(pkt, source=True):
         # https://tools.ietf.org/html/rfc2464#section-4
         return LINK_LOCAL_PREFIX[:8] + addr[:3] + b"\xff\xfe" + addr[3:]
     elif type(underlayer) == Dot15d4Data:
-        addr = underlayer.src_addr if source else underlayer.dest_addr
+        if source:
+            addr = underlayer.src_addr
+            addrmode = underlayer.underlayer.fcf_srcaddrmode
+        else:
+            addr = underlayer.dest_addr
+            addrmode = underlayer.underlayer.fcf_destaddrmode
         addr = struct.pack(">Q", addr)
-        if underlayer.underlayer.fcf_destaddrmode == 3:  # Extended/long
+        if addrmode == 3:  # Extended/long
             tmp_ip = LINK_LOCAL_PREFIX[0:8] + addr
             # Turn off the bit 7.
             return tmp_ip[0:8] + struct.pack("B", (orb(tmp_ip[8]) ^ 0x2)) + tmp_ip[9:16]  # noqa: E501
-        elif underlayer.underlayer.fcf_destaddrmode == 2:  # Short
+        elif addrmode == 2:  # Short
             return (
                 LINK_LOCAL_PREFIX[0:8] +
                 b"\x00\x00\x00\xff\xfe\x00" +

test/scapy/layers/dot15d4.uts

@@ -375,6 +375,16 @@ packet.show2()
 assert packet[LoWPAN_IPHC].src == 'fe80::ff:fe00:0'
 assert packet[LoWPAN_IPHC].dst == 'fe80::ff:fe00:ffff'
 
+= SixLoWPAN - Advanced 4 - SAM=3/SAC=0 decompression
+
+packet = Dot15d4(b'A\xc8t\xcd\xab\xff\xff\xcf\r\xa4\xfe\xffn\x95\xe4z;:\x1a\x9b\x01\xd4^\x1e\xdc\x02\xba\x95\x7f\x00\x00\x00\x01\x00\x00\x00\x00\x00\x00\xe6\x95n\xff\xfe\xaf\xff\xfd\x08\x1e@@\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x01\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x03\x16\x80\x10\xff\xff\xff\xff\x00\x01\x00\x00\x00\x00\x00\x00\xe6\x95n\xff\xfe\xa4\x05O')
+assert packet[LoWPAN_IPHC].sac == 0
+assert packet[LoWPAN_IPHC].sam == 3
+assert packet[LoWPAN_IPHC].dac == 0
+assert packet[LoWPAN_IPHC].dam == 3
+assert packet[LoWPAN_IPHC].src == "fe80::e695:6eff:fea4:dcf"
+assert packet[LoWPAN_IPHC].dst == "ff02::1a"
+
 = SixLoWPAN - Using ICMP
 
 #ICMP: Neighbour Solicitation