Fixes for Geneve option flags/length field (#17)

* Fix Geneve option flags/length field decoding

The option header length is 5 bits, not 4, so fix the bit shift for
decoding flags, and fix the mask for decoding the length. This also adds
a unit test for decoding a max length option (124 byte payload).

* Fix Geneve option flags/length field serialization

OR-assigning the flag bits into the byte offset for the Geneve option
header flags/length field produced corrupt packets because the buffer
returned by PrependBytes is not guaranteed to be zeroed out. Since this
is the first time that offset is touched, just assign instead.

---------

Co-authored-by: Simeon Miteff <simeon.miteff@corelight.com>

layers/geneve.go

@@ -62,8 +62,8 @@ func decodeGeneveOption(data []byte, gn *Geneve, df gopacket.DecodeFeedback) (*G
 
 	opt.Class = binary.BigEndian.Uint16(data[0:2])
 	opt.Type = data[2]
-	opt.Flags = data[3] >> 4
-	opt.Length = (data[3]&0xf)*4 + 4
+	opt.Flags = data[3] >> 5
+	opt.Length = (data[3]&0x1f)*4 + 4
 
 	if len(data) < int(opt.Length) {
 		df.SetTruncated()
@@ -181,7 +181,7 @@ func (gn *Geneve) SerializeTo(b gopacket.SerializeBuffer, opts gopacket.Serializ
 		bytes[offset] = o.Type
 
 		offset += 1
-		bytes[offset] |= o.Flags << 5
+		bytes[offset] = o.Flags << 5
 		bytes[offset] |= ((o.Length - 4) >> 2) & 0x1f
 
 		offset += 1

layers/geneve_test.go

@@ -58,6 +58,66 @@ var testPacketGeneve3 = []byte{
 	0x2c, 0x2d, 0x2e, 0x2f, 0x30, 0x31, 0x32, 0x33, 0x34, 0x35, 0x36, 0x37,
 }
 
+var testMaxSizeOption = []byte{
+	0x00, 0x01, 0x02, 0x03, 0x04, 0x05, 0x06, 0x07, 0x08, 0x09, 0x0a, 0x0b, 0x0c, 0x0d, 0x0e, 0x0f,
+	0x10, 0x11, 0x12, 0x13, 0x14, 0x15, 0x16, 0x17, 0x18, 0x19, 0x1a, 0x1b, 0x1c, 0x1d, 0x1e, 0x1f,
+	0x20, 0x21, 0x22, 0x23, 0x24, 0x25, 0x26, 0x27, 0x28, 0x29, 0x2a, 0x2b, 0x2c, 0x2d, 0x2e, 0x2f,
+	0x30, 0x31, 0x32, 0x33, 0x34, 0x35, 0x36, 0x37, 0x38, 0x39, 0x3a, 0x3b, 0x3c, 0x3d, 0x3e, 0x3f,
+	0x40, 0x41, 0x42, 0x43, 0x44, 0x45, 0x46, 0x47, 0x48, 0x49, 0x4a, 0x4b, 0x4c, 0x4d, 0x4e, 0x4f,
+	0x50, 0x51, 0x52, 0x53, 0x54, 0x55, 0x56, 0x57, 0x58, 0x59, 0x5a, 0x5b, 0x5c, 0x5d, 0x5e, 0x5f,
+	0x60, 0x61, 0x62, 0x63, 0x64, 0x65, 0x66, 0x67, 0x68, 0x69, 0x6a, 0x6b, 0x6c, 0x6d, 0x6e, 0x6f,
+	0x70, 0x71, 0x72, 0x73, 0x74, 0x75, 0x76, 0x77, 0x78, 0x79, 0x7a, 0x7b,
+}
+
+var testPacketGeneve4 = []byte{
+	0x00, 0x1b, 0x21, 0x3c, 0xac, 0x30, // dst mac
+	0x00, 0x1b, 0x21, 0x3c, 0xab, 0x64, // src mac
+	0x08, 0x00, // ethertype, len
+
+	// IPv4
+	0x45, 0x00,
+	0x01, 0x06, // total length
+	0xdf, 0xad, // identification
+	0x40, 0x00, // flags, fragment offset
+	0x40,       // TTL
+	0x11,       // proto UDP
+	0x32, 0xaf, // header checksum
+	0x14, 0x00, 0x00, 0x01, // src IP
+	0x14, 0x00, 0x00, 0x02, // dst IP
+
+	// UDP?
+	0x31, 0x4a, // src port
+	0x17, 0xc1, // dst port
+	0x00, 0xf2, // length
+	0x00, 0x00, // checksum
+
+	// Start of geneve header
+	0x20, // top 2 bits = version 0, bottom 6 bits = options.len = 32 = 0x20 (32 * 4 = 128 bytes)
+	0x40, 0x65, 0x58, 0x00, 0x00, 0x0a, 0x00,
+
+	// Start of geneve option header (4 bytes)
+	0x00, 0x00, 0x80, 0x1f, // option[0].len = 31 = 0x1f (31 * 4 = 124 bytes)
+
+	// Geneve option payload (124 bytes)
+	0x00, 0x01, 0x02, 0x03, 0x04, 0x05, 0x06, 0x07, 0x08, 0x09, 0x0a, 0x0b, 0x0c, 0x0d, 0x0e, 0x0f,
+	0x10, 0x11, 0x12, 0x13, 0x14, 0x15, 0x16, 0x17, 0x18, 0x19, 0x1a, 0x1b, 0x1c, 0x1d, 0x1e, 0x1f,
+	0x20, 0x21, 0x22, 0x23, 0x24, 0x25, 0x26, 0x27, 0x28, 0x29, 0x2a, 0x2b, 0x2c, 0x2d, 0x2e, 0x2f,
+	0x30, 0x31, 0x32, 0x33, 0x34, 0x35, 0x36, 0x37, 0x38, 0x39, 0x3a, 0x3b, 0x3c, 0x3d, 0x3e, 0x3f,
+	0x40, 0x41, 0x42, 0x43, 0x44, 0x45, 0x46, 0x47, 0x48, 0x49, 0x4a, 0x4b, 0x4c, 0x4d, 0x4e, 0x4f,
+	0x50, 0x51, 0x52, 0x53, 0x54, 0x55, 0x56, 0x57, 0x58, 0x59, 0x5a, 0x5b, 0x5c, 0x5d, 0x5e, 0x5f,
+	0x60, 0x61, 0x62, 0x63, 0x64, 0x65, 0x66, 0x67, 0x68, 0x69, 0x6a, 0x6b, 0x6c, 0x6d, 0x6e, 0x6f,
+	0x70, 0x71, 0x72, 0x73, 0x74, 0x75, 0x76, 0x77, 0x78, 0x79, 0x7a, 0x7b,
+
+	// Remainder of packet
+	0xfe, 0x71, 0xd8, 0x83, 0x72, 0x4f,
+	0xb6, 0x9e, 0xd2, 0x49, 0x51, 0x48, 0x08, 0x00, 0x45, 0x00, 0x00, 0x54, 0xbd, 0xa2, 0x40, 0x00,
+	0x40, 0x01, 0x41, 0x04, 0x1e, 0x00, 0x00, 0x01, 0x1e, 0x00, 0x00, 0x02, 0x08, 0x00, 0x2c, 0x54,
+	0x29, 0x52, 0x00, 0x17, 0xf1, 0xa2, 0xce, 0x54, 0x00, 0x00, 0x00, 0x00, 0x17, 0x78, 0x0c, 0x00,
+	0x00, 0x00, 0x00, 0x00, 0x10, 0x11, 0x12, 0x13, 0x14, 0x15, 0x16, 0x17, 0x18, 0x19, 0x1a, 0x1b,
+	0x1c, 0x1d, 0x1e, 0x1f, 0x20, 0x21, 0x22, 0x23, 0x24, 0x25, 0x26, 0x27, 0x28, 0x29, 0x2a, 0x2b,
+	0x2c, 0x2d, 0x2e, 0x2f, 0x30, 0x31, 0x32, 0x33, 0x34, 0x35, 0x36, 0x37,
+}
+
 func TestDecodeGeneve1(t *testing.T) {
 	p := gopacket.NewPacket(testPacketGeneve1, LinkTypeLinuxSLL, gopacket.Default)
 	if p.ErrorLayer() != nil {
@@ -150,6 +210,42 @@ func TestDecodeGeneve3(t *testing.T) {
 	}
 }
 
+func TestDecodeGeneve4(t *testing.T) {
+	p := gopacket.NewPacket(testPacketGeneve4, LinkTypeEthernet, gopacket.Default)
+	if p.ErrorLayer() != nil {
+		t.Error("Failed to decode packet:", p.ErrorLayer().Error())
+	}
+	checkLayers(p, []gopacket.LayerType{
+		LayerTypeEthernet, LayerTypeIPv4, LayerTypeUDP, LayerTypeGeneve,
+		LayerTypeEthernet, LayerTypeIPv4, LayerTypeICMPv4, gopacket.LayerTypePayload,
+	}, t)
+	if got, ok := p.Layer(LayerTypeGeneve).(*Geneve); ok {
+		want := &Geneve{
+			BaseLayer: BaseLayer{
+				Contents: testPacketGeneve4[42:178],
+				Payload:  testPacketGeneve4[178:276],
+			},
+			Version:        0x0,
+			OptionsLength:  124 + 4,
+			OAMPacket:      false,
+			CriticalOption: true,
+			Protocol:       EthernetTypeTransparentEthernetBridging,
+			VNI:            0xa,
+			Options: []*GeneveOption{
+				{
+					Class:  0x0,
+					Type:   0x80,
+					Length: 124 + 4,
+					Data:   testMaxSizeOption,
+				},
+			},
+		}
+		if !reflect.DeepEqual(want, got) {
+			t.Errorf("Geneve layer mismatch, \nwant %#v\ngot  %#v\n", want, got)
+		}
+	}
+}
+
 func BenchmarkDecodeGeneve1(b *testing.B) {
 	for i := 0; i < b.N; i++ {
 		gopacket.NewPacket(testPacketGeneve1, LinkTypeEthernet, gopacket.NoCopy)