multi: enforce strict TLV length checks

This commit improves TLV decoding safety and consistency across multiple
packages by enforcing fixed-length requirements and adding unit tests to
prevent malformed TLV records from being accepted.

Changes include:

- lnwire:
  * Enforce 8-byte length in Fee TLV decoder.
  * Enforce PubNonceSize in Musig2Nonce TLV decoder.
  * Enforce 8-byte length in ShortChannelID TLV decoder.
  * Added roundtrip and invalid length tests for Fee, Musig2Nonce,
    and ShortChannelID records.

- routing/route:
  * Enforce Vertex TLV length (33 bytes).
  * Added encode/decode and invalid length tests for Vertex.

- tlv:
  * Enforce correct length in DBytes33 decoder (33 bytes).
  * Added tests ensuring all fixed-size primitive decoders reject
    incorrect TLV lengths.

By strictly validating TLV lengths, we prevent malformed or corrupted
TLV records from being silently accepted, improving protocol safety.

lnwire/musig2.go

@@ -51,7 +51,7 @@ func nonceTypeEncoder(w io.Writer, val interface{}, _ *[8]byte) error {
 func nonceTypeDecoder(r io.Reader, val interface{}, _ *[8]byte,
 	l uint64) error {
 
-	if v, ok := val.(*Musig2Nonce); ok {
+	if v, ok := val.(*Musig2Nonce); ok && l == musig2.PubNonceSize {
 		_, err := io.ReadFull(r, v[:])
 		return err
 	}

lnwire/musig2_test.go

@@ -0,0 +1,62 @@
+package lnwire
+
+import (
+	"testing"
+
+	"github.com/btcsuite/btcd/btcec/v2/schnorr/musig2"
+	"github.com/stretchr/testify/require"
+)
+
+// makeNonce creates a test Musig2Nonce with sequential byte values for testing
+// TLV encoding/decoding.
+func makeNonce() Musig2Nonce {
+	var n Musig2Nonce
+	for i := range n {
+		n[i] = byte(i)
+	}
+
+	return n
+}
+
+// TestMusig2NonceEncodeDecode tests that we're able to properly encode and
+// decode Musig2Nonce within TLV streams.
+func TestMusig2NonceEncodeDecode(t *testing.T) {
+	t.Parallel()
+
+	nonce := makeNonce()
+
+	var extraData ExtraOpaqueData
+	require.NoError(t, extraData.PackRecords(&nonce))
+
+	var extractedNonce Musig2Nonce
+	_, err := extraData.ExtractRecords(&extractedNonce)
+	require.NoError(t, err)
+
+	require.Equal(t, nonce, extractedNonce)
+}
+
+// TestMusig2NonceTypeDecodeInvalidLength ensures that decoding a Musig2Nonce
+// TLV with an invalid length (anything other than 66 bytes) fails with an
+// error.
+func TestMusig2NonceTypeDecodeInvalidLength(t *testing.T) {
+	t.Parallel()
+
+	nonce := makeNonce()
+
+	var extraData ExtraOpaqueData
+	require.NoError(t, extraData.PackRecords(&nonce))
+
+	// Corrupt the TLV length field to simulate malformed input.
+	// Byte 1 contains the varint size encoding. Since 66 bytes fits into
+	// a single varint byte, we can directly modify extraData[1].
+	extraData[1] = musig2.PubNonceSize + 1
+
+	var out Musig2Nonce
+	_, err := extraData.ExtractRecords(&out)
+	require.Error(t, err)
+
+	extraData[1] = musig2.PubNonceSize - 1
+
+	_, err = extraData.ExtractRecords(&out)
+	require.Error(t, err)
+}

lnwire/short_channel_id.go

@@ -92,7 +92,8 @@ func DShortChannelID(r io.Reader, val interface{}, buf *[8]byte,
 
 	if v, ok := val.(*ShortChannelID); ok {
 		var scid uint64
-		err := tlv.DUint64(r, &scid, buf, 8)
+		// tlv.DUint64 forces the length to be 8 bytes.
+		err := tlv.DUint64(r, &scid, buf, l)
 		if err != nil {
 			return err
 		}

lnwire/short_channel_id_test.go

@@ -62,3 +62,28 @@ func TestScidTypeEncodeDecode(t *testing.T) {
 	require.Contains(t, tlvs, AliasScidRecordType)
 	require.Equal(t, aliasScid, aliasScid2)
 }
+
+// TestScidTypeDecodeInvalidLength ensures that decoding a ShortChannelID TLV
+// with an invalid length (anything other than 8 bytes) fails with an error.
+func TestScidTypeDecodeInvalidLength(t *testing.T) {
+	t.Parallel()
+
+	aliasScid := ShortChannelID{
+		BlockHeight: 1, TxIndex: 1, TxPosition: 1,
+	}
+
+	var extraData ExtraOpaqueData
+	require.NoError(t, extraData.PackRecords(&aliasScid))
+
+	// Corrupt the TLV length field to simulate malformed input.
+	extraData[1] = 8 + 1
+
+	var out ShortChannelID
+	_, err := extraData.ExtractRecords(&out)
+	require.Error(t, err)
+
+	extraData[1] = 8 - 1
+
+	_, err = extraData.ExtractRecords(&out)
+	require.Error(t, err)
+}

lnwire/typed_fee.go

@@ -41,7 +41,7 @@ func feeEncoder(w io.Writer, val interface{}, buf *[8]byte) error {
 // feeDecoder is a custom TLV decoder for the fee record.
 func feeDecoder(r io.Reader, val interface{}, buf *[8]byte, l uint64) error {
 	v, ok := val.(*Fee)
-	if !ok {
+	if !ok || l != 8 {
 		return tlv.NewTypeForDecodingErr(val, "lnwire.Fee", l, 8)
 	}
 

lnwire/typed_fee_test.go

@@ -38,3 +38,28 @@ func testTypedFee(t *testing.T, fee Fee) { //nolint: thelper
 
 	require.Equal(t, fee, extractedFee)
 }
+
+// TestTypedFeeTypeDecodeInvalidLength ensures that decoding a Fee TLV
+// with an invalid length (anything other than 8 bytes) fails with an error.
+func TestTypedFeeTypeDecodeInvalidLength(t *testing.T) {
+	t.Parallel()
+
+	fee := Fee{
+		BaseFee: 1, FeeRate: 1,
+	}
+
+	var extraData ExtraOpaqueData
+	require.NoError(t, extraData.PackRecords(&fee))
+
+	// Corrupt the TLV length field to simulate malformed input.
+	extraData[3] = 8 + 1
+
+	var out Fee
+	_, err := extraData.ExtractRecords(&out)
+	require.Error(t, err)
+
+	extraData[3] = 8 - 1
+
+	_, err = extraData.ExtractRecords(&out)
+	require.Error(t, err)
+}

routing/route/route.go

@@ -112,7 +112,7 @@ func encodeVertex(w io.Writer, val interface{}, _ *[8]byte) error {
 }
 
 func decodeVertex(r io.Reader, val interface{}, _ *[8]byte, l uint64) error {
-	if b, ok := val.(*Vertex); ok {
+	if b, ok := val.(*Vertex); ok && l == VertexSize {
 		_, err := io.ReadFull(r, b[:])
 		return err
 	}

routing/route/route_test.go

@@ -8,6 +8,7 @@ import (
 	"github.com/btcsuite/btcd/btcec/v2"
 	"github.com/lightningnetwork/lnd/lnwire"
 	"github.com/lightningnetwork/lnd/record"
+	"github.com/lightningnetwork/lnd/tlv"
 	"github.com/stretchr/testify/require"
 )
 
@@ -430,3 +431,57 @@ func TestBlindedHopFee(t *testing.T) {
 	require.Equal(t, lnwire.MilliSatoshi(0), route.HopFee(3))
 	require.Equal(t, lnwire.MilliSatoshi(0), route.HopFee(4))
 }
+
+// makeVertex creates a test Vertex with sequential byte values for testing
+// TLV encoding/decoding.
+func makeVertex() Vertex {
+	var v Vertex
+	for i := range v {
+		v[i] = byte(i)
+	}
+
+	return v
+}
+
+// TestVertexTLVEncodeDecode tests that we're able to properly encode and decode
+// Vertex within TLV streams.
+func TestVertexTLVEncodeDecode(t *testing.T) {
+	t.Parallel()
+
+	vertex := makeVertex()
+
+	var extraData lnwire.ExtraOpaqueData
+	require.NoError(t, extraData.PackRecords(&vertex))
+
+	var vertex2 Vertex
+	tlvs, err := extraData.ExtractRecords(&vertex2)
+	require.NoError(t, err)
+
+	require.Contains(t, tlvs, tlv.Type(0))
+	require.Equal(t, vertex, vertex2)
+}
+
+// TestVertexTypeDecodeInvalidLength ensures that decoding a Vertex TLV
+// with an invalid length (anything other than 33) fails with an error.
+func TestVertexTypeDecodeInvalidLength(t *testing.T) {
+	t.Parallel()
+
+	vertex := makeVertex()
+
+	var extraData lnwire.ExtraOpaqueData
+	require.NoError(t, extraData.PackRecords(&vertex))
+
+	// Corrupt the TLV length field to simulate malformed input.
+	// Byte 1 contains the varint size encoding. Since 33 bytes fits into
+	// a single varint byte, we can directly modify extraData[1].
+	extraData[1] = VertexSize + 1
+
+	var out Vertex
+	_, err := extraData.ExtractRecords(&out)
+	require.Error(t, err)
+
+	extraData[1] = VertexSize - 1
+
+	_, err = extraData.ExtractRecords(&out)
+	require.Error(t, err)
+}

tlv/primitive.go

@@ -257,7 +257,7 @@ func EBytes33(w io.Writer, val interface{}, _ *[8]byte) error {
 // DBytes33 is a Decoder for 33-byte arrays. An error is returned if val is not
 // a *[33]byte.
 func DBytes33(r io.Reader, val interface{}, _ *[8]byte, l uint64) error {
-	if b, ok := val.(*[33]byte); ok {
+	if b, ok := val.(*[33]byte); ok && l == 33 {
 		_, err := io.ReadFull(r, b[:])
 		return err
 	}

tlv/primitive_test.go

@@ -7,6 +7,7 @@ import (
 
 	"github.com/btcsuite/btcd/btcec/v2"
 	"github.com/lightningnetwork/lnd/tlv"
+	"github.com/stretchr/testify/require"
 )
 
 var testPK, _ = btcec.ParsePubKey([]byte{0x02,
@@ -253,3 +254,89 @@ func TestPrimitiveEncodings(t *testing.T) {
 			prim, prim2)
 	}
 }
+
+// TestPrimitiveWrongLength asserts that fixed-size primitive decoders fail
+// with ErrTypeForDecoding when given an incorrect TLV length.
+func TestPrimitiveWrongLength(t *testing.T) {
+	prim := primitive{
+		u8:      0x01,
+		u16:     0x0201,
+		u32:     0x02000001,
+		u64:     0x0200000000000001,
+		b32:     [32]byte{0x02, 0x01},
+		b33:     [33]byte{0x03, 0x01},
+		b64:     [64]byte{0x02, 0x01},
+		pk:      testPK,
+		boolean: true,
+	}
+
+	type item struct {
+		enc fieldEncoder
+		dec fieldDecoder
+	}
+
+	items := []item{
+		{
+			fieldEncoder{&prim.u8, tlv.EUint8},
+			fieldDecoder{new(byte), tlv.DUint8, 1},
+		},
+		{
+			fieldEncoder{&prim.u16, tlv.EUint16},
+			fieldDecoder{new(uint16), tlv.DUint16, 2},
+		},
+		{
+			fieldEncoder{&prim.u32, tlv.EUint32},
+			fieldDecoder{new(uint32), tlv.DUint32, 4},
+		},
+		{
+			fieldEncoder{&prim.u64, tlv.EUint64},
+			fieldDecoder{new(uint64), tlv.DUint64, 8},
+		},
+		{
+			fieldEncoder{&prim.b32, tlv.EBytes32},
+			fieldDecoder{new([32]byte), tlv.DBytes32, 32},
+		},
+		{
+			fieldEncoder{&prim.b33, tlv.EBytes33},
+			fieldDecoder{new([33]byte), tlv.DBytes33, 33},
+		},
+		{
+			fieldEncoder{&prim.b64, tlv.EBytes64},
+			fieldDecoder{new([64]byte), tlv.DBytes64, 64},
+		},
+		{
+			fieldEncoder{&prim.pk, tlv.EPubKey},
+			fieldDecoder{new(*btcec.PublicKey), tlv.DPubKey, 33},
+		},
+		{
+			fieldEncoder{&prim.boolean, tlv.EBool},
+			fieldDecoder{new(bool), tlv.DBool, 1},
+		},
+	}
+
+	for _, it := range items {
+		var buf [8]byte
+		var b bytes.Buffer
+		err := it.enc.encoder(&b, it.enc.val, &buf)
+		require.NoError(t, err, "encode %T", it.enc.val)
+		data := b.Bytes()
+
+		// Generate two wrong lengths: expected-1 (if >0) and
+		// expected+1.
+		wrongs := []uint64{it.dec.size + 1}
+		if it.dec.size > 0 {
+			wrongs = append(wrongs, it.dec.size-1)
+		}
+
+		for _, l := range wrongs {
+			r := bytes.NewReader(data)
+			err := it.dec.decoder(r, it.dec.val, &buf, l)
+			require.ErrorAs(
+				t, err, &tlv.ErrTypeForDecoding{},
+				"decoder %T should reject wrong length "+
+					"%d (expected %d)",
+				it.dec.decoder, l, it.dec.size,
+			)
+		}
+	}
+}