Cover a couple of more cases on data track runt packet handling. (#4741)

* Cover a couple of more cases on data track runt packet handling.

* Guard data track header parser against extensions-size integer wraparound.

Widen the extensions-size arithmetic to int so a 0xFFFF wire value no
longer wraps in uint16, and reject any packet whose computed hdrSize
exceeds the buffer before slicing the payload.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

---------

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

pkg/rtc/datatrack/packet.go

@@ -126,11 +126,14 @@ func (h *Header) Unmarshal(buf []byte) (int, error) {
 	h.Timestamp = binary.BigEndian.Uint32(buf[timestampOffset : timestampOffset+timestampLength])
 
 	if h.HasExtensions {
-		extensionsSize := (binary.BigEndian.Uint16(buf[extensionsSizeOffset:extensionsSizeOffset+extensionsSizeLength])+1)*4 - extensionsSizeLength
+		if len(buf) < extensionsSizeOffset+extensionsSizeLength {
+			return 0, fmt.Errorf("%w: %d < %d", errHeaderSizeInsufficient, len(buf), extensionsSizeOffset+extensionsSizeLength)
+		}
+		extensionsSize := (int(binary.BigEndian.Uint16(buf[extensionsSizeOffset:extensionsSizeOffset+extensionsSizeLength]))+1)*4 - extensionsSizeLength
 		hdrSize += extensionsSizeLength
 
 		extensionHeaderSize := extensionIDLength + extensionSizeLength
-		remainingSize := int(extensionsSize)
+		remainingSize := extensionsSize
 		idx := extensionsSizeOffset + extensionsSizeLength
 		for remainingSize != 0 {
 			// read extension header
@@ -140,6 +143,9 @@ func (h *Header) Unmarshal(buf []byte) (int, error) {
 			id := buf[idx]
 			if id == 0 {
 				// end of extensions, padding has started
+				if len(buf[idx:]) < remainingSize {
+					return 0, fmt.Errorf("%w: %d/%d < %d", errExtensionSizeInsufficient, remainingSize, len(buf[idx:]), remainingSize)
+				}
 				hdrSize += remainingSize
 				break
 			}
@@ -163,7 +169,7 @@ func (h *Header) Unmarshal(buf []byte) (int, error) {
 			idx += size
 			hdrSize += size
 		}
-		h.ExtensionsSize = extensionsSize - uint16(remainingSize)
+		h.ExtensionsSize = uint16(extensionsSize - remainingSize)
 	}
 
 	return hdrSize, nil
@@ -271,6 +277,9 @@ func (p *Packet) Unmarshal(buf []byte) error {
 	if err != nil {
 		return err
 	}
+	if hdrSize > len(buf) {
+		return fmt.Errorf("%w: %d < %d", errBufferSizeInsufficient, len(buf), hdrSize)
+	}
 
 	p.Payload = buf[hdrSize:]
 	return nil

pkg/rtc/datatrack/packet_test.go

@@ -256,4 +256,39 @@ func TestPacket(t *testing.T) {
 		err = unmarshaled.Unmarshal(badPacket)
 		require.Error(t, err)
 	})
+
+	t.Run("oversized extension padding does not panic", func(t *testing.T) {
+		var unmarshaled Packet
+		// HasExtensions set, extensionsSize describes more bytes than present,
+		// terminated by a 0x00 padding id -> hdrSize would exceed len(buf)
+		badPacket := []byte{
+			0x04, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
+			0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
+		}
+		err := unmarshaled.Unmarshal(badPacket)
+		require.Error(t, err)
+	})
+
+	t.Run("extensions size wraparound does not panic", func(t *testing.T) {
+		var unmarshaled Packet
+		// 0xFFFF extensions-size field wraps (raw+1)*4 uint16 arithmetic to a huge
+		// remainingSize; the 0x00 padding id must not push hdrSize past len(buf)
+		badPacket := []byte{
+			0x04, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
+			0x00, 0x00, 0x00, 0x00, 0xff, 0xff, 0x00,
+		}
+		err := unmarshaled.Unmarshal(badPacket)
+		require.Error(t, err)
+	})
+
+	t.Run("truncated extensions size field does not panic", func(t *testing.T) {
+		var unmarshaled Packet
+		// HasExtensions set but buffer too short to hold the extensionsSize field
+		badPacket := []byte{
+			0x04, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
+			0x00, 0x00, 0x00, 0x00, 0x00,
+		}
+		err := unmarshaled.Unmarshal(badPacket)
+		require.Error(t, err)
+	})
 }