rlp: fix list bounds check overflow (found by go-fuzz)

The list size checking overflowed if the size information
for a value was bigger than the list. This is resolved by
always performing the check before reading.

rlp/decode.go

@@ -820,6 +820,16 @@ func (s *Stream) Kind() (kind Kind, size uint64, err error) {
 func (s *Stream) readKind() (kind Kind, size uint64, err error) {
 	b, err := s.readByte()
 	if err != nil {
+		if len(s.stack) == 0 {
+			// At toplevel, Adjust the error to actual EOF. io.EOF is
+			// used by callers to determine when to stop decoding.
+			switch err {
+			case io.ErrUnexpectedEOF:
+				err = io.EOF
+			case ErrValueTooLarge:
+				err = io.EOF
+			}
+		}
 		return 0, 0, err
 	}
 	s.byteval = 0
@@ -876,9 +886,6 @@ func (s *Stream) readUint(size byte) (uint64, error) {
 		return 0, nil
 	case 1:
 		b, err := s.readByte()
-		if err == io.EOF {
-			err = io.ErrUnexpectedEOF
-		}
 		return uint64(b), err
 	default:
 		start := int(8 - size)
@@ -899,10 +906,9 @@ func (s *Stream) readUint(size byte) (uint64, error) {
 }
 
 func (s *Stream) readFull(buf []byte) (err error) {
-	if s.limited && s.remaining < uint64(len(buf)) {
-		return ErrValueTooLarge
+	if err := s.willRead(uint64(len(buf))); err != nil {
+		return err
 	}
-	s.willRead(uint64(len(buf)))
 	var nn, n int
 	for n < len(buf) && err == nil {
 		nn, err = s.r.Read(buf[n:])
@@ -915,23 +921,32 @@ func (s *Stream) readFull(buf []byte) (err error) {
 }
 
 func (s *Stream) readByte() (byte, error) {
-	if s.limited && s.remaining == 0 {
-		return 0, io.EOF
+	if err := s.willRead(1); err != nil {
+		return 0, err
 	}
-	s.willRead(1)
 	b, err := s.r.ReadByte()
-	if len(s.stack) > 0 && err == io.EOF {
+	if err == io.EOF {
 		err = io.ErrUnexpectedEOF
 	}
 	return b, err
 }
 
-func (s *Stream) willRead(n uint64) {
+func (s *Stream) willRead(n uint64) error {
 	s.kind = -1 // rearm Kind
-	if s.limited {
-		s.remaining -= n
-	}
+
 	if len(s.stack) > 0 {
+		// check list overflow
+		tos := s.stack[len(s.stack)-1]
+		if n > tos.size-tos.pos {
+			return ErrElemTooLarge
+		}
 		s.stack[len(s.stack)-1].pos += n
 	}
+	if s.limited {
+		if n > s.remaining {
+			return ErrValueTooLarge
+		}
+		s.remaining -= n
+	}
+	return nil
 }

rlp/decode_test.go

@@ -119,6 +119,10 @@ func TestStreamErrors(t *testing.T) {
 		{"8158", calls{"Uint", "Uint"}, nil, io.EOF},
 		{"C0", calls{"List", "ListEnd", "List"}, nil, io.EOF},
 
+		{"", calls{"List"}, withoutInputLimit, io.EOF},
+		{"8158", calls{"Uint", "Uint"}, withoutInputLimit, io.EOF},
+		{"C0", calls{"List", "ListEnd", "List"}, withoutInputLimit, io.EOF},
+
 		// Input limit errors.
 		{"81", calls{"Bytes"}, nil, ErrValueTooLarge},
 		{"81", calls{"Uint"}, nil, ErrValueTooLarge},
@@ -426,6 +430,13 @@ var decodeTests = []decodeTest{
 		ptr:   new([]io.Reader),
 		error: "rlp: type io.Reader is not RLP-serializable",
 	},
+
+	// fuzzer crashes
+	{
+		input: "c330f9c030f93030ce3030303030303030bd303030303030",
+		ptr:   new(interface{}),
+		error: "rlp: element is larger than containing list",
+	},
 }
 
 func uintp(i uint) *uint { return &i }