rlp: check top-level value sizes against input limit

This is a preliminary fix for #420 (SEC-18 RLP decoder unsafe
allocation). If a sane input limit is set on the rlp.Stream,
it should no longer be possible to cause huge []byte allocations.

rlp/decode.go

@@ -9,6 +9,7 @@ import (
 	"io"
 	"math/big"
 	"reflect"
+	"strings"
 )
 
 var (
@@ -70,14 +71,22 @@ type Decoder interface {
 // Non-empty interface types are not supported, nor are booleans,
 // signed integers, floating point numbers, maps, channels and
 // functions.
+//
+// Note that Decode does not set an input limit for all readers
+// and may be vulnerable to panics cause by huge value sizes. If
+// you need an input limit, use
+//
+//     NewStream(r, limit).Decode(val)
 func Decode(r io.Reader, val interface{}) error {
-	return NewStream(r).Decode(val)
+	// TODO: this could use a Stream from a pool.
+	return NewStream(r, 0).Decode(val)
 }
 
 // DecodeBytes parses RLP data from b into val.
 // Please see the documentation of Decode for the decoding rules.
 func DecodeBytes(b []byte, val interface{}) error {
-	return NewStream(bytes.NewReader(b)).Decode(val)
+	// TODO: this could use a Stream from a pool.
+	return NewStream(bytes.NewReader(b), uint64(len(b))).Decode(val)
 }
 
 type decodeError struct {
@@ -470,10 +479,12 @@ var (
 	ErrExpectedList   = errors.New("rlp: expected List")
 	ErrCanonInt       = errors.New("rlp: expected Int")
 	ErrElemTooLarge   = errors.New("rlp: element is larger than containing list")
+	ErrValueTooLarge  = errors.New("rlp: value size exceeds available input length")
 
 	// internal errors
-	errNotInList = errors.New("rlp: call of ListEnd outside of any list")
-	errNotAtEOL  = errors.New("rlp: call of ListEnd not positioned at EOL")
+	errNotInList    = errors.New("rlp: call of ListEnd outside of any list")
+	errNotAtEOL     = errors.New("rlp: call of ListEnd not positioned at EOL")
+	errUintOverflow = errors.New("rlp: uint overflow")
 )
 
 // ByteReader must be implemented by any input reader for a Stream. It
@@ -496,7 +507,13 @@ type ByteReader interface {
 //
 // Stream is not safe for concurrent use.
 type Stream struct {
-	r       ByteReader
+	r ByteReader
+
+	// number of bytes remaining to be read from r.
+	remaining uint64
+	limited   bool
+
+	// auxiliary buffer for integer decoding
 	uintbuf []byte
 
 	kind    Kind   // kind of value ahead
@@ -507,12 +524,26 @@ type Stream struct {
 
 type listpos struct{ pos, size uint64 }
 
-// NewStream creates a new stream reading from r.
-// If r does not implement ByteReader, the Stream will
-// introduce its own buffering.
-func NewStream(r io.Reader) *Stream {
+// NewStream creates a new decoding stream reading from r.
+//
+// If r implements the ByteReader interface, Stream will
+// not introduce any buffering.
+//
+// For non-toplevel values, Stream returns ErrElemTooLarge
+// for values that do not fit into the enclosing list.
+//
+// Stream supports an optional input limit. If a limit is set, the
+// size of any toplevel value will be checked against the remaining
+// input length. Stream operations that encounter a value exceeding
+// the remaining input length will return ErrValueTooLarge. The limit
+// can be set by passing a non-zero value for inputLimit.
+//
+// If r is a bytes.Reader or strings.Reader, the input limit is set to
+// the length of r's underlying data unless an explicit limit is
+// provided.
+func NewStream(r io.Reader, inputLimit uint64) *Stream {
 	s := new(Stream)
-	s.Reset(r)
+	s.Reset(r, inputLimit)
 	return s
 }
 
@@ -520,7 +551,7 @@ func NewStream(r io.Reader) *Stream {
 // at an encoded list of the given length.
 func NewListStream(r io.Reader, len uint64) *Stream {
 	s := new(Stream)
-	s.Reset(r)
+	s.Reset(r, len)
 	s.kind = List
 	s.size = len
 	return s
@@ -574,8 +605,6 @@ func (s *Stream) Raw() ([]byte, error) {
 	return buf, nil
 }
 
-var errUintOverflow = errors.New("rlp: uint overflow")
-
 // Uint reads an RLP string of up to 8 bytes and returns its contents
 // as an unsigned integer. If the input does not contain an RLP string, the
 // returned error will be ErrExpectedString.
@@ -667,14 +696,36 @@ func (s *Stream) Decode(val interface{}) error {
 }
 
 // Reset discards any information about the current decoding context
-// and starts reading from r. If r does not also implement ByteReader,
-// Stream will do its own buffering.
-func (s *Stream) Reset(r io.Reader) {
+// and starts reading from r. This method is meant to facilitate reuse
+// of a preallocated Stream across many decoding operations.
+//
+// If r does not also implement ByteReader, Stream will do its own
+// buffering.
+func (s *Stream) Reset(r io.Reader, inputLimit uint64) {
+	if inputLimit > 0 {
+		s.remaining = inputLimit
+		s.limited = true
+	} else {
+		// Attempt to automatically discover
+		// the limit when reading from a byte slice.
+		switch br := r.(type) {
+		case *bytes.Reader:
+			s.remaining = uint64(br.Len())
+			s.limited = true
+		case *strings.Reader:
+			s.remaining = uint64(br.Len())
+			s.limited = true
+		default:
+			s.limited = false
+		}
+	}
+	// Wrap r with a buffer if it doesn't have one.
 	bufr, ok := r.(ByteReader)
 	if !ok {
 		bufr = bufio.NewReader(r)
 	}
 	s.r = bufr
+	// Reset the decoding context.
 	s.stack = s.stack[:0]
 	s.size = 0
 	s.kind = -1
@@ -700,6 +751,8 @@ func (s *Stream) Kind() (kind Kind, size uint64, err error) {
 		tos = &s.stack[len(s.stack)-1]
 	}
 	if s.kind < 0 {
+		// don't read further if we're at the end of the
+		// innermost list.
 		if tos != nil && tos.pos == tos.size {
 			return 0, 0, EOL
 		}
@@ -709,8 +762,19 @@ func (s *Stream) Kind() (kind Kind, size uint64, err error) {
 		}
 		s.kind, s.size = kind, size
 	}
-	if tos != nil && tos.pos+s.size > tos.size {
-		return 0, 0, ErrElemTooLarge
+	// Make sure size is reasonable. This is done always
+	// so Kind returns the same error when called multiple times.
+	if tos == nil {
+		// At toplevel, check that the value is smaller
+		// than the remaining input length.
+		if s.limited && s.size > s.remaining {
+			return 0, 0, ErrValueTooLarge
+		}
+	} else {
+		// Inside a list, check that the value doesn't overflow the list.
+		if tos.pos+s.size > tos.size {
+			return 0, 0, ErrElemTooLarge
+		}
 	}
 	return s.kind, s.size, nil
 }
@@ -778,6 +842,9 @@ 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
+	}
 	s.willRead(uint64(len(buf)))
 	var nn, n int
 	for n < len(buf) && err == nil {
@@ -791,6 +858,9 @@ func (s *Stream) readFull(buf []byte) (err error) {
 }
 
 func (s *Stream) readByte() (byte, error) {
+	if s.limited && s.remaining == 0 {
+		return 0, io.EOF
+	}
 	s.willRead(1)
 	b, err := s.r.ReadByte()
 	if len(s.stack) > 0 && err == io.EOF {
@@ -801,6 +871,9 @@ func (s *Stream) readByte() (byte, error) {
 
 func (s *Stream) willRead(n uint64) {
 	s.kind = -1 // rearm Kind
+	if s.limited {
+		s.remaining -= n
+	}
 	if len(s.stack) > 0 {
 		s.stack[len(s.stack)-1].pos += n
 	}

rlp/decode_test.go

@@ -36,7 +36,8 @@ func TestStreamKind(t *testing.T) {
 	}
 
 	for i, test := range tests {
-		s := NewStream(bytes.NewReader(unhex(test.input)))
+		// using plainReader to inhibit input limit errors.
+		s := NewStream(newPlainReader(unhex(test.input)), 0)
 		kind, len, err := s.Kind()
 		if err != nil {
 			t.Errorf("test %d: Kind returned error: %v", i, err)
@@ -70,29 +71,63 @@ func TestNewListStream(t *testing.T) {
 }
 
 func TestStreamErrors(t *testing.T) {
+	withoutInputLimit := func(b []byte) *Stream {
+		return NewStream(newPlainReader(b), 0)
+	}
+	withCustomInputLimit := func(limit uint64) func([]byte) *Stream {
+		return func(b []byte) *Stream {
+			return NewStream(bytes.NewReader(b), limit)
+		}
+	}
+
 	type calls []string
 	tests := []struct {
 		string
 		calls
+		newStream func([]byte) *Stream // uses bytes.Reader if nil
 		error
 	}{
-		{"", calls{"Kind"}, io.EOF},
-		{"", calls{"List"}, io.EOF},
-		{"", calls{"Uint"}, io.EOF},
-		{"C0", calls{"Bytes"}, ErrExpectedString},
-		{"C0", calls{"Uint"}, ErrExpectedString},
-		{"81", calls{"Bytes"}, io.ErrUnexpectedEOF},
-		{"81", calls{"Uint"}, io.ErrUnexpectedEOF},
-		{"BFFFFFFFFFFFFFFF", calls{"Bytes"}, io.ErrUnexpectedEOF},
-		{"89000000000000000001", calls{"Uint"}, errUintOverflow},
-		{"00", calls{"List"}, ErrExpectedList},
-		{"80", calls{"List"}, ErrExpectedList},
-		{"C0", calls{"List", "Uint"}, EOL},
-		{"C801", calls{"List", "Uint", "Uint"}, io.ErrUnexpectedEOF},
-		{"C8C9", calls{"List", "Kind"}, ErrElemTooLarge},
-		{"C3C2010201", calls{"List", "List", "Uint", "Uint", "ListEnd", "Uint"}, EOL},
-		{"00", calls{"ListEnd"}, errNotInList},
-		{"C40102", calls{"List", "Uint", "ListEnd"}, errNotAtEOL},
+		{"C0", calls{"Bytes"}, nil, ErrExpectedString},
+		{"C0", calls{"Uint"}, nil, ErrExpectedString},
+		{"89000000000000000001", calls{"Uint"}, nil, errUintOverflow},
+		{"00", calls{"List"}, nil, ErrExpectedList},
+		{"80", calls{"List"}, nil, ErrExpectedList},
+		{"C0", calls{"List", "Uint"}, nil, EOL},
+		{"C8C9010101010101010101", calls{"List", "Kind"}, nil, ErrElemTooLarge},
+		{"C3C2010201", calls{"List", "List", "Uint", "Uint", "ListEnd", "Uint"}, nil, EOL},
+		{"00", calls{"ListEnd"}, nil, errNotInList},
+		{"C401020304", calls{"List", "Uint", "ListEnd"}, nil, errNotAtEOL},
+
+		// Expected EOF
+		{"", calls{"Kind"}, nil, io.EOF},
+		{"", calls{"Uint"}, nil, io.EOF},
+		{"", calls{"List"}, nil, io.EOF},
+		{"8105", calls{"Uint", "Uint"}, nil, io.EOF},
+		{"C0", calls{"List", "ListEnd", "List"}, nil, io.EOF},
+
+		// Input limit errors.
+		{"81", calls{"Bytes"}, nil, ErrValueTooLarge},
+		{"81", calls{"Uint"}, nil, ErrValueTooLarge},
+		{"81", calls{"Raw"}, nil, ErrValueTooLarge},
+		{"BFFFFFFFFFFFFFFFFFFF", calls{"Bytes"}, nil, ErrValueTooLarge},
+		{"C801", calls{"List"}, nil, ErrValueTooLarge},
+
+		// Test for input limit overflow. Since we are counting the limit
+		// down toward zero in Stream.remaining, reading too far can overflow
+		// remaining to a large value, effectively disabling the limit.
+		{"C40102030401", calls{"Raw", "Uint"}, withCustomInputLimit(5), io.EOF},
+		{"C4010203048102", calls{"Raw", "Uint"}, withCustomInputLimit(6), ErrValueTooLarge},
+
+		// Check that the same calls are fine without a limit.
+		{"C40102030401", calls{"Raw", "Uint"}, withoutInputLimit, nil},
+		{"C4010203048102", calls{"Raw", "Uint"}, withoutInputLimit, nil},
+
+		// Unexpected EOF. This only happens when there is
+		// no input limit, so the reader needs to be 'dumbed down'.
+		{"81", calls{"Bytes"}, withoutInputLimit, io.ErrUnexpectedEOF},
+		{"81", calls{"Uint"}, withoutInputLimit, io.ErrUnexpectedEOF},
+		{"BFFFFFFFFFFFFFFF", calls{"Bytes"}, withoutInputLimit, io.ErrUnexpectedEOF},
+		{"C801", calls{"List", "Uint", "Uint"}, withoutInputLimit, io.ErrUnexpectedEOF},
 
 		// This test verifies that the input position is advanced
 		// correctly when calling Bytes for empty strings. Kind can be called
@@ -109,12 +144,15 @@ func TestStreamErrors(t *testing.T) {
 
 			"Bytes", // past final element
 			"Bytes", // this one should fail
-		}, EOL},
+		}, nil, EOL},
 	}
 
 testfor:
 	for i, test := range tests {
-		s := NewStream(bytes.NewReader(unhex(test.string)))
+		if test.newStream == nil {
+			test.newStream = func(b []byte) *Stream { return NewStream(bytes.NewReader(b), 0) }
+		}
+		s := test.newStream(unhex(test.string))
 		rs := reflect.ValueOf(s)
 		for j, call := range test.calls {
 			fval := rs.MethodByName(call)
@@ -124,8 +162,12 @@ testfor:
 				err = lastret.(error).Error()
 			}
 			if j == len(test.calls)-1 {
-				if err != test.error.Error() {
-					t.Errorf("test %d: last call (%s) error mismatch\ngot:  %s\nwant: %v",
+				want := "<nil>"
+				if test.error != nil {
+					want = test.error.Error()
+				}
+				if err != want {
+					t.Errorf("test %d: last call (%s) error mismatch\ngot:  %s\nwant: %s",
 						i, call, err, test.error)
 				}
 			} else if err != "<nil>" {
@@ -137,7 +179,7 @@ testfor:
 }
 
 func TestStreamList(t *testing.T) {
-	s := NewStream(bytes.NewReader(unhex("C80102030405060708")))
+	s := NewStream(bytes.NewReader(unhex("C80102030405060708")), 0)
 
 	len, err := s.List()
 	if err != nil {
@@ -166,7 +208,7 @@ func TestStreamList(t *testing.T) {
 }
 
 func TestStreamRaw(t *testing.T) {
-	s := NewStream(bytes.NewReader(unhex("C58401010101")))
+	s := NewStream(bytes.NewReader(unhex("C58401010101")), 0)
 	s.List()
 
 	want := unhex("8401010101")
@@ -284,11 +326,6 @@ var decodeTests = []decodeTest{
 		ptr:   new([5]byte),
 		error: "rlp: input string too long for [5]uint8",
 	},
-	{
-		input: "850101",
-		ptr:   new([5]byte),
-		error: io.ErrUnexpectedEOF.Error(),
-	},
 
 	// byte array reuse (should be zeroed)
 	{input: "850102030405", ptr: &sharedByteArray, value: [5]byte{1, 2, 3, 4, 5}},
@@ -401,11 +438,17 @@ func TestDecodeWithByteReader(t *testing.T) {
 	})
 }
 
-// dumbReader reads from a byte slice but does not
-// implement ReadByte.
-type dumbReader []byte
+// plainReader reads from a byte slice but does not
+// implement ReadByte. It is also not recognized by the
+// size validation. This is useful to test how the decoder
+// behaves on a non-buffered input stream.
+type plainReader []byte
+
+func newPlainReader(b []byte) io.Reader {
+	return (*plainReader)(&b)
+}
 
-func (r *dumbReader) Read(buf []byte) (n int, err error) {
+func (r *plainReader) Read(buf []byte) (n int, err error) {
 	if len(*r) == 0 {
 		return 0, io.EOF
 	}
@@ -416,15 +459,14 @@ func (r *dumbReader) Read(buf []byte) (n int, err error) {
 
 func TestDecodeWithNonByteReader(t *testing.T) {
 	runTests(t, func(input []byte, into interface{}) error {
-		r := dumbReader(input)
-		return Decode(&r, into)
+		return Decode(newPlainReader(input), into)
 	})
 }
 
 func TestDecodeStreamReset(t *testing.T) {
-	s := NewStream(nil)
+	s := NewStream(nil, 0)
 	runTests(t, func(input []byte, into interface{}) error {
-		s.Reset(bytes.NewReader(input))
+		s.Reset(bytes.NewReader(input), 0)
 		return s.Decode(into)
 	})
 }
@@ -518,7 +560,7 @@ func ExampleDecode() {
 
 func ExampleStream() {
 	input, _ := hex.DecodeString("C90A1486666F6F626172")
-	s := NewStream(bytes.NewReader(input))
+	s := NewStream(bytes.NewReader(input), 0)
 
 	// Check what kind of value lies ahead
 	kind, size, _ := s.Kind()