encoding/gob: more hardening for lengths of input strings.

Fixes #3160.

R=r
CC=golang-dev
https://golang.org/cl/5716046

src/pkg/encoding/gob/codec_test.go

@@ -1455,11 +1455,14 @@ func TestFuzz(t *testing.T) {
 func TestFuzzRegressions(t *testing.T) {
 	// An instance triggering a type name of length ~102 GB.
 	testFuzz(t, 1328492090837718000, 100, new(float32))
+	// An instance triggering a type name of 1.6 GB.
+	// Commented out because it takes 5m to run.
+	//testFuzz(t, 1330522872628565000, 100, new(int))
 }
 
 func testFuzz(t *testing.T, seed int64, n int, input ...interface{}) {
-	t.Logf("seed=%d n=%d\n", seed, n)
 	for _, e := range input {
+		t.Logf("seed=%d n=%d e=%T", seed, n, e)
 		rng := rand.New(rand.NewSource(seed))
 		for i := 0; i < n; i++ {
 			encFuzzDec(rng, e)

src/pkg/encoding/gob/decode.go

@@ -392,12 +392,12 @@ func decUint8Slice(i *decInstr, state *decoderState, p unsafe.Pointer) {
 		}
 		p = *(*unsafe.Pointer)(p)
 	}
-	n := int(state.decodeUint())
+	n := state.decodeUint()
-	if n < 0 {
+	if n > uint64(state.b.Len()) {
-		errorf("negative length decoding []byte")
+		errorf("length of []byte exceeds input size (%d bytes)", n)
 	}
 	slice := (*[]uint8)(p)
-	if cap(*slice) < n {
+	if uint64(cap(*slice)) < n {
 		*slice = make([]uint8, n)
 	} else {
 		*slice = (*slice)[0:n]
@@ -417,7 +417,11 @@ func decString(i *decInstr, state *decoderState, p unsafe.Pointer) {
 		}
 		p = *(*unsafe.Pointer)(p)
 	}
-	b := make([]byte, state.decodeUint())
+	n := state.decodeUint()
+	if n > uint64(state.b.Len()) {
+		errorf("string length exceeds input size (%d bytes)", n)
+	}
+	b := make([]byte, n)
 	state.b.Read(b)
 	// It would be a shame to do the obvious thing here,
 	//	*(*string)(p) = string(b)
@@ -647,7 +651,11 @@ func (dec *Decoder) ignoreMap(state *decoderState, keyOp, elemOp decOp) {
 // decodeSlice decodes a slice and stores the slice header through p.
 // Slices are encoded as an unsigned length followed by the elements.
 func (dec *Decoder) decodeSlice(atyp reflect.Type, state *decoderState, p uintptr, elemOp decOp, elemWid uintptr, indir, elemIndir int, ovfl error) {
-	n := int(uintptr(state.decodeUint()))
+	nr := state.decodeUint()
+	if nr > uint64(state.b.Len()) {
+		errorf("length of slice exceeds input size (%d elements)", nr)
+	}
+	n := int(nr)
 	if indir > 0 {
 		up := unsafe.Pointer(p)
 		if *(*unsafe.Pointer)(up) == nil {
@@ -702,6 +710,9 @@ func (dec *Decoder) decodeInterface(ityp reflect.Type, state *decoderState, p ui
 		*(*[2]uintptr)(unsafe.Pointer(p)) = ivalue.InterfaceData()
 		return
 	}
+	if len(name) > 1024 {
+		errorf("name too long (%d bytes): %.20q...", len(name), name)
+	}
 	// The concrete type must be registered.
 	typ, ok := nameToConcreteType[name]
 	if !ok {