stackitem: limit deserialization to MaxDeserialized items

Follow neo-project/neo#2531. Even though it's not strictly required (our node
handles problematic script just fine) we better be compliant wrt
deserialization behavior. MaxDeserialized is introduced to avoid moving
MaxStackSize which is a VM parameter.

pkg/vm/stackitem/serialization.go

@@ -9,6 +9,10 @@ import (
 	"github.com/nspcc-dev/neo-go/pkg/io"
 )
 
+// MaxDeserialized is the maximum number one deserialized item can contain
+// (including itself).
+const MaxDeserialized = 2048
+
 // ErrRecursive is returned on attempts to serialize some recursive stack item
 // (like array including an item with reference to the same array).
 var ErrRecursive = errors.New("recursive item")
@@ -25,6 +29,13 @@ type serContext struct {
 	seen         map[Item]sliceNoPointer
 }
 
+// deserContext is an internal deserialization context.
+type deserContext struct {
+	*io.BinReader
+	allowInvalid bool
+	limit        int
+}
+
 // Serialize encodes given Item into the byte slice.
 func Serialize(item Item) ([]byte, error) {
 	sc := serContext{
@@ -179,21 +190,36 @@ func Deserialize(data []byte) (Item, error) {
 // as a function because Item itself is an interface. Caveat: always check
 // reader's error value before using the returned Item.
 func DecodeBinary(r *io.BinReader) Item {
-	return decodeBinary(r, false)
+	dc := deserContext{
+		BinReader:    r,
+		allowInvalid: false,
+		limit:        MaxDeserialized,
+	}
+	return dc.decodeBinary()
 }
 
 // DecodeBinaryProtected is similar to DecodeBinary but allows Interop and
 // Invalid values to be present (making it symmetric to EncodeBinaryProtected).
 func DecodeBinaryProtected(r *io.BinReader) Item {
-	return decodeBinary(r, true)
+	dc := deserContext{
+		BinReader:    r,
+		allowInvalid: true,
+		limit:        MaxDeserialized,
+	}
+	return dc.decodeBinary()
 }
 
-func decodeBinary(r *io.BinReader, allowInvalid bool) Item {
+func (r *deserContext) decodeBinary() Item {
 	var t = Type(r.ReadB())
 	if r.Err != nil {
 		return nil
 	}
 
+	r.limit--
+	if r.limit < 0 {
+		r.Err = errTooBigElements
+		return nil
+	}
 	switch t {
 	case ByteArrayT, BufferT:
 		data := r.ReadVarBytes(MaxSize)
@@ -216,7 +242,7 @@ func decodeBinary(r *io.BinReader, allowInvalid bool) Item {
 		}
 		arr := make([]Item, size)
 		for i := 0; i < size; i++ {
-			arr[i] = decodeBinary(r, allowInvalid)
+			arr[i] = r.decodeBinary()
 		}
 
 		if t == ArrayT {
@@ -231,8 +257,8 @@ func decodeBinary(r *io.BinReader, allowInvalid bool) Item {
 		}
 		m := NewMap()
 		for i := 0; i < size; i++ {
-			key := decodeBinary(r, allowInvalid)
-			value := decodeBinary(r, allowInvalid)
+			key := r.decodeBinary()
+			value := r.decodeBinary()
 			if r.Err != nil {
 				break
 			}
@@ -242,12 +268,12 @@ func decodeBinary(r *io.BinReader, allowInvalid bool) Item {
 	case AnyT:
 		return Null{}
 	case InteropT:
-		if allowInvalid {
+		if r.allowInvalid {
 			return NewInterop(nil)
 		}
 		fallthrough
 	default:
-		if t == InvalidT && allowInvalid {
+		if t == InvalidT && r.allowInvalid {
 			return nil
 		}
 		r.Err = fmt.Errorf("%w: %v", ErrInvalidType, t)