vm: add instruction correctness check

See neo-project/neo-vm#392.

pkg/vm/context.go

@@ -3,6 +3,7 @@ package vm
 import (
 	"encoding/binary"
 	"errors"
+	"fmt"
 	"math/big"
 
 	"github.com/nspcc-dev/neo-go/pkg/crypto/hash"
@@ -109,6 +110,9 @@ func (c *Context) Next() (opcode.Opcode, []byte, error) {
 
 	var instrbyte = c.prog[c.ip]
 	instr := opcode.Opcode(instrbyte)
+	if !opcode.IsValid(instr) {
+		return instr, nil, fmt.Errorf("incorrect opcode %s", instr.String())
+	}
 	c.nextip++
 
 	var numtoread int

pkg/vm/opcode/opcode.go

@@ -219,3 +219,9 @@ const (
 	ISTYPE  Opcode = 0xD9
 	CONVERT Opcode = 0xDB
 )
+
+// IsValid returns true if the opcode passed is valid (defined in the VM).
+func IsValid(op Opcode) bool {
+	_, ok := _Opcode_map[op] // We rely on stringer here, it has a map anyway.
+	return ok
+}

pkg/vm/opcode/opcode_test.go

@@ -28,3 +28,10 @@ func TestFromString(t *testing.T) {
 	require.NoError(t, err)
 	require.Equal(t, MUL, op)
 }
+
+func TestIsValid(t *testing.T) {
+	require.True(t, IsValid(ADD))
+	require.True(t, IsValid(CONVERT))
+	require.False(t, IsValid(0xff))
+	require.False(t, IsValid(0xa5))
+}