JVM 权限逃逸技术

前言: 仅研究 JDK 9+, JDK 8- 无研究意义

从 Java 9 开始, Java 引入了一个新的概念, 模块(Module). 模块的存在, 限制了反射技术, 在 JDK 16 中, 直接反射越权修改 java.base 甚至会得到错误 java.lang.reflect.InaccessibleObjectException, 对于某些需要的 devops 而言意味着无法完成预期操作

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在阅读 java.lang.reflect.AccessibleObject 源码后, 有如下代码片段

/**
 * If the given AccessibleObject is a {@code Constructor}, {@code Method}
 * or {@code Field} then checks that its declaring class is in a package
 * that can be accessed by the given caller of setAccessible.
 */
 void checkCanSetAccessible(Class<?> caller) {
     // do nothing, needs to be overridden by Constructor, Method, Field
 }

 final void checkCanSetAccessible(Class<?> caller, Class<?> declaringClass) {
     checkCanSetAccessible(caller, declaringClass, true);
 }

 private boolean checkCanSetAccessible(Class<?> caller,
                                       Class<?> declaringClass,
                                       boolean throwExceptionIfDenied) {
     if (caller == MethodHandle.class) {
         throw new IllegalCallerException();   // should not happen
     }

     Module callerModule = caller.getModule();
     Module declaringModule = declaringClass.getModule();

     if (callerModule == declaringModule) return true;
     if (callerModule == Object.class.getModule()) return true;
     if (!declaringModule.isNamed()) return true;

     String pn = declaringClass.getPackageName();
     int modifiers;
     if (this instanceof Executable) {
         modifiers = ((Executable) this).getModifiers();
     } else {
         modifiers = ((Field) this).getModifiers();
     }

     // class is public and package is exported to caller
     boolean isClassPublic = Modifier.isPublic(declaringClass.getModifiers());
     if (isClassPublic && declaringModule.isExported(pn, callerModule)) {
         // member is public
         if (Modifier.isPublic(modifiers)) {
             logIfExportedForIllegalAccess(caller, declaringClass);
             return true;
         }

         // member is protected-static
         if (Modifier.isProtected(modifiers)
             && Modifier.isStatic(modifiers)
             && isSubclassOf(caller, declaringClass)) {
             logIfExportedForIllegalAccess(caller, declaringClass);
             return true;
         }
     }

     // package is open to caller
     if (declaringModule.isOpen(pn, callerModule)) {
         logIfOpenedForIllegalAccess(caller, declaringClass);
         return true;
     }

     if (throwExceptionIfDenied) {
         // not accessible
         String msg = "Unable to make ";
         if (this instanceof Field)
             msg += "field ";
         msg += this + " accessible: " + declaringModule + " does not \"";
         if (isClassPublic && Modifier.isPublic(modifiers))
             msg += "exports";
         else
             msg += "opens";
         msg += " " + pn + "\" to " + callerModule;
         InaccessibleObjectException e = new InaccessibleObjectException(msg);
         if (printStackTraceWhenAccessFails()) {
             e.printStackTrace(System.err);
         }
         throw e;
     }
     return false;
 }

有两个关键判断逻辑: declaringModule.isExported(pn, callerModule), declaringModule.isOpen(pn, callerModule)

阅读 Module.java 后发现有 implAddExports 方法, 通过 IDEA 查找调用引用发现了 java.lang.System 有访问此方法的 JDK Internal API

private static void setJavaLangAccess() {
    // Allow privileged classes outside of java.lang
    SharedSecrets.setJavaLangAccess(new JavaLangAccess() {
        public Module defineModule(ClassLoader loader,
                                   ModuleDescriptor descriptor,
                                   URI uri) {
            return new Module(null, loader, descriptor, uri);
        }
        public Module defineUnnamedModule(ClassLoader loader) {
            return new Module(loader);
        }
        public void addReads(Module m1, Module m2) {
            m1.implAddReads(m2);
        }
        public void addReadsAllUnnamed(Module m) {
            m.implAddReadsAllUnnamed();
        }
        public void addExports(Module m, String pn, Module other) {
            m.implAddExports(pn, other);
        }
        public void addExportsToAllUnnamed(Module m, String pn) {
            m.implAddExportsToAllUnnamed(pn);
        }
        public void addOpens(Module m, String pn, Module other) {
            m.implAddOpens(pn, other);
        }
        public void addOpensToAllUnnamed(Module m, String pn) {
            m.implAddOpensToAllUnnamed(pn);
        }
        public void addOpensToAllUnnamed(Module m, Set<String> concealedPackages, Set<String> exportedPackages) {
            m.implAddOpensToAllUnnamed(concealedPackages, exportedPackages);
        }
        public void addUses(Module m, Class<?> service) {
            m.implAddUses(service);
        }
    });
}

找到了 JDK 提供的后门之后, 我们只需要调用 SharedSecrets.getJavaLangAccess().addExports 就能开后门了….
不对，目前还无法调用 SharedSecrets, 还需要一些手段….

在 java.lang.reflect 中翻到了一个特别的东西, java.lang.reflect.Proxy, 她是破局的关键中心

抱着好奇的心里, 我尝试了使用 Proxy 实现 jdk.internal.access 中的一个接口玩玩

public static void main(String[] args) throws Exception {
    var obj = Proxy.newProxyInstance(
            Usffsa.class.getClassLoader(),
            new Class[]{Class.forName("jdk.internal.access.JavaLangAccess")},
            new InvocationHandler() {
                @Override
                public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
                    return null;
                }
            }
    );
    System.out.println(obj);
}

没想到, 运行成功了(eg: 没有对应权限(Exported)是不能实现对应接口的), 迎接着激动的心情, 输出了更多的详细信息

System.out.println(obj);
System.out.println(obj.getClass());
System.out.println(obj.getClass().getModule());
System.out.println(Object.class.getModule().isExported("jdk.internal.access", obj.getClass().getModule()));

null
class com.sun.proxy.jdk.proxy1.$Proxy0
module jdk.proxy1
true

破局点找到了, java.lang.reflect.Proxy 拥有打开模块访问的权利, 然后尝试对该模块进行注入

public static void main(String[] args) throws Exception {
    var ccl = new ClassLoader(Usffsa.class.getClassLoader()) {
        Class<?> defineClass(byte[] code) {
            return defineClass(null, code, 0, code.length);
        }
    };
    var obj = Proxy.newProxyInstance(
            ccl,
            new Class[]{Class.forName("jdk.internal.access.JavaLangAccess")},
            (proxy, method, args1) -> null
    );
    var writer = new ClassWriter(0); // org.objectweb.asm.ClassWriter
    writer.visit(Opcodes.V1_8, 0,
            obj.getClass().getPackageName().replace('.', '/') + "/Test0",
            null,
            "java/lang/Object",
            null
    );
    var injectedClass = ccl.defineClass(writer.toByteArray());
    System.out.println("Proxy     Module  : " + obj.getClass().getModule());
    System.out.println("Injected  Module  : " + injectedClass.getModule());
    System.out.println("Is Same Module    : " + (injectedClass.getModule() == obj.getClass().getModule()));
}

Proxy     Module  : module jdk.proxy1
Injected  Module  : module jdk.proxy1
Is Same Module    : true

至此已经破开了 JVM 的模块限制的死局, 实际应用可参考 [Karlatemp/UnsafeAccessor]