153 lines
8.9 KiB
Typst
153 lines
8.9 KiB
Typst
#import "../lib.typ": todo, APK, DEX, JAR, OAT, eg
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== Code Transformation <sec:th-trans>
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#todo[Define code loading and reflection somewhere]
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#todo[This is a draft, clean this up]
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#todo[Reflectif call? Reflection call?]
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In this section, we will see how we can transform the application code to make dynamic codeloading and reflexif calls analysable by static analysis tools.
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=== Reflection <sec:th-trans-ref>
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In Android, reflection can be used to do two things: instanciate a class, or call a method.
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Either way, reflection starts by retreiving the `Class` object representing the class to use.
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This class is usually retrived using a `ClassLoader` object, but can also be retrieved directly from the classloader of the class defining the calling method.
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// elaborate? const-class dalvik instruction / MyClass.class in java?
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One the class is retrieve, it can be instanciated using the deprecated method `Class.newInstance()`, like shown in @lst:-th-expl-cl-new-instance, or a specific method can be retrieved.
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The current approche to instanciate a class is to retrieve the specific `Constructor` object, then calling `Constructor.newInstance(..)` like in @lst:-th-expl-cl-cnstr.
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Similarly, to call a method, the `Method` object must be retrieved, then called using `Method.invoke(..)`, like shown in @lst:-th-expl-cl-call.
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Although the process seems to differ between class instanciation and method call from the Java stand point, the runtime opperations are very similar.
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When instanciating an object with `Object obj = cst.newInstance("Hello Void")`, the constructor method `<init>(Ljava/lang/String;)V`, represented by the `Constructor` `cst`, is called on the object `obj`.
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#figure(
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```java
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ClassLoader cl = MainActivity.class.getClassLoader();
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Class clz = cl.loadClass("com.example.Reflectee");
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Object obj = clz.newInstance();
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```,
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caption: [Instanciating a class using `Class.newInstance()`]
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) <lst:-th-expl-cl-new-instance>
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#figure(
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```java
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Constructor cst = clz.getDeclaredConstructor(String.class);
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Object obj = cst.newInstance("Hello Void");
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```,
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caption: [Instanciating a class using `Constructor.newInstance(..)`]
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) <lst:-th-expl-cl-cnstr>
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#figure(
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```java
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Method mth = clz.getMethod("myMethod", String.class);
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Object[] args = {(Object)"an argument"}
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String retData = (String) mth.invoke(obj, args);
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```,
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caption: [Calling a method using reflection]
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) <lst:-th-expl-cl-call>
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To allow static analysis tools to analyse an application that use reflection, we want to replace the reflection call by the bytecode that does the actual calls.
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One of the main reason to use reflection is to access classes not from the application.
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Although allows the use classes that do not exist in the application in bytecode, at runtime, if the classes are not found in the current classloader, the application will crash.
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Similarly, some analysis tools might have trouble analysis application calling non existing classes.
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@sec:th-trans-cl deals with the issue of adding dynamically loaded bytecode to the application.
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A notable issue is that a specific reflection call can call different methods.
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@lst:th-worst-case-ref illustrate a worst case scenario where any method can be call at the same reflection call.
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In those situation, we cannot garanty that we know all the methods that can be called (#eg the name of the method called could be retrieved from a remote server).
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#figure(
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```java
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Object myInvoke(Object obj, Method mth, Object[] args) throws .. {
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return mth.invoke(obj, args);
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}
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```,
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caption: [A reflection call that can call any method]
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) <lst:th-worst-case-ref>
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To handle those situation, instead of entirely removing the reflection call, we can modify the application code to test if the `Method` (or `Constructor`) object match any expected method, and if yes, directly call the method.
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If the object does not match any expected method, the code can fallback to the original reflection call.
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@lst:-th-expl-cl-call-trans demonstrate this transformation on @lst:-th-expl-cl-call.
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It should be noted that we do the transformation at the bytecode level, the code in the listing correspond to the output of JADX #todo[Ref to list of common tools?] reformated for readability.
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The method check is done in a separate method injected inside the application to avoid clutering the application too much.
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Because Java (and thus Android) uses polymorphic methods, we cannot just check the method name and its class, but also the whole method signature.
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We chose to limit the transformation to the specific instruction that call `Method.invoke(..)`.
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This drastically reduce the risks of breaking the application, but leads to a lot of type casting.
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Indeed, the reflection call uses the generic `Object` class, but actual methods usually use specific classes (#eg `String`, `Context`, `Reflectee`) or scalar types (#eg `int`, `long`, `boolean`).
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This means that the method parameters and object on which the method is called must be downcast to their actual type before calling the method, then the returned value must be upcasted back to an `Object`.
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Scalar types especially require special attention.
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Java (and Android) distinguish between scalar type and classes, and they cannot be mixed: a scalar cannot be cast into an `Object`.
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However, each scalar type has an associated class that can be use when doing reflection.
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For example, the scalar type `int` is associated with the class `Integer`, the method `Integer.valueOf()` can convert an `int` scalar to an `Integer` object, and the method `Integer.intValue()` convert back an `Integer` object to an `int` scalar.
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Each time the method called by reflection used scalars, the scalar-object convertion must be made before calling it.
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And finally, because the instruction following the reflection call expect an `Object`, the return value of the method must be cast into an `Object`.
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This back and forth between types might confuse some analysis tools.
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This could be improved in futur works by analysing the code around the reflection call.
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For example, if the result of the reflection call is imediatly cast into the expected type (#eg in @lst:-th-expl-cl-call, the result is cast to a `String`), they should not be any need to cast it to Object in between.
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Similarly, it is common to have the method parameter arrays generated just before the reflection call never be used again (This is due to `Method.invoke(..)` beeing a varargs method: the array can be generated by the compiler at compile time).
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In those cases, the parameters could be used directly whithout the detour inside an array.
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#figure(
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```java
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class T {
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static boolean check_is_reflectee_mymethod_e398(Method mth) {
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Class<?>[] paramTys = mth.getParameterTypes();
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return (
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meth.getName().equals("myMethod") &&
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paramTys.length == 1 &&
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paramTys[0].descriptorString().equals(
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String.class.descriptorString()
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) &&
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mth.getReturnType().descriptorString().equals(
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String.class.descriptorString()
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) &&
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mth.getDeclaringClass().descriptorString().equals(
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Reflectee.class.descriptorString()
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)
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)
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}
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}
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...
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Method mth = clz.getMethod("myMethod", String.class);
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Object[] args = {(Object)"an argument"}
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Object objRet;
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if (T.check_is_reflectee_mymethod_e398abf7d3ce6ede(mth)) {
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objRet = (Object) ((Reflectee) obj).myMethod((String)args[0]);
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} else {
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objRet = mth.invoke(obj, args);
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}
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String retData = (String) objRet;
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```,
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caption: [@lst:-th-expl-cl-call after the de-reflection transformation]
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) <lst:-th-expl-cl-call-trans>
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=== Code loading <sec:th-trans-cl>
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#todo[custom class loaders]
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An application can dynamically import code from several format like #DEX, #APK, #JAR or #OAT, either stored in memory or in a file.
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Because it is an internal, platform dependant format, we elected to ignore the #OAT format.
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Practically, #JAR and #APK files are zip files containing #DEX files.
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This means that we only need to find a way to integrate #DEX files to the application.
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We elected to simply add the dex files to the application, using the multi-dex feature introduced by the SDK 21 now used by all applications.
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This gives access to the dynamically loaded code to static analysis tool.
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#todo[add drawing of dex insertion]
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We decided to leave untouched the original code that load the bytecode.
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At runtime, although the bytecode is already present in the application, the application will still dynamically load the code.
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This ensure that the application keep working as intended even if the transformation we applied are incomplete.
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Specifically, to call dynamically loaded code, an application needs to use reflection, and we saw in @sec:th-trans-ref that we need to keep reflecton calls, and in order to keep reflection calls, we need the classloader created when loading bytecode.
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=== Class Collisions <sec:th-class-collision>
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=== Pitfalls
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#todo[interupting try blocks: catch block might expect temporary registers to still stored the saved value]
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