Paulo Marques, Bruno Cabral Dependable Systems Group University of Coimbra, Portugal RAIL: Code Instrumentation for.NET

Code Instrumentation The ability to modify an application after it has been compiled but before (or during) its execution Application Scenarios: Security Verifications, Dynamic Code Optimizers, Profiling, Fault Injection, AOP, among others

RAIL Runtime Assembly Instrumentation Library An API that allows CLR assemblies to be manipulated and instrumented before they are loaded and executed Currently, one of the main high-level code instrumentation libraries for.NET

But whats this RAIL API anyway? Simple example: How can you be sure that the application that youve downloaded from the Internet is not searching through your files??

RAIL Suppose that you can open the application executable and substitute all class references from File to SecureFileAccess!!!... File theSecret; theSecret = new File(secret.doc);... data = theSecret.read(); internet.send(...); Original File (but in binary code!) RAIL... SecureFileAccess theSecret; theSecret = new SecureFileAccess(secret.doc);... data = theSecret.read(); internet.send(...); New File (also in binary code...)

class SecureFileAccess { File theRealFile; boolean accessPermited; File theRealFile; boolean accessPermited; SecureFileAccess(String filename) { logfile.write(The foo is accessing {0}, name);... accessPermited = User.readPermitAccess(); theRealFile = new File(filename); } SecureFileAccess(String filename) { logfile.write(The foo is accessing {0}, name);... accessPermited = User.readPermitAccess(); theRealFile = new File(filename); } read() { if (accessPermited) { theRealFile.read(); } } read() { if (accessPermited) { theRealFile.read(); } }} theSecret Proxy class Real File Real reference to the file

RAIL Suppose that you can open the application executable and substitute all class references from File to SecureFileAccess!!!... File theSecret; theSecret = new File(secret.doc);... data = theSecret.read(); internet.send(...); Original File (but in binary code!) RAIL... SecureFileAccess theSecret; theSecret = new SecureFileAccess(secret.doc);... data = theSecret.read(); internet.send(...); New File (also in binary code...) // Load assembly into memory RAssemblyDef myAssembly = RAssemblyDef.LoadAssembly("Download.exe"); // Creates references for the old and new types to be used RType oldType = myAssembly.RModuleDef.GetType("File"); RType newType = myAssembly.RModuleDef.GetType("SecureFileAccess"); // Creates a reference replacer and apply the substitution ReferenceReplacer replacer = new ReferenceReplacer(oldType, newType); myAssembly.Accept(replacer);

Development Model

What can be done with RAIL? Iterate over code, injecting and removing code Replace Type references Add epilogues and prologues to methods Redirect method accesses and calls Redirect field and property accesses Redirect field access to properties Redirect field read and write access to methods Manipulate custom attributes Copy-Paste Types and Methods and IL code across assemblies Manipulate Exception Blocks Integration with CODEDOM

Operating System program.exe/dll PE Header MetadataIL ILx86 Source Code Compile Assembly JIT-compiler RAIL.cs

Structure of an Assembly

Object-Oriented Representation (diagram not complete)

RAILs Internal Structure Fully-configurable: Can use third-party libraries

// Source code to use string myProxy using system; class SecureFileAccess { File theRealFile; boolean accessPermited; (...) }"; // Define the code in an assembly RAssemblyDef dynamicAssembly = RAssemblyDef.CreateRAssemblyFromCode(myProxy, false); (...) // Creates references for the old and new types to be used RType oldType = myAssembly.RModuleDef.GetType("File"); RType newType = dynamicAssembly.RModuleDef.GetType("SecureFileAccess"); // Creates a reference replacer and apply the substitution ReferenceReplacer replacer = new ReferenceReplacer(oldType, newType); myAssembly.Accept(replacer); Example Using CodeDom

Conclusion High Level of Abstraction No need for handling all the internal details of PEs At the same time, has the ability to manipulate IL code directly Object-oriented Model for representation of all assembly modules Flexible MSIL instruction handling Use of Design Patterns One of the main high-level code instrumentation libraries for.NET

Questions?