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Systems for Safety and Dependability by David Evans

Explore methods for protecting systems from malicious attacks, buggy programs, and user errors to ensure safety and dependability. Learn about Naccio for policy-directed code safety and LCLint for static checking.

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Systems for Safety and Dependability by David Evans

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  1. Systems for Safety and Dependability David Evans http://www.cs.virginia.edu/~evans/ University of Virginia Department of Computer Science

  2. What Are You Afraid Of? • Malicious attacks • Russian New Year, Melissa, Chernobyl, Java thread attack, etc. • Buggy programs • Can cause harm accidentally • Can be exploited by attackers • User mistakes/bad interfaces • tar –cf * Safety and Dependability

  3. Menu Naccio: Policy-Directed Code Safety How do you prevent bad programs from doing bad things? naccio.cs.virginia.edu LCLint: Annotation-Assisted Static Checking How do you help good people not write bad programs? lclint.cs.virginia.edu Safety and Dependability

  4. Naccio Motivation • Weaknesses in existing code safety systems: • Limited range of policies • Policy definition is ad hoc and platform dependent • Enforcement is tied to a particular architecture • Can we solve them without sacrificing efficiency or convenience? Yes! Safety and Dependability

  5. Naccio Overview Program • General method for defining policies • Abstract resources • Platform independent • System architecture for enforcing policies • Prototypes for JavaVM classes, Win32 executables Naccio Safety Policy Safe Program Safety and Dependability

  6. Problem User’s View System View Program tar cf * WriteFile (fHandle, …) System Library Policy Platform Interface OS Kernel Resources Files Disk Safety and Dependability

  7. Safety Policy Definition • Resource descriptions: abstract operational descriptions of resources (files, network, …) • Platform interface: mapping between system events (Java API calls) and abstract resources • Resource use policy: constraints on manipulating those resources Safety and Dependability

  8. Resource Description • global resourceRFileSystem • openRead (file: RFile) • Called before file is opened for reading • openWrite (file: RFile) • Called before existing file is opened for writing • write (file: RFile, nbytes: int) • Called before nbytes are written to file • … // other operations for observing properties of files, deleting, etc. • resourceRFile • RFile (pathname: String) • Constructs object corresponding to pathname Safety and Dependability

  9. Java PFI Excerpt wrapper java.io.FileOutputStream requires RFileMap; stateRFile rfile; wrapper void write (byte b[]) if (rfile != null) RFileSystem.write (rfile, b.length); %%% // original method call … // wrappers needed for constructors, other write // methods, close and getFD Safety and Dependability

  10. Resource Use Policy • policyLimitWrite • LimitBytesWritten (1000000), NoOverwrite • propertyLimitBytesWritten (n: int) • requires TrackBytesWritten; • checkRFileSystem.write (file: RFile, nbytes: int) • if (bytes_written > n) violation (“Writing more than …”); • stateblockTrackBytesWritten • addfieldRFileSystem.bytes_written: int = 0; • precodeRFileSystem.write (file: RFile, nbytes: int) • bytes_written += nbytes; Safety and Dependability

  11. Enforceable Policies • Can enforce any policy that can be defined • What can be defined depends on resource operations • Resource operations depend on platform interface • Any manipulation done through API calls • Cannot constrain CPU usage • Solutions possible: insert calls • Portable policies use standard resources Safety and Dependability

  12. Outline Program • System architecture • Defining policies • Enforcing policies • Architecture • Results – JavaVM, Win32 Naccio Safety Policy Safe Program Safety and Dependability

  13. Naccio Architecture Per policy Per application Safety policy definition Program Application transformer Policy compiler Policy description file • Version of program that: • Uses policy-enforcing system library • Satisfies low-level code safety Policy-enforcing system library Platforms in development: JavaVM– program is collection of Java classes Win32 – program is Win32 executable and DLLs Safety and Dependability

  14. Policy Compiler Resource use policy Platform independent analyses Resource descriptions Policy-enforcing system library • Implementations of resource operations • Perform checking described by resource use policy • Modifies Java byte codes • Call abstract resource operations as directed by platform interface Platform interface Describes Java API Policy description file Platform dependent analyses and code generation System library Java library classes Safety and Dependability

  15. Implementing Resources policy LimitWrite NoOverwrite, LimitBytesWritten (1000000) stateblock TrackBytesWritten addfieldRFileSystem.bytes_written: int; precodeRFileSystem.write (file: RFile, nbytes: int) bytes_written += nbytes; property LimitBytesWritten (n: int) checkRFileSystem.write (file: RFile, nbytes: int) if (bytes_written > n) violation (“Attempt …); RFileSystem RFile Resource descriptions Resource use policy Policy compiler Resource implementations package naccio.p253.resource; class RFileSystem { static int bytes_written = 0; static void write (RFile file, int nbytes) { bytes_written += nbytes; if (bytes_written > 1000000) Check.violation (“LimitWrite”, “Attempt to write …); } …

  16. Rewriting Classes class FileOutputStream { … public void write (byte b[]) { writeBytes (b, 0, b.length); } } wrapper java.io.FileOutputStream stateRFile rfile; wrapper void write (byte b[]) if (rfile != null) RFileSystem.write (rfile, b.length); %%% // original method call System library classes Platform interface Policy compiler Wrapped library classes class FileOutputStream { naccio.p253.resource.RFile rfile; … // orig_write – same implementation as old write method void write (byte b[]) { if (rfile != null) naccio.p253.resource.RFileSystem.write (rfile, b.length); orig_write (b); } Safety and Dependability

  17. Optimizations • Only implement resource operation if it: • May produce a violation • Modifies state used elsewhere • Only wrap library method if it: • Calls implemented resource operation • Modifies state used meaningfully • Alters behavior • Simple dataflow dependency analysis • Not done yet: inline methods and state to remove resource overhead Safety and Dependability

  18. Program Collection of Java classes Program Transformer Policy description file • Version of program that: • Uses policy-enforcing library • • Replace class names in constant pool • • Wrappers for dynamic class loading methods • Satisfies low-level code safety • • Use Java byte code verifier • • Wrappers on reflection methods Safety and Dependability

  19. What’s different for Win32? • Program is Win32 executable and DLLs • Platform interface describes Win32 API • Policy compiler • Generate DLLs instead of Java classes • Application transformer • Replace DLL names in import table • Low-level code safety is platform-specific • SFI for jumps, PFI wrappers to protect memory • Scan for kernel traps • Policies can be reused Safety and Dependability

  20. Results - JavaVM • Preparation time minimal • Overhead depends on policy and application • Substantially faster than JDK • Policy decisions made at transform time • Can optimize out unnecessary checking • Details in [Evans99] Naccio tar from www.ice.com Safety and Dependability

  21. Results – Win32 • Can enforce policies on Microsoft Word • Caveats: • Subset of Win32 API • Doesn’t deal with low-level code safety yet (need to implement SFI) • Details in [Twyman99] pkzip Safety and Dependability

  22. Related Work • Software fault isolation [Wahbe et al, 93] • Similar enforcement mechanisms • Execution monitoring [Schneider] • Ariel Project [Pandey, Hashii] • Alternative: verify properties • Proof-carrying code [Necula, Lee] • Typed Assembly Language [Morrisett] Safety and Dependability

  23. Naccio Summary • Method for defining large class of policies • Using abstract resources • General architecture for code safety • Encouraging results so far • Win32: need to implement low-level safety • JavaVM: needs to be attacked • For more information: • IEEE Security & Privacy 99 (Oakland) • http://naccio.cs.virginia.edu Safety and Dependability

  24. Annotation-Assisted Static Checking all Formal Verifiers Bugs Detected LCLint Compilers none Low Unfathomable Effort Required Safety and Dependability

  25. Approach • Programmers add annotations (formal specifications) • Simple and precise • Describe programmers intent: • Types, memory management, data hiding, aliasing, modification, nullness, etc. • LCLint detects inconsistencies between annotations and code • Simple (fast!) dataflow analyses Safety and Dependability

  26. Sample Annotation: only extern only char *gptr; extern only out null void *malloc (int); • Reference (return value) owns storage • No other persistent (non-local) references to it • Implies obligation to transfer ownership • Transfer ownership by: • Assigning it to an external only reference • Return it as an only result • Pass it as an only parameter: e.g., extern void free (only void *); Safety and Dependability

  27. Example • extern only null void *malloc (int); in library 1 int dummy (void) { 2 int *ip= (int *) malloc (sizeof (int)); 3 *ip = 3; 4 return *ip; 5 } LCLint output: dummy.c:3:4: Dereference of possibly null pointer ip: *ip dummy.c:2:13: Storage ip may become null dummy.c:4:14: Fresh storage ip not released before return dummy.c:2:43: Fresh storage ip allocated Safety and Dependability

  28. Checking Examples • Encapsulation – abstract types (rep exposure), global variables, documented modifications • Memory management – leaks, dead references • De-referencing null pointers, dangerous aliasing, undefined behavior Safety and Dependability

  29. Unsoundness & Incompleteness are Good! • Okay to miss errors • Report as many as possible • Okay to issue false warnings • But don’t annoy the user to too many • Make it easy to configure checking and override warnings • Design tradeoff – do more ambitious checking the best you can Safety and Dependability

  30. LCLint Status • Public distribution since 1993 • Effective checking >100K line programs (checks about 1K lines per second) • Detects lots of real bugs in real programs (including itself, of course) • More information: lclint.cs.virginia.edu PLDI ’96, FSE’94 Safety and Dependability

  31. Where do we go from here? • Motivating Example: Take an e-commerce site and prove that credit card information is never stored or transmitted unencrypted • Meta-annotations [David LaRochelle] • Allow users to define new annotations and associated checking • Generalize framework • Support static checking for multiple source languages in a principled way • Integrate static and run-time checking to enable completeness guarantees Safety and Dependability

  32. Summary • A little redundancy goes a long way • Naccio: • Describe high-level behavioral constraints in an abstract way • Check them automatically at run-time • LCLint: • Describe programmer intent in a precise way • Check them statically at compile-time Safety and Dependability

  33. Credits • Naccio Win32 Implementation: Andrew Twyman • LCLint LCL: Yang Meng Tan, John Guttag, Jim Horning • Funding DARPA, NSF, ONR Safety and Dependability

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