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On improved tool support for Cocoa developers

I started writing some tweets, that were clearly taking up too much room. They started like this:

My own thoughts: tool support is very important to good software engineering. 3.3.1 is not a big inhibitor to novel tools. /cc @rentzsch

then this:

There’s still huge advances to make in automating design, bug-hunting/squashing and traceability/accountability, for instance.

(The train of thought was initiated by the Dog Spanner’s [c4 release]; post.)

In terms of security tools, the Cocoa community needs to catch up with where Microsoft are before we need to start wondering whether Apple might be holding us back. Yes, I have started working on this, I expect to have something to show for it at NSConference MINI. However, I don’t mind whether it’s you or me who gets the first release, the important thing is that the tools should be available for all of us. So I don’t mind sharing my impression of where the important software security engineering tools for Mac and iPhone OS developers will be in the next few years.

Requirements comprehension

My first NSConference talk was on understanding security requirements, and it’s the focus of Chapter 1 of Professional Cocoa Application Security. The problem is, most of you aren’t actually engineers of security requirements, you’re engineers of beautiful applications. Where do you dump all of that security stuff while you’re focussing on making the beautiful app? It’s got to be somewhere that it’s still accessible, somewhere that it stays up to date, and it’s got to be available when it’s relevant. In other words, this information needs to be only just out of your way. A Pages document doesn’t really cut it.

Now over in the Windows world, they have Microsoft Threat Modeling Tool, which makes it easy to capture and organise the security requirements. But stops short of providing any traceability or integration with the rest of the engineering process. It’d be great to know how each security requirement impacts each class, or the data model, etc.

Bug-finding

The Clang analyser is just the start of what static analysis can do. Many parts of Cocoa applications are data-driven, and good analysis tools should be able to inspect the relationship between the code and the data. Other examples: currently if you want to ensure your UI is hooked up properly, you manually write tests that inspect the outlets, actions and bindings you set up in the XIB. If you want to ensure your data model is correct, you manually write tests to inspect your entity descriptions and relationships. Ugh. Code-level analysis can already reverse-engineer test conditions from the functions and methods in an app, they ought to be able to use the rest of the app too. And it ought to make use of the security model, described above.

I have recently got interested in another LLVM project called KLEE, a symbolic execution tool. Current security testing practices largely involve “fuzzing”, or choosing certain malformed/random input to give to an app and seeing what it does. KLEE can take this a step further by (in effect) testing any possible input, and reporting on the outcomes for various conditions. It can even generate automated tests to make it easy to see what effect your fixes are having. Fuzzing will soon become obsolete, but we Mac people don’t even have a good and conventional tool for that yet.

Bug analysis

Once you do have fuzz tests or KLEE output, you start to get crash reports. But what are the security issues? Apple’s CrashWrangler tool can take a stab at analysing the crash logs to see whether a buffer overflow might potentially lead to remote code execution, but again this is just the tip of the iceberg. Expect KLEE-style tools to be able to report on deviations from expected behaviour and security issues without having to wait for a crash, just as soon as we can tell the tool what the expected behaviour is. And that’s an interesting problem in itself, because really the specification of what you want the computer to do is your application’s source code, and yet we’re trying to determine whether or not that is correct.

Safe execution

Perhaps the bitterest pill to swallow for long time Objective-C programmers: some time soon you will be developing for a managed environment. It might not be as high-level as the .Net runtime (indeed my money is on the LLVM intermediate representation, as hardware-based managed runtimes have been and gone), but the game has been up for C arrays, memory dereferencing and monolithic process privileges for years. Just as garbage collectors have obsoleted many (but of course not all) memory allocation problems, so environment-enforced buffer safety can obsolete buffer overruns, enforced privilege checking can obsolete escalation problems and so on. We’re starting to see this kind of safety retrofitted to compiled code using stack guards and the like, but by the time the transition is complete (if it ever is), expect your application’s host to be unrecognisable to the app as an armv7 or x86_64, even if the same name is still used.

{ 1 } Comments

  1. alastair | May 12, 2010 at 10:54 pm | Permalink

    Don’t really agree with the bit at the bottom. There’s no need for a “managed” environment to provide safe buffers and so on. Various programming languages have provided that kind of safety with native code generation for ages now; the problem is really that we’re stuck with C and its derivatives, which weren’t designed to be safe.

    Historically there were good reasons for that. Machines were slow, and some of the other languages imposed an excessive burden on the CPU in exchange for the increased safety or convenience.

    These days, though, all but the lowest level code *could* use bounds-checked arrays and the like. But there’s really no need to jump to a virtual machine for that functionality.