Unleashing use-before-initialization vulnerabilities in the Linux kernel [pdf]
cc.gatech.edu
cc.gatech.edu
The more time goes by, and the more ubiquitous our computing devices become, the crazier it seems not to take this approach. It sounds like Fuchsia is (might be?) a step in the right direction… I'm excited to see what happens there.
Only FOSS project actively soliciting and sometimes integrating best-of-breed components from high-assurance field is Genode OS framework. They recently went AGPL, though, for non-proprietary license. That's not promising... The others stopped getting any maintenance or their people and/or I.P. get poached into corporate circles.
Otherwise, I don't know of anything happening with big companies and high-assurance tech. Only smartcard vendors seem to do it regularly for specific components in the hardware (esp MPU's) or software (esp JavaCard or MULTOS).
We have so much power it's obscene. We're producing SHA-1 collisions without damaging or analyzing the algorithm, just using brute force. Why not take that approach to kernel security?
I'd sooner trust code whose quintillion input and output states have been exhaustively brute-forced over a few months (so that literally every possible path and value that code can take has ACTUALLY been run), than code which has been formally proven to be correct but without having been subject to such a test.
No.
Take a simple function that has two 64-bit numbers as input and one output; your sixteen 4Ghz cores would take over 11 billion times the current age of the universe to verify it.
You can, instead, do things like symbolic execution... and it turns out, that kind of thing is often a big part of "formally proven correct".
Nowadays they use instrumentation in the fuzzed/tested code to have guidance in what direction the input should be modified to get more cases the code covered.
Good example: http://lcamtuf.coredump.cx/afl/
Uh oh. Another OpenSSL bug.
If you're running OpenSSL in a container or VM, there's a good chance of repeatability.
afaik openssl removed a bunch of those uninitialized uses, not sure if all of them. But openssl never exclusively relied on them. There once was a "famous" bug, but it affected only openssl in debian, not openssl itself, where such an occurrence was fixed in a wrong way.
Plus (at least in Rust) in order to ever do anything useful, you have to use unsafe anyway, usually in the most complex bits of code that have the most potential for subtle memory errors. Personally that's the biggest problem I have with it: it seems like it helps avoid issues a lot, but only in the easy cases where I'm very confident I can avoid them anyway. In the really tricky subtle cases (or even fairly basic things like cyclic data structures, which are not niche) the compiler throws up its hands. Those, honestly, are the cases where I want compiler help.
That's not to say I don't think it's useful. In fact, making all the easy cases trivial does mean you can focus your effort, analysis, thought, design, whiteboard space, etc. on the trickier cases. But it's not anywhere near as useful as it is sometimes claimed to be.
Until we have perfect AI writing all our code, it's going to be pretty difficult to fix the other cases. That's what code reviews and extensive unit tests are for.
Could you possibly go into further detail?
I feel that Rust is envisioned by its followers as the just-unveiled never-before-known perfect solution to memory safety and related problems, when in reality C and other languages continue to have similar issues because the issues themselves are not solved problems. So I'm holding Rust at arm's length until someone can thoroughly and objectively debunk the views that I have.
But I'd love to be able to argue effectively. I agree with all the statements you made two levels up. Can you provide further detail to counter the parent comment?
The C programs I encounter in the wild tend to be both insecure and not memory-safe. The difference between regular bugs in memory-safe programs and memory-safety bugs is that with a regular bug your program is still evaluating the code you wrote; perhaps you forgot a permission check, or you made a silly mistake in code that you didn't think was security-relevant but which a user might be using to keep their house doors locked. These are frustrating, but can usually be understood once found by established debugging techniques like bisecting a codebase with a test case.
With a memory-safety bug, your program runs the buggy code and, if you're unlucky, will diverge from the semantics you associate with its source code. Whereas a C programmer thinks of "x->name = foo;" as making an assignment of a struct field, if "x" is an invalid pointer, this code might write to an unrelated integer variable, modify 4 or 8 bytes of a string buffer, cause a segfault (this is nice, since we find out our program is buggy), alter a function pointer (to produce a segfault muuch later in code which is itself correct), or have any number of other effects.
This is much more hard to debug, and it's effectively impossible to mitigate the impact on your code of memory safety bugs outside your code. If you're working in a memory-safe language and call a function which you do not know to be implemented correctly, the worst thing it can do is issue an unexpected system call, which shows up very quickly in debugging. It can't mangle unrelated variables to make the program crash in correct, bug-free portions of code.
As noted, Rust is not, as a whole, memory-safe. Unsafe code can set up bombs that will cause segfaults and other mischief in safe code, just like the situation in C if we substitute 'unsafe' for 'incorrect' and 'safe' for correct. But, in Rust, if you see misbehavior you do not have to audit your entire codebase. You only need to audit the code explicitly marked as unsafe. The vast majority of computation in Rust is outside of unsafe blocks, and using unsafe in the middle of a complicated, subtle algorithm raises big warning bells in code review.
With respect to "C and other languages [having] similar issues", I don't think this is the case. Remote native code execution is a security issue only present in memory-unsafe programs. We do see remote (high-level) code execution vulnerabilities in programming languages with "eval"-like faculties, but it's not hard to audit your code and all its dependencies for calls to the built-in "eval", whereas auditing C code for memory safety is incredibly difficult.
All of this is in some sense a consequence of the principle of least privilege: we want to make the world more secure by avoiding having an omnipresent "do absolutely anything" escape hatch, so that we can trust modules in proportion to the privileges they need. Rust and memory safety aren't a silver bullet, but I believe they're a cost-effective step toward correctness.
The other half of Rust is that having algrebraic datatypes and a good typesystem helps you help yourself avoid many logic mistakes, and having an expressive language with generics helps you write less code (which correlates with having fewer bugs). This is why Haskell programmers like to claim they see fewer bugs than other languages. It's much harder to make an objective argument here, because availability of tools for preventing bugs isn't the same as lack of bugs. There are C codebases with fewer bugs than Haskell codebases; pure bug count depends on a multitude of factors in the development process and only formal verification can prove your program to be correct relative to its specification.
And then your specification still might not mean what you wanted it to mean! There aren't any silver bullets, but Rust is a useful tool that you should consider making part of your toolkit, and there's been a lot of work to try to make it able to do all the things people do in C.
(Some bits have been elided for brevity and to avoid the HN comment length limit; do note I understand the points you've made in spite of shortening some parts)
> The difference between regular bugs in memory-safe programs and memory-safety bugs is that with a regular bug your program is still evaluating the code you wrote; (...) [t]hese are frustrating, but can usually be understood once found by established debugging techniques like bisecting a codebase with a test case.
Right. (It's sad the only truly generic way to apply this approach is with fuzzing.)
> With a memory-safety bug, (...) it's effectively impossible to mitigate the impact on your code of memory safety bugs outside your code.
Ooohh. I see - faulty image parsing letting you break out of Chrome, or faulty font parsing letting you run amok in the NT kernel (yay!).
> As noted, Rust is not, as a whole, memory-safe. (...) But, in Rust, if you see misbehavior you do not have to audit your entire codebase. You only need to audit the code explicitly marked as unsafe. The vast majority of computation in Rust is outside of unsafe blocks, and using unsafe in the middle of a complicated, subtle algorithm raises big warning bells in code review.
That makes sense.
> With respect to "C and other languages [having] similar issues", I don't think this is the case. Remote native code execution is a security issue only present in memory-unsafe programs.
It's so sad the Burroughs-style architecture (https://news.ycombinator.com/item?id=14073079) fizzled out :(
> We do see remote (high-level) code execution vulnerabilities in programming languages with "eval"-like faculties, but it's not hard to audit your code and all its dependencies for calls to the built-in "eval", whereas auditing C code for memory safety is incredibly difficult.
Unless it's JavaScript - https://news.ycombinator.com/item?id=14013155
(Related: http://stackoverflow.com/questions/42776357/can-i-effectivel...)
> All of this is in some sense a consequence of the principle of least privilege: we want to make the world more secure by avoiding having an omnipresent "do absolutely anything" escape hatch, so that we can trust modules in proportion to the privileges they need. (...)
Right. On the one hand that [escape hatch mentality] gets you Symbolics Lisp; on the other hand those Lisp machines never ran banks or multiuser systems :P
> The other half of Rust is that having algrebraic datatypes and a good typesystem helps you help yourself avoid many logic mistakes, and having an expressive language with generics helps you write less code (which correlates with having fewer bugs).
The only problem there is that the verbosity of the code can cause oldschool hackers to flounder: http://esr.ibiblio.org/?p=7294
> This is why Haskell programmers like to claim they see fewer bugs than other languages. It's much harder to make an objective argument here, because availability of tools for preventing bugs isn't the same as lack of bugs.
Can't disagree there. Objectivity FTW.
> There are C codebases with fewer bugs than Haskell codebases; pure bug count depends on a multitude of factors in the development process and only formal verification can prove your program to be correct relative to its specification.
> And then your specification still might not mean what you wanted it to mean! There aren't any silver bullets, but Rust is a useful tool that you should consider making part of your toolkit, and there's been a lot of work to try to make it able to do all the things people do in C.
Thanks for this, it was an interesting read.
If anything, Rust badly needs a rockstar documentation team who can canonically communicate "the state of 4 minutes ago!" and keep everyone updated in real time. A herculean effort, most definitely, but it would ensure people had correct ideas about the language from the start, which would be cool.
I also think I have a theory as to why the parent commentator ("Except compilers are also...") has the views they have.
Rust requires you to be extremely verbose in describing every last iota of abstraction (and you have to have the right number of levels) - you can't just start banging around in the kitchen with random bits of hardware like you can in C, or hammer strings together like you can in Perl, or do Python's "executable pseudocode" thing, etc. You have to be neurotically specific (it feels like it when coming from the familiarity of another language that most decidedly does not feel like that).
I think that oldschool people encountering this see it as "too much work", because for the level of effort required, they expect the compiler to do more work for them than it's doing (for example, wrangling cyclic data structures). And so they go "I have to do all this?! And I only get that as a result?!" and throw their hands up in the air and find it hard not to criticise.
So this folds back into the documentation problem - people are collectively memoizing the "Rust is awesome!" from all the impressionable foghorn types. The foghorns need to be turned down a little and the nuance made a little clearer.
PS. I must admit that when I saw your "x->name" my brain immediately went "wait, what's x's current value?" - I'm still learning C (I understand the basics, but I'm still slow with it in many areas), and I'm happy I instinctively reacted like that :)
Anecdotally, I've rewritten fair amounts of code from Go to Rust and it seems like Rust takes about 60% the lines of Go, which is mostly due to library reuse for control-flow constructs (higher-order functions rely heavily on generics to be usable and not merely confusing footguns) and simplified error handling. The biggest pain point when porting is usage of goroutines, because these dictate the threading and communication architecture of your Rust code, and unlike goroutines it isn't cheap to spawn large numbers of real threads. This would be better with a coroutine library, but I haven't tried that approach in a ported codebase.
I do think explaining the language clearly and precisely is a big challenge and hope we can make it happen.
It's true that Rust forces you to be explicit and precise in many places C doesn't, but coming from a long time of programming C I loved to see that Rust was enforcing universally the guidelines that C projects slowly develop after years of developer experience: explicitly state memory ownership (in comments, in C), explicitly state which shared memory is protected by which locks (again, in comments or variable names in C), call destructor functions for all live local variables before returning from a function (done automatically by the Rust compiler), and so on. Rust is, in many ways, shorthand for well-written C. It makes you jump through hoops to express many sloppy C practices.
If you're just learning C, after you understand the basics I think the most important thing to understand is the abstract machine around which the language is defined; that's how you learn to reason about UB and which optimizations and transformations compilers will or won't do, along with how to guide the compiler to the translation you want. C isn't defined against a single huge byte array of memory, and pointers aren't indices into such!
> Related to "Unless it's JavaScript", what I mean by "not hard" is that a conservative static analysis can determine whether your code can possibly reference the built-in eval function. (...)
Oooh, like that. I get what you mean now.
> Anecdotally, I've rewritten fair amounts of code from Go to Rust and it seems like Rust takes about 60% the lines of Go, which is mostly due to library reuse for control-flow constructs (higher-order functions rely heavily on generics to be usable and not merely confusing footguns) and simplified error handling.
Hmm.
> The biggest pain point when porting is usage of goroutines, because these dictate the threading and communication architecture of your Rust code, and unlike goroutines it isn't cheap to spawn large numbers of real threads. This would be better with a coroutine library, but I haven't tried that approach in a ported codebase.
I'm curious if this is because there's no consensus on which coroutine library to use yet. The example I linked last message (http://esr.ibiblio.org/?p=7294) also whined about Rust not having a select() primitive or equivalent functionality in the core language - https://github.com/rust-lang/rust/issues/27800 remains open since Aug 2015 as of my writing this comment, but I don't know if there are any standardized (ideally stdlib-level) solutions known outside of this communication channel.
> I do think explaining the language clearly and precisely is a big challenge and hope we can make it happen.
Cool :D
> It's true that Rust forces you to be explicit and precise in many places C doesn't, but coming from a long time of programming C I loved to see that Rust was enforcing universally the guidelines that C projects slowly develop after years of developer experience: explicitly state memory ownership (in comments, in C), explicitly state which shared memory is protected by which locks (again, in comments or variable names in C), call destructor functions for all live local variables before returning from a function (done automatically by the Rust compiler), and so on.
That's fair. I get the impression Rust's vision is to be the go-to language for low-level enterprise-scale (ie, long-term) stuff, while also being accessible and suitable for hobbyist scripting. I can completely appreciate that.
> Rust is, in many ways, shorthand for well-written C. It makes you jump through hoops to express many sloppy C practices.
That's pretty cool. :>
> If you're just learning C, after you understand the basics I think the most important thing to understand is the abstract machine around which the language is defined; that's how you learn to reason about UB and which optimizations and transformations compilers will or won't do, along with how to guide the compiler to the translation you want.
I've only very recently started to notice this and grasp the importance of the difference between the hardware and what C abstracts that hardware as; thanks very much for the recommendation, I'll focus more closely on it. On that note, I find it remarkable how platform-agnostic yet hardware-accessible the C abstract machine is; I'm not sure whether to be mildly scared at the engineering ingenuity of the language or merely duly impressed that the developers just happened to stumble upon a really good design :)