Memory safety strikes again, it seems (overflow in a C string due to complex parameter parsing rules).
$ man doas | wc -l
58
$ man doas.conf | wc -l
101
$ man sudo | wc -l
741
$ man sudoers | wc -l
3254
$ man sudoers | grep -C1 despair
The sudoers file grammar will be described below in Extended Backus-Naur
Form (EBNF). Don't despair if you are unfamiliar with EBNF; it is fairly
simple, and the definitions below are annotated.
http://ftp.netbsd.org/pub/pkgsrc/current/pkgsrc/security/doa...
http://ftp.netbsd.org/pub/pkgsrc/current/pkgsrc/security/sud...
My reaction to this vulnerability was mild amusement, then later wondering if I should go discredit the inevitable Rust brigade. We don't have to rewrite everything in Rust to get better security. We just have to use simpler tools.
Yes, it would still be vulnerable to logic errors, like the last famous sudo bug where you pass -1 as the UID. But it wouldn't be vulnerable to this. (And this isn't the first memory safety bug to be found in sudo.)
Yes, sudo's complexity is useless for 99.99% of its users. But wouldn't it be nice if the result were merely a gross feeling rather than a security hole?
This is not true. Complexity breeds bugs, including security bugs, and memory safety doesn't change that. Your example is a good one - here's another: doas once failed to limit the environment variables which are passed to the child process, which could be used to nefariously influence the program running (e.g. with LD_PRELOAD). How would Rust prevent that oversight? It wouldn't.
A simpler program will generally be more secure than a complicated one, no matter what language either is written in. Furthermore, rewriting an established program from one language to another will always introduce more bugs than it fixes, and more severely the more complex the program is. The single best way to improve security is to reduce the attack surface, and the single best way to do that is to reduce the complexity of your system.
"Yes, it would still be vulnerable to logic errors... But it wouldn't be vulnerable to this. "
I think you'll find in disagreeing with the comment on logic errors you just said the same thing the comment did about logic errors.
Also I think the generalization that rewriting an established bit of code in a new language in a secure language is a bit too general. clearly Firefox not only set out to make Rust for this purpose but it's not had an explosion in vulnerabilities with the modules it has replaced. Quite the opposite actually. Nor has every tool or app rewritten become a security failure compared to the original. I do think it's something that can easily be screwed up though, especially if someone rushes through by focusing on functionality duplication instead of building a more secure version of something.
Regardless, both "using a memory safe language results in a more safe program" and "having a minimum attack sufrace results in a more safe program" can be true. There is no need to make it a choice of A or B.
I think you'll find that my comment explicitly acknowledges this and expands on it with another example. Are we done telling each other to read the things we're writing?
>Firefox not only set out to make Rust for this purpose but it's not had an explosion in vulnerabilities with the modules it has replaced.
You're setting the bar pretty high with an "explosion" of vulnerabilities here. Rust programs have vulnerabilities, including rewrites. They also have other kinds of bugs, often ones which were not present in the code that they're replacing. You need only browse your nearest convenient RiiR bug tracker to find evidence of this.
Let me restate my thesis in mathematical terms. If we presume that 1 in 100 lines of production code has a bug in it, regardless of language (generous, I know), and that 1 in 10 bugs in C programs are memory corruption related, then saving 10% of those bugs by rewriting it in Rust would take a 10,000 line codebase from 100 bugs to 90 bugs. A 1,000 line codebase, still written in C and without the advantage of memory safety, would have only 10.
In today's example, sudo is a caricature of runaway complexity. Rust is often touted as a panacea, but C has very little to do with why sudo is insecure. Sudo is comically overengineered and that level of overengineering has no place in a security context. This is the larger issue that needs to be addressed, not Rust.
Both Chrome & Microsoft found about 70% of bugs to be memory safety related. I've heard similar numbers out of FB as well. The math looks a little different with that data.
https://www.chromium.org/Home/chromium-security/memory-safet...
https://www.zdnet.com/article/microsoft-70-percent-of-all-se...
There's another argument I could make, too. Look at the bug tracker for the program you want to rewrite in Rust, examining the historical bugs. You'll find that there are often hundreds or thousands of mistakes that they made and already fixed in the original codebase. If you're rewriting it from scratch, can you be sure you won't make just as many? A stable, maintained codebase with a low throughput of changes tends to have fewer bugs over time, as the lack of churn avoids introducing new bugs and the application of time susses out all of the existing bugs. Rewriting the whole thing from scratch has a very high rate of churn, introducing a whole new slew of bugs on its own.
Now, a small codebase, focused on delivering its key value-adds without distractions, kept stable and at a low-churn rate over a long period of time: no matter what language you use, this is the best recipe for reliability and security.
it is literally impossible to write "a small codebase focused on its key value-adds without distractions" in a language that doesn't have strings and requires you to build a dictionary from scratch
What? Rust has so few libraries of significance that it still depends on C for security-critical areas like SSL.
>it is literally impossible to write "a small codebase focused on its key value-adds without distractions" in a language that doesn't have strings and requires you to build a dictionary from scratch
Strings are misunderstood, I'm not going to get into it here. My dictionaries in C usually clock in at about two dozen lines of code. The complexity doesn't go away because your language does it for you.
https://paste.sr.ht/~sircmpwn/3122d4a27a8e5312462e2329bf7ed6...
Actually managed to get it to exactly 2 dozen lines of code, not including the header, which isn't bad for an off-the-cuff remark.
You'd naturally expand or shrink this with whatever subset of map functions you require, like key/value enumeration, object deletion, resizing, whatever. It depends on your use-case. I don't believe in generic code.
I agree that rewrites have the serious potential to introduce new bugs and the cost is rarely worth it if the codebase is actually that stable and low througput, but the reality is that most aren't. A one time high cost in exchange for introducing 70% less bugs over a period of N years starts to look like a good trade off.
Yes, complexity is the root of all evil. I can get onboard with the whole statement except the "no matter what language you use". If you have the ability to use any language that enforces memory safety, we should use it.
Rewrites do bring the chance to Royally Screw it Up™ so it's certainly not simply a product of "it is now written in <Language X> therefore safe" but as it said not only have projects shown the security didn't fall apart but they have shown the opposite.
I agree you don't get there by a bunch of yolo rewrites to whatever is hip though, it has to be a planned effort that isn't rushed. Much in the same way quickly writing a small replacement utility does not inherently make it more secure or reliable than an existing significantly more complex utility. Even just trying to shave some functionality off the existing code is rife with "but how does removing this piece affect the app remaining logic" and takes time and effort to do right.
Both methods do have to be done right and both do greatly help security but there is nothing about picking a memory safe language or making a significantly narrower focused utility that preclude each other.
Maybe but not necessarily; it's reasonable to assume that Microsoft and FaceBook put non-zero effort into designing around, programming around, testing, looking for and fixing memory safety related issues in their C code. It could be the case that not having to care so much about those frees up some non-trivial amount of attention and time which could be spent on the other classes of problems.
The first bug lets any user cause doas to read out of bounds of an array, though not in a way that's exploitable.
Well, it's arguably a bug in libc. If you run doas with a completely empty argv (argc = 0, so not even an executable name; the two systems I tried, Linux and macOS, both let you do this), getopt will exit with optind = 1. Then when doas does;
argv += optind;
argc -= optind;
doas will then dereference argv, and generally act as if you tried to execute a command consisting of the environment variables. However, the environment variables are not secret, and doas doesn't behave any differently than if you just passed the environment variables as normal command-line arguments, so this is not exploitable.
On an OS where argv is not followed by environ or a similar array of character pointers, doas might crash instead, although since it only reads from those pointers rather than writing to them, this still probably wouldn't be exploitable.
The second bug would compromise memory safety if things were slightly different. The bug is in configuration file parsing. Even if it did compromise memory safety, it would not actually be exploitable, because doas normally only parses the trusted systemwide configuration file. It can be asked to parse a configuration file passed on the command line, but it drops privileges before doing so. This is a good example of layered defense, so kudos to doas for that! Still, I thought the bug was worth mentioning.
The bug is a traditional sort of integer overflow. parse.y grows the array of rules with
maxrules *= 2;
However, because maxrules happens to be signed, before doubling produces a smaller-than-expected positive value, it will first produce a negative value. This will then get sign-extended when converting to size_t (assuming 32-bit int and 64-bit size_t), and reallocarray's overflow check will trigger, causing reallocarray to return NULL. doas interprets that as out-of-memory and handles it cleanly.
(On a system where sizeof(int) == sizeof(size_t), things are a bit different, but it will just run out of memory before maxrules gets that high.)
Moral of the story? Well, as I see it:
Simplicity and layered defense, both featured in doas, are both effective ways to avoid vulnerabilities. But guaranteed memory safety, which would require a different implementation language, is also an effective way to avoid vulnerabilities. You aren't forced to pick and choose. Why not demand all three?
As for the integer overflow case, it's also highly unlikely to be exploitable, even if it were unsigned - the system would have to, as I'm sure you can infer, have tens of millions of rules before this was an issue. It's quite within the realm of reason, in my opinion, to declare this an acceptable trade-off. The rest of your explanation shows that even if this weren't the case, the bug wouldn't be exploitable.
Anyway, I like your comment, but I'd recommend a different moral to this story: in the space of 47 minutes you were able to conduct a reasonably thorough audit on the doas codebase. Wanna give that a shot for sudo now?
This is true, but for a language where dynamically sized arrays are a standard data type, the most natural thing to do is to start by collecting the arguments into an array (maybe copying the strings at this point, maybe not). All further argument parsing is done with the array and is thus bounds-checked. I checked Rust's standard library and sure enough, it follows this pattern. Though, I could imagine some hypothetical startup code messing up the argc=0 case if it tried to separate argv[0] from the rest of the arguments while constructing the array.
> Anyway, I like your comment, but I'd recommend a different moral to this story: in the space of 47 minutes you were able to conduct a reasonably thorough audit on the doas codebase. Wanna give that a shot for sudo now?
Fair point. (And I didn't downvote you.) But in my opinion, that just confirms my view: ideally you want both simplicity and memory safety.
I have trouble imagining how aborting leads to a security vulnerability? That's literally running no code, the opposite of running arbitrary code.
Aborting is fine in any language. Criticisms of C here would come about because C doesn't abort when it should (null pointer deref, array out of bounds, etc), not the inverse.
As for your second find. It already got fixed: https://marc.info/?l=openbsd-cvs&m=161176698927944&w=2
> Complexity breeds bugs, including security bugs, and memory safety doesn't change that.
Yes, memory safety changes that radically.
> A simpler program will generally be more secure than a complicated one, no matter what language either is written in.
Disagree, but the statement is really weak anyways, especially since 'complexity' is an ill-defined term. More features? Cyclomatic?
> urthermore, rewriting an established program from one language to another will always introduce more bugs than it fixes, and more severely the more complex the program is.
Should be obvious to anyone that this isn't true.
> The single best way to improve security is to reduce the attack surface,
Not true, but it's a great way to start.
I'm not sure of any definition of complexity you could appeal to which makes my argument weak.
>>rewriting an established program from one language to another will always introduce more bugs than it fixes, and more severely the more complex the program is.
>Should be obvious to anyone that this isn't true.
The opposite is painfully obvious: (1) Writing code causes bugs. (2) Rewriting an established project involves writing more code than leaving it would. (3) Writing all of that new code will introduce new bugs which were not present in the original.
I'm not saying that a rewrite is never justified, but rather that the argument that we should rewrite in Rust simply to avoid bugs has little weight.
Here[1] is a link to a slideshow of a talk on the F# language, with a case study from EON PowerGen company rewriting the core of an application evaluating revenue due from their balancing services contracts nationwide in the UK. It was originally 350,000 lines of C# developed by 8 people in 5 years and incomplete. It was redeveloped by 3 people (2 had never used F# before) in 30,000 lines, complete in 1 year.
They claim zero bugs in the F# redeveloped system (page 29). This example also gets a mention in a Don Syme (F# language designer) talk in 2018[2] with the PowerGen employee in the audience.
The PDF cites a testimonial from Kaggle saying they're moving more and more of their application into F# which is "shorter, easier to read, easier to refactor, and because of strong typing, contains far fewer bugs".
[1] https://www.microsoft.com/en-us/research/wp-content/uploads/...
I'm puzzled that we don't have a memory-safe ABI (e.g. amd64-safe) and runtime for C so we could just compile things with
clang -safe sudo.c
There are a number of proof-of-concept memory-safe compilers for C using fat pointers, etc., but memory safety hasn't made it into gcc or clang. 64-bit CPUs can help because you can repurpose address bits. Even Pascal (which is largely isometric to C) supported a degree of memory safety via array bounds checking. I believe Ada compilers also support memory safety. PL/I was actually memory safe and is why Multics never had any buffer overflows. Obviously Rust is memory safe, but for a lot of legacy C code it is impractical to rewrite everything in Rust but eminently practical to recompile it with memory safety turned on.
Also to ship this in a Linux distro you'd need two builds of many packages. Tons of tools would need to be updated to work with the new ABI. It would be a nightmare.
Furthermore, a new fat-pointer ABI would not address lifetime errors like use-after-free, so what would your plan be there? Boehm GC? More complexity, more overhead, more compatibility issues.
So in practice this is not that appealing, which is why we don't do it.
Maybe it shouldn't be doing that? Isn't that the whole point of something like a -safe flag? Increase security as you go along. Yes it is going to take time. The best time to plant a tree is 20 years ago, next best is today.
If you have to make all kinds of changes to the code you might as well just translate it into a different language.
Any C code that needs changing to deal with fat-pointers is probably already UB in C (or at best, has some implementation-defined behaviour).
That's because the representation of pointers themselves is undefined (so you can't get a valid result by looking at those). Pointer/integer casts (either direction) are implementation defined. And accesses via pointers to anything beyond their bounds is already UB.
There's some good and interesting discussion of what's involved in all of this on: https://www.ralfj.de/blog/2020/12/14/provenance.html
And there's already bodies of work within the Rust (and C/C++) communities around the concepts/technologies that would need to be developed to achieve something like a memory-safe UNIX TCB.
But it comes with costs. Someone has to learn Rust and then convert all of these programs. And it also has the issue that Rust programs are only memory safe if the unsafe keyword is not used anywhere in the program (correct me if wrong?). So it looks like the effort to do such thing, while noble, and valiant, is essentially an experiment with an uncertain pay-off that could turn out to be small or large.
Much more interesting (to my mind, anyway) is something like Miri. The rust interpreter, which uses fat-pointers to make things (more? completely? someone more informed can correct me..) memory-safe by inserting some relatively lightweight run-time checks.
And, then again, if such a thing could be compiled rather than interpreted (some things similar to this already exist, like C with fat-pointers). And if the source language was C (or something like it) or C++ (or some future C++) then the human aspect of re-training a generation of programmers goes from being a very big hurdle, to a much lower one.
At that point the benefits go up quite a bit, and the costs come down quite a bit. And I think that might be a promising path to overcoming the sort of human/political hurdles/inertia involved in rewriting the world :)
Someone has to learn compiler engineering and then design and implement a 'safe' ABI. Unlike learning Rust, this is probably worthy of a research paper.
> Rust programs are only memory safe if the unsafe keyword is not used anywhere in the program
If you use unsafe, then you take some of the responsibility for maintaining memory safety. However, you can audit the unsafe parts of the code, and it will compose with the compiler-provided guarantees for the rest of the code. Besides, one can easily avoid unsafe code for safety-critical tools like these.
> Much more interesting (to my mind, anyway) is something like Miri. The rust interpreter, which uses fat-pointers to make things (more? completely? someone more informed can correct me..) memory-safe by inserting some relatively lightweight run-time checks.
Miri does not support most interaction with the outside world [1]. It is focused more on detecting UB in unsafe code when it is exercised by tests, than on having your code running in production through Miri. Moreover, I wouldn't call a thousand-fold slowdown [2] "relatively lightweight".
[1]: https://github.com/rust-lang/miri#miri [2]: https://www.reddit.com/r/rust/comments/hosvqu/will_the_miri_...
Yes, all good points. What I'm getting at is that it seems like nobody has yet re-written sudo in this safe way. And it's not just a matter of re-writing it. If (when) someone comes along with this re-write there's "if this person goes away, will we find someone else to maintain it?" and all those other very conservative social forces at play.
I think any new programming language community has these sorts of adoption hurdles to face. And I'm sure the rust community is working hard to build up that pool of developers and I think that's all really positive so I don't want to sound like I'm subtracting from it at all. I'm just also an interested spectator of PL and systems programming research/new directions :)
> If you use unsafe, then you take some of the responsibility for maintaining memory safety. However, you can audit the unsafe parts of the code, and it will compose with the compiler-provided guarantees for the rest of the code. Besides, one can easily avoid unsafe code for safety-critical tools like these.
Thanks, yep. That's why I think that generally Rust is a good idea, and rewriting the TCB in it is a worthwhile project. In regards to safety it looks like a step in the right direction. We're just quibbling about the cost/benefit analysis of how big of a step it is compared to above-mentioned issues that all new programming languages face. Personally, I've no doubt that even with all that factored in, it's still a net positive.
> Miri does not support most interaction with the outside world [1]. It is focused more on detecting UB in unsafe code when it is exercised by tests, than on having your code running in production through Miri. Moreover, I wouldn't call a thousand-fold slowdown [2] "relatively lightweight"
Thanks for the clarifications, you're definitely more up to speed on that project than I am! But yeah, what I meant there was not that the implementation of miri was something to use as-is, more that it's an interesting direction in PL/systems programming research (imo). And some of the ideas there, especially where runtime cost _in principle_ can be made to be relatively lightweight are interesting. I've seen some other research where C implementations with bounds-checking have been implemented part-statically and where the remaining checks are done at run-time with fat-pointers.
OK, bounds checking isn't memory safety, but the paper was a while ago. Maybe it was this one https://www.comp.nus.edu.sg/~ryap/Projects/LowFat/ ?
So I mean, it sounds like you might be able to get to a place where you can use some bits of unsafe in rust, but maybe the program overall could still be safe because the compiler can have a mode where run-time checks (which can be statically eliminated in a lot of cases) are included.
But hey, I'm just a relatively amateur outside-observer of all this, maybe that's a totally impossible pipe dream? :)
You will use existing libraries that contain unsafe code, but you should be able to stick to popular well-tested libraries, which means it will be very difficult for an attacker to find a new exploitable bug in those libraries to attack your tools.
The kernel won't allow you to setuid scripts, there is a reason for this: it's very easy to leave glaring security holes while doing so.
My main point was that you could rewrite sudo in all sorts of languages, but saying "just rewrite it in Perl" (assuming it worked) isn't a enough justification to make it happen. Nobody is going to re-create their own project in Perl, Rust, etc just to eliminate buffer overflows. If somebody wants sudo in Rust, they'll have to do it themselves, and it still might never replace the original.
If someone would get serious about security, auditing every setuid binary would certainly be something on their list (if they use any). If they really want the functionality, rewriting it to cover just enough of the required functionality wouldn't be unheard of.
Or said another way, is the lack of CVEs for doas an indication it is more secure or just less (ab?)used?
People don't add these features for the fun of it; they're present because they solve a specific use case.
I use doas as well; it's a neat little program that covers many of the common use cases, but it doesn't cover all of them. Usually you should use the simplest tool that solves your problem, but sometimes your problem is complex and thus your tool will be complex.
That is not to say that sudo can't perhaps be simpler; the first version was released in 1985 and there are probably things that can be improved, but sudo really isn't written by idiots who just add features because they have nothing better to do.
If doas doesn't cover your edge case, then the responsible thing is to make a new tool which covers just that, and not to shove your complexity into a critical security component that the other 99% of the userbase doesn't need. Remember Heartbleed? The entire internet shat its pants because of a vulnerability in a feature that no one uses.
Failing to uphold that principle over and over again leads to broken, unreliable, insecure programs. This is why everything is on fire. Not C.
The last major sudo bug was in the PAM code (which lead to the creation of doas), which is something many people don't need, but also something that many others do need.
And writing separate tools would be the equal (or more!) lines of code and an equal amount of bugs in total (or probably more, since people will be reinventing stuff and there will be fewer reviewers per line of code). This isn't reducing complexity, it's just spreading it out.
>writing separate tools would be the equal (or more!) lines of code and an equal amount of bugs in total
In total, yes, but crucially, not all on your system at the same time.
I'm curious whether you run OpenBSD, since you mentioned you use doas. Do you have any thoughts on OpenBSD?
But I don’t see a point in using systems languages for the usual UNIX tools, they could be rewritten in a more secure language. In the rare case performance is important, there is FFI, but they are usually IO-bound so there is not much point.
But the two are pieces of the same puzzle (defence in depth). Ideally, SUID binaries should be formally specified/verified (ada+spark?), though you could still have bugs in the specification.
And I'd argue that if you need more specific features than just `sudo`'s core functionality, you should probably just make your own setuid binary. That still exposes you to making the same mistakes, so better keep the complexity low, and still rely on a memory-safe language. Using proved, lightweight libraries helps getting an implementation correct.
As much as I like C (which is the language I'm most proficient with), it just gives you too many ways to shoot yourself in the foot, and IMO isn't really the best suited for something where:
- performance doesn't really matter
- memory safety, typechecking are critical.
You could always get away with a transpiler for a DSL, or a compiler that injects more checks, but better suited tools are available anyway.