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OpenBSD's Pledge and Unveil from Python

85 points6 hoursnullprogram.com
comex3 hours ago

> This means no checking for OpenBSD specifically but instead feature sniffing for their presence.

Indeed it sniffs for any functions named pledge() and unveil() that exist in any library loaded into the process… and then assumes that, if they exist, they have not only the same purpose but also the exact same signatures as the corresponding functions from OpenBSD. ctypes cannot validate function signatures, so if they have different signatures, you get undefined behavior. I wouldn’t recommend this approach.

saghul2 hours ago

What would be the right way to do it? A plain old Python C extension?

masklinn2 hours ago

Or using API-mode cffi which basically does that for you, though it’s still not quite safe you can combine `cdef` and `set_source` to re-export exactly what you’re looking for. `set_source` will basically create an intermediate module under your control.

Sadly AFAIK you always need a `cdef` which defines the binding between Python and C, I don’t think you can tell cffi to get this information from a real header file. But by providing a custom source you can more easily ensure the `cdef` and the function for it match correctly, with `set_source` bridging to the real underlying functions.

One drawback of using API-level CFFI is it requires a C compiler (and probably all sorts of dev packages / headers), whereas ABI-level use doesn’t.

AndyMcConachie2 hours ago

I would check to make sure I was running on an OpenBSD system first before anything else.

You could use sys.platform()

https://docs.python.org/3/library/sys.html#sys.platform

masklinn5 hours ago

> Python functions that accept paths, such as open, generally accept either strings or bytes.

Or a pathlib.Path, hence os.fspath.

In fact for this specific use case there’s even better:

> os.fsencode(filename)

> Encode path-like filename to the filesystem encoding with 'surrogateescape' error handler, or 'strict' on Windows; return bytes unchanged.

travisgriggs5 hours ago

I was not familiar with these syscalls. Are they primarily in a “if you build it they will come” stage right now? Or is there an upswing of usage in the BSD world? Does the Linux kernel have any analog to pledge and unveil?

moonchild4 hours ago

> BSD world

Pledge/unveil are openbsd-specific. Freebsd has capsicum which, like seccomp (mentioned else-thread) is much more complex and flexible.

Pledge/unveil are manifestly more successful, being used pervasively (though not really by programs which do not primarily target openbsd). Seccomp and capsicum are barely used at all (likely due to their higher complexity), and capsicum usage was even removed from a few freebsd utilities. At the same time, there have been some valid criticisms of pledge/unveil by capsicum people; IIRC something to do with its restrictions not persisting following an exec?

toast025 minutes ago

> Freebsd has capsicum which, like seccomp (mentioned else-thread) is much more complex and flexible.

Capsicum may be more flexible in some ways, but it's also less flexible in others.

After you a process entera capsicum mode, it can't open new sockets, except by accepting on an existing listen socket or by receiving them on a unix socket, sent by a cooperating non-capsicum process. This means you can't capsicum a TLS proxy like hitch, which would be a great thing to capsicum since the operation is pretty simple and OpenSSL is scary.

foxfluff1 hour ago

> At the same time, there have been some valid criticisms of pledge/unveil by capsicum people; IIRC something to do with its restrictions not persisting following an exec?

The validity of that criticism is so-so. It's a common complaint from people who are trying to build an externally imposed sandbox that dictates what a program can do.

But that's not what pledge and unveil are. They're more of an internally imposed set of constraints: the program just announces what it's going to do. After that, if it breaks the contract (due to a bug or malicious intervention), the system has license to kill it.

The program knows what it is going to do, so it can write the contract for itself. But it doesn't know what some other program is going to do, so it doesn't make sense for these restrictions to persist after exec. The program-to-be-exec'd should have its own pledges.

In reality it's even more complicated than that: programs often need to perform some "privileged" operations before they are ready to put on their straight jacket. It'd be very hard for program A to say that program B starts with privileges X, Y, Z, and then after instruction Q drops Y and Z. And program B might require more privileges than what A had when it called exec, so again it's just not going to work for this at all. If A's privileges were to persist, then it would have to have some way to elevate them or it'd never drop them in the first place. Both seem like a bad deal.

It's just a completely different mechanism, but people think sandbox sandbox sandbox and if that's all one can think of, pledge and unveil might seem like a terrible tool for that. They're not a tool for sandboxing untrusted programs.

ingve4 hours ago

They are also available (and used) in SerenityOS:

https://awesomekling.github.io/pledge-and-unveil-in-Serenity...

dbt005 hours ago

they only exist on OpenBSD. They're used a lot for default OpenBSD binaries, but they are mostly hopeless for openbsd developers/porters to try to apply to portable software after the fact if the upstream devs aren't supporting that work.

(There are other mechanisms like containers or freebsd' jail that try to accomplish the same thing, but those tend to be "lots of functionality inside the sandbox" solutions, whereas openbsd is mostly aiming for "allow nothing besides the minimum in the sandbox".

(edited for clarity, thanks for the nudge ghoward).

ghoward5 hours ago

I upvoted, but I wanted to correct a small thing.

> ...they are mostly hopeless to try to apply to portable software after the fact if the devs aren't constantly testing with it.

They don't have to be hopeless. They were pretty easy ([1], [2]) to add to my bc.

[1]: https://git.yzena.com/gavin/bc/commit/3e8cd345de9d5b65ac7c33...

[2]: https://git.yzena.com/gavin/bc/commit/b6c65bb44c910c054bedfa...

dbt005 hours ago

Yes. That's awesome!

I was referring to the OpenBSD devs trying to hack pledges into other people's software by trying to guess what capabilities it needs while porting. That's not a good thing to try to do -- see also all the various attempts at guessing seccomp profiles in the Linux world, which has the same problem.

baq3 hours ago

looks like unveil() has an rough analog in Linux 5.13: https://landlock.io/, there was even a HN discussion which I missed: https://news.ycombinator.com/item?id=27215563

masklinn5 hours ago

> I was not familiar with these syscalls. Are they primarily in a “if you build it they will come” stage right now?

It’s an openbsd feature they built for themselves and deployed throughout the base system.

chousuke4 hours ago

To my understanding, you can use existing Linux kernel facilities like seccomp to build equivalents for pledge and unveil. The latter are less flexible but also easier to use, which is a point in their favour; ease of use matters for security.

robmusial5 hours ago

seccomp in linux is _roughly_ similar to pledge. they aim to accomplish the same goals but pledge is more straightforward. think of it like sudo vs doas.

Godel_unicode4 hours ago

Seccomp profiles are typically applied externally to a process in my experience though. I think this is more analogous to using capset() to remove capabilities in highly permissioned processes. For instance dnsmasq makes use of this ability to be minimally permissioned.