OpenBSD's Pledge and Unveil from Python

https://nullprogram.com/blog/2021/09/15/

by todsacerdoti • 5 years ago

142 40 5 years 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?

dbt002• 5 years 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).

ghoward2• 5 years 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...

robmusial• 5 years 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_unicode• 5 years 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.

masklinn3• 5 years 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.

chousuke• 5 years 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.

moonchild• 5 years 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?

foxfluff• 5 years 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.

toast02• 5 years 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.

the8472• 5 years ago

A major limitation of seccomp is that you can't turn off inheritance. With pledge inheritance is opt-in.

This has major consequences because it makes it possible for a process to secure itself without affecting its ability to spawn child processes. You could apply seccomp in-process or use a spawn-helper before securing it. But more realistically it means that one applies an external seccomp profile to a group of processes (e.g. a container) that contains the union of all needed syscalls.

So while seccomp is more flexible in some sense (being able to run BPF filters on every syscall) its architecture leads to it being applied in a less flexible way.

xnormal• 5 years ago

Pledge/unveil are manifestly more successful, being used pervasively (though not really by programs which do not primarily target openbsd).

Because of their simplicity, they are easily added as a patch to a ported application. There are probably more ports using them than programs that target openbsd.

protomyth• 5 years ago

It really doesn't make much sense to compare pledge/unveil to seccomp and capsicum or any other sandboxing solution.

Pledge and Unveil are really part of the program's specification. They are much closer in practice to asserts, pre-conditions, or contracts depending on the programming language you have used. They basically make sure that the program you are writing doesn't do anything stupid with bad inputs from the outside world. Its part of the development process, and not very hard to add to an existing program.

ingve• 5 years ago

They are also available (and used) in SerenityOS:

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

baq• 5 years 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

rbanffy2• 5 years ago

Would be useful to have a unified API…

masklinn3• 5 years 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.

comex• 5 years 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.

saghul• 5 years ago

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

masklinn3• 5 years 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• 5 years 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

jancsika• 5 years ago

Just ask OpenBSD to provide a pledge_sig and unveil_sig that simply return a C string where each character is the type of the corresponding argument.

E.g., if pledge_sig returns "ss" you know you're dealing either with the bona fide OpenBSD pledge or an evil demon.

Chris2048• 5 years ago

so at what level does this happen? The forked python interpreter and its children (but not any parent process)?

Rendello• 5 years ago

I love Python and OpenBSD. I've been using `pyopenbsd` [1] to access pledge/unveil in some chat bots. They may not be the most well-coded, secure, or stable bots, but having a one- [2] to three-line [3] change to harden them so much is great for me.

1. https://github.com/yuce/pyopenbsd

2. https://github.com/rendello/TTD2_Bot/blob/dev/src/main.py#L1...

3. https://github.com/rendello/pipe_bot/blob/develop/src/main.p...

junon• 5 years ago

This was written by Chris "skeeto" Wellons, a computer scientist for which I've personally grown a lot of admiration. Check out his Github if you're interested, he does a lot of really interesting things and is worth following.

Personal favorite is the branchless utf-8 decoder which I've used quite a few times in lexers and other projects.

https://github.com/skeeto?tab=repositories&sort=stargazers

mleonhard2• 5 years ago

We would be better off if every program on every operating system was required to call these functions on startup. Even better if the data was encoded into the binary, to be read by the kernel before starting the process.

xojoc• 5 years ago

Other discussions: https://discussions.xojoc.pw/?url=https%3A%2F%2Fnullprogram....

OpenBSD's Pledge and Unveil from Python

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