All that's required to emulate "no inheritance" is a broker.

jart · on July 15, 2022

Yes but we can't always assume a broker is installed. For instance I distribute a lot of foss software. I don't want to have to ask my users to install and adopt these other tools in order to use my code. I don't want to have to ask people to do things like run my security daemon as root. What I like about this new pledge.com program is it doesn't need root. I could just incorporate it into a build process or automated tooling for instance, and it would just work, and wouldn't inconvenience anyone. The only thing it needs is a Linux Kernel from the last 12 years.

staticassertion · on July 15, 2022

You can package the broker yourself and launch it before you pledge.

Keep in mind that if you can:

1. write to a file 2. Make that file executable (or write to an already-executable file, or an executable that provides scripting/ code exec natively)

And you have the `exec` capability, the sandbox is trivial to bypass in the face of code execution in the pledge'd process.

jart · on July 16, 2022

Yes that's true. It's something that I considered. I was reluctant at first to restrict changing the executable bit since OpenBSD lets this happen:

    int main(int argc, char *argv[]) {
      CHECK_NE(-1, pledge("stdio rpath wpath cpath exec", 0));
      CHECK_NE(-1, open("/tmp/doge.exe", O_CREAT | O_WRONLY | O_TRUNC, 0755));
      return 0;
    }

But if other people are noticing it too, then I think I might change the tool to simply disallow creating new executables. Since that makes more sense to me. But naturally would make even more sense for OpenBSD.

staticassertion · on July 17, 2022

The bypass is so significant that I kind of wouldn't bother. To me I think that it's probably best to just assume that this tpe of sandboxing is not capable of resisting an attacker with code execution, which is fine. It could instead be for things like path traversal attacks in web servers, or other design flaws that would allow "tricking" the application into performing actions you don't want.

I mean it's probably a good idea to close the trivial version of the bypass by disallowing setting exec on files (although you need to check the path because you may want to set it on a directory), but if you can execute `chmod`, write to the i-node directly, write to any other executable, write to your own executable, etc, that's just a full bypass.

the8472 · on July 16, 2022

But that's the issue. In the example I gave we already have a broker (the root process spawning ssh shells). But we want to restrict the broker too to make it more difficult to exploit. To do that we need to pledge without inheritance.

staticassertion · on July 17, 2022

I wouldn't call sshd a broker, at least, it's not exclusively one.