alt Hacker NewsOne question I've always had about these capability systems is: why isn't there a way to set capabilities from the parent process when execing? Why trust a program to set its own capabilities? I know that having a process set capabilities on itself doesn't break existing tools, but it seems like if you really wanted a robust system it would make sense to have the parent process, the user's shell for example, set the capabilities on its children, and have those capabilities be inheritable so the child could spawn other processes with the same or fewer capabilities (if it's allowed to do that at all). Is there an existing system that works this way, in or outside of the UNIX family? Or maybe some research paper written on the subject? I'd love to know.
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I've only really messed with capsicum. You can certainly cap_enter between fork and exec, but depending on exactly what your target does, it's really not simple to do anything meaningful beyond the basic capsicum mode without changes to the program.
The way capabilities usually work is you more or less turn off the usual do whatever you want syscalls, and have to do restricted things through FDs that have the capability to do them. So like, no more open any path, you have to use openat with a FD in your directory of interest. But that requires the program to understand how to use the capabilities and how to be passed them. It's not something that you can just impose.
My understanding of SELinux, is it can be imposed on a program without the knowledge of the program, because it's more or less matching rules for syscalls... rather than giving a restricted FD to use with openat, you restrict the options for open.
This is essentially what containers are. Bubblewrap / Docker / Podman. I think the primary issue is very few applications on Desktop systems are actually designed with sandboxing in mind unlike say something on a phone.
Answering without reading TFA here. But I am familiar with capsicum.
But I am pretty sure you CAN get your capabilities from a patent process using capsicum, since they are just file descriptors.
You may be interested in OpenBSD's pledge[1][2][3].
> Why trust a program to set its own capabilities?
An example may be that a program starts needing a wide range of capabilties but can then ratchet down to a reduced set once running, aka "privdrop".
> why isn't there a way to set capabilities from the parent process when execing?
There have been replies on other systems so just to stick with pledge which provides the abiliy to set "execpromises" to do this.
[1] https://man.openbsd.org/pledge
[2] https://www.openbsd.org/papers/eurobsdcon2017-pledge.pdf
[3] https://www.openbsd.org/papers/BeckPledgeUnveilBSDCan2018.pd...