#structured preemption

a rant that ended up flowing into "structured privileges"

Thinking about the execve(2) interface of UNIX, particularly with elevated privileges - especially in combination with the UNIX philosophy of little tools that do small jobs well.

This generalizes, actually - the same ideas of privilege segregation could apply within a process if languages and runtimes took advantage of modern kernels' support for it - at least in Linux, where processes and threads are basically the same kernel primitive, with just a few configuration differences.

Anyway, it bothers me that the default behavior is that if I start a root program, everything it executes is also root. In some sense, the ideal default for a program with a privileged effective UID and an unprivileged real ID is that the execve system call drops privileges - and if you want to keep privileges, that should be an explicit flag.

Even more ideally, this wouldn't be a flag that you, as the program with privileged effective UID or effective capability set or whatever, get to just set however you want - this should be some sort of object, like a file descriptor or a 128-bit random key or a pointer into read-only mapped memory, which is given to you by the caller who gave you elevated privileges. So your options are either "drop privileges" or "keep any privileges that your caller allowed you to keep" - notably absent from your options is an unconditional "keep privileges".

Of course, one problem here is that at some levels of abstraction, forking another process should be an implementation detail - when I call a program as root, I shouldn't have to know "this program wants to fork a child", let alone details like which child programs, how many, how deep the process tree goes, and so on.

So execve isn't exactly the right boundary here, at least not always. But here are a couple examples where the execve boundary is user-facing and explicit, and is the right boundary:

Suppose I want to run a privileged `find` program - because it needs to look inside directories that only root can read, and then I want to run some un-privileged command per file path - the most secure thing, especially if I don't have time to audit the command's source and build and binary for problems, is to prefix my exec arguments for find with something like sudo -iu "$USER" so that the child process of find drops privileges back down to my user before running the command.

For another example, suppose I want to run something like fzf on a list of paths, and then I want fzf to use `bat` to preview those files. bat could need read-only root privileges if some of those files are only root-readable, but there's no great way to give those privileges to just bat instead of fzf - at best, I run sudo once before running fzf, sudo caches my credentials, and then I set the preview argument of fzf to include a sudo call. But this isn't friendly to building larger programs: if the fzf call is inside some larger script, that script now has to either unconditionally include that sudo call, or include it or not based on either a flag that is passed in or if it already has special privileges.

Going back to how this generalizes, this same problem happens with libraries - instead of calling into another program, you call into some library, but either way, you probably don't have time to audit the whole thing, and it often doesn't need all the elevated privileges that you have - if I've got code to parse YAML inside my process, those instructions don't need any privileges besides access to the input bytes and enough memory to return the parsed result, and yet I have to go out of my way to isolate them and drop privileges if I want to be safe from some exploitable parser bug.

Ideally, code should have to go out of its way to explicitly take privileges that it either needs or can optionally use, as a kind of parameter. The whole system, from the ground up, would be built with APIs which have a bias towards dropping privileges and which require every privilege to be passed in from higher up in the call tree. Then the path of least resistance aligns with security: if you're writing code which doesn't need privileges, you don't go out of your way to take them from the caller - if you need privileges to do something, you have no choice but to reveal it in the API.

The only wrinkle here is that people should be able to write middleware which doesn't know or care about privileges, but is transparent to them. The simplest solution is to do what Go did with its context type - force middleware to take that parameter, and pass it through just in case the thing it calls needs it. This is... better than privilege use being invisible, but it's still pretty bad, especially when we generalize it to a granular "permissions" object, since middleware would need an arbitrarily permissive type: instead of knowing if we're calling something that requires privileges, we are back to calling something that might use any of the privileges we fed into it, and we can't know without inspecting the middleware.

In languages with templates, the permissions could be expressed as a template parameter, and then it's at least clear that the middleware takes whatever permissions its callee requires - and someone higher up in the call stack is defining the callee, so they have the responsibility and knowledge of those permissions. But this still isn't really enough, that's just a nice way to make the type system transparently propagate information about elevated privileges to the places where a developer is most served by knowing it.

But at runtime, we need a distinction between a usable privilege, and an unusable "sealed" privilege which is just being passed through code from a higher caller to a transitive callee which has what they need to unseal it.

This has been on my mind for a couple years now, at least. The overarching theme here is that ideally software has a reified representation of all interesting privileges, which ideally is both visible in the type system and actually enforced at runtime, and that code should be forced to be explicit whenever it uses or passes privileges to do anything.