Feb 01, 2010

POSIX capabilities for python

I bet everyone who did any sysadmin tasks for linux/*bsd/whatever, stumbled upon the need to elevate privileges for some binary or script.

And most of the time if there's any need for privileges at all, it's for the ones that only root has: changing uid/gid on files, full backup, moving stuff owned by root/other-uids, signaling daemons, network tasks, etc.

Most of these tasks require only a fragment of root's power, so capabilities(7) is a nice way to get what you need without compromising anything. Great feat of caps is that they aren't inherited on exec, which seem to beat most of vulnerabilities for scripts, which don't usually suffer from C-like code shortcomings, provided the interpreter itself is up-to-date.

However, I've found that support for capabilities in linux (gentoo in my case, but that seem to hold true for other distros) is quite lacking. While they've been around for quite a while, even simpliest ping util still has suid bit instead of single cap_net_*, daemons get root just to bind a socket on a privileged port and service scripts just to send signal some pid.

For my purposes, I needed to backup FS with rsync, synchronize data between laptops and control autofs/mounts, all that from py scripts, and using full root for any of these tasks isn't necessary at all.

First problem is to give limited capabilities to a script.

One way to get them is to get everything from sudo or suid bit (aka get root), then drop everything that isn't needed, which is certainly better than having root all the time, but still excessive, since I don't need full and inheritable root at any point.

Another way is to inherit caps from cap-enabled binary. Just like suid, but you don't need to get all of them, they won't have to be inheritable and it doesn't have to be root-or-nothing. This approach looks a way nicer than the first one, so I decided to stick with it.

For py script, it means that the interpreter has to inherit some caps from something else, since it wouldn't be wise to give caps to all py scripts indiscriminatively. "some_caps=i" (according to libcap text representation format, see cap_to_text(3)) or even "all=i" are certainly better.

To get caps from nothing, a simple C wrapper would suffice, but I'm a bit too lazy to write one for every script I run so I wrote one that gets all the caps and drops them to the subset that script file's inherited set. More on this (a bit unrelated) subject here.

That leads to the point there py code starts with some permitted, but not immediately effective, set of capabilities.

Tinkering with caps in C is possible via libcap and libcap-ng, and the only module for py seem to be cap-ng bindings. And they do suck.

Not only it's a direct C calls translation, but the interface is sorely lacking as well. Say, you need something extremely simple: to remove cap from some set, to activate permitted caps as effective or copy them to inherited set... well, no way to do that, what a tool. Funny thing, libcap can't do that in any obvious way either!

So here goes my solution - dumped whole cap-manipulation interface of both libs apart from dump-restore from/to string functions, wrote simple py-C interface to it and wrapped them in python OO interface - Caps class.

And the resulting high-level py code to make permitted caps effective goes like this:


To make permitted caps inheritable:


And the rest of the ops is just like this:


Well, friendly enough for me, and less than hundred lines of py code (which does all the work apart from load-save) for that.

While the code is part of a larger toolkit (fgc), it doesn't depend on any other part of it - just C module and py wrapper.

Of course, I was wondering why no-one actually wrote something like this before, but looks like not many people actually use caps at all, even though it's worth it, supported by the fact that while I've managed to find the bug in .32 and .33-rc* kernel, preventing prehaps one of the most useful caps (cap_dac_read_search) from working ;(

Well, whatever.

Guess I'll write more about practical side and my application of this stuff next time.