Inconsistency between os.getgroups and os.system('groups') after os.setgroups()

[Heiko Wundram]

Am 25.03.2012 23:32, schrieb jeff:
> After the os.setgroups, os.getgroups says that the process is not in
> any groups, just as you would expect... I can suppress
> membership in the root group only by doing os.setgid and os.setuid
> before the os.system call (in which case I wind up in the group of 
> the
> new user instead of root), but I have to be able to get back to root
> privilege so I can't use setgid and setuid.

Simply not possible (i.e., you can't drop root privileges, be it by 
setuid()/setgid() or removing yourself from groups with setgroups()), 
and later reacquire them _in the same process_. See the discussion of 
how to implement privilege separation at

http://www.citi.umich.edu/u/provos/ssh/privsep.html

(which discusses how this is implemented in OpenSSH) by running 
multiple processes which communicate through IPC mechanisms, and each of 
those drops the rights it requires. Using IPC to implement 
reduced-privilege process spawning has a long history; also, Postfix 
comes to mind as an "early" adopter of a privilege separation mechanism.

-- 
--- Heiko.