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Re: dropping setuid/setgid privileges

From: Bruno Haible
Subject: Re: dropping setuid/setgid privileges
Date: Fri, 12 Jun 2009 01:29:58 +0200
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James Youngman wrote:
> > For example, the user can write to a file that he
> > does not own but which is chgrp'ed to a group that is contained among
> > his supplementary groups. The program may need to write to such a file.
> > If it has only the user's uid and gid, it cannot do it. So it needs
> > also to acquire all supplementary groups of the user, right?
> One option would be for the program to be setgid instead of setuid, if
> it's the group membership that's important.

Sorry, you lost me. I was answering Sergey, discussing the case of an
executable that is not setuid/setgid but is run by root. (For
executables that are setuid/setgid and are run by the user, the process'
supplementary groups contain the user's extra groups, hence nothing to do.)

> > Shouldn't the program also call setgroups (possibly indirectly through
> > initgroups), in order to make sure that it can write any file that the
> > user can write to?
> That is usually necessary but not always sufficient, for example see
> http://blogs.sun.com/peteh/date/20050614

What do you mean by "not always sufficient", other than kernel bugs and
implementation limits? Assuming a small number of supplementary groups,
all a process needs to have in order to access all files that a user has
access to is that
  - the process' uid = the user's uid,
  - the process' gid and supplementary groups together contain all groups
    to which the user belongs.


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