Re: [PATCH 2/2] groups: Allow unprivileged processes to use setgroups to drop groups

[Josh Triplett <josh-iaAMLnmF4UmaiuxdJuQwMA@public.gmane.org>]

On Sun, Nov 16, 2014 at 08:32:30AM -0500, Theodore Ts'o wrote:
> On Sat, Nov 15, 2014 at 09:08:07PM -0600, Eric W. Biederman wrote:
> > That may be a bug with the user namespace permission check.  Perhaps we
> > shouldn't allow dropping groups that aren't mapped in the user
> > namespace.
> 
> I'm not saying that we can't change the behavior of whether or not a
> user can drop a group permission.  I'm just saying that we need to do
> so consciously.

Agreed.

> The setgroups()/getgroups() ABI isn't part of
> POSIX/SuSv3 so we wouldn't be breaking POSIX compatibility, for those
> people who care about that.

POSIX.1-2001 actually specifies getgroups, but not setgroups.  In any
case, yes, POSIX doesn't say anything about this behavior.

> The bigger deal is that it's very different from how BSD 4.x has
> handled things, which means there is two decades of history that we're
> looking at here.  And there are times when taking away permissions in
> an expected fashion can cause security problems.  (As a silly example;
> some architect at Digital wrote a spec that said that setuid must
> return EINVAL for values greater than 32k --- back in the days when
> uid's were a signed short.  The junior programmer who implemented this
> for Ultrix made the check for 32,000 decimal.  Guess what happened
> when /bin/login got a failure with setuid when it wasn't expecting one
> --- since root could never get an error with that system call, right?

Ignored it and kept going, starting the user's shell as root?

I'd guess that a similar story motivated the note in the Linux manpages
for setuid, setresuid, and similar, saying "Note: there are cases where
setuid() can fail even when the caller is UID 0; it is a grave security
error to omit checking for a failure return from setuid().".

(Also, these days, glibc marks setuid and similar with the
warn_unused_result attribute.)

> And MIT Project Athena started ran out of lower numbered uid's and
> froshlings started getting assigned uid's > 32,000....)
> 
> In this particular case, the change is probably a little less likely
> to cause serious problems, although the fact that sudo does allow
> negative group assignments is an example of another potential
> breakage.
> 
> OTOH, I'm aware of how this could cause major problems to the concept
> of allowing an untrusted user to set up their own containers to
> constrain what program with a possibly untrusted provinance might be
> able to do.  I can see times when I might want to run in a container
> where the user didn't have access to groups that I have access to by
> default --- including groups such as disk, sudo, lpadmin, etc.
> 
> If we do want to make such a change, my suggestion is to keep things
> *very* simple.  Let it be a boot-time option whether or not users are
> allowed to drop group permissions, and let it affect all possible ways
> that users can drop groups.  And we can create a shell script that
> will search for the obvious ways that a user could get screwed by
> enabling this, which we can encourage distributions to package up for
> their end users.  And then we document the heck out of the fact that
> this option exists, and when/if we want to make it the default, so
> it's perfectly clear and transparent to all what is happening.

An option sounds sensible to me.  I think a sysctl makes more sense,
though.  I'll add one in v4.

What did you have in mind about the shell script? Something like:
grep -r !% /etc/sudoers /etc/sudoers.d
?

- Josh Triplett