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

Andy Lutomirski
2014-11-16T15:42:56Z

On Sun, Nov 16, 2014 at 5:32 AM, Theodore Ts'o <tytso@mit.edu> 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. 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.

It may make sense to reach out to some place like oss-security.

FWIW, I think we should ask, at the same time, about:

- Dropping supplementary groups.
- Switching gid/egid/sgid to a supplementary group.
- Denying ptrace of a process with supplementary groups that the
tracer doesn't have.

Also, I much prefer a sysctl to a boot option. Boot options are nasty
to configure in many distributions.

--Andy

>
> 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?
> 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.
>
> One of the things that scare me about the addition of the forced
> capability "setuid" binary was that the feature was just silently slid
> in, and we didn't do a good job reaching out to the tripwire and
> rootkit and other such programs out there, such that several years
> after we enabled capability support, most sysadmins and security
> scanning programs are still apparently obivious to the this very
> convenient feature that we gave to rootkit and malware authors. Now
> we can say that we're just adding new features, and we owe no debt
> other parts of the ecosystem --- this is the attitude used by upower
> and other freedesktop.org components when they made incompatible
> changes made available by new features provided by systemd[1].
>
> [1] http://m.memegen.com/u7o1tk.jpg
>
> But as kernel developers, who pride ourselves on not breaking
> userspace, I think we should try for a higher standard than this. And
> perhaps this change with allowing groups to be dropped --- which is
> admittedly a useful thing to be able to allow --- is a good place to
> start trying to model a better way of doing things. We should try to
> be responsible about how we add new features, and think of all of the
> potential downstream consequences to the _all_ of the ecosystem.
>
> - Ted
>
> P.S. And we really should try reaching out to the security scanners
> about capabilities, too....

--
Andy Lutomirski
AMA Capital Management, LLC