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

From: Theodore Ts'o
Date: Sun Nov 16 2014 - 08:32:43 EST

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.

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

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 components when they made incompatible
changes made available by new features provided by systemd[1].


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....
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