Re: [CFT][PATCH] userns: Avoid problems with negative groups

On Thu, Nov 27, 2014 at 9:21 PM, Eric W. Biederman
[off-list ref] wrote:

Andy Lutomirski [off-list ref] writes:

quoted

quoted

This change should break userspace by the minimal amount needed
to fix this issue.

This should fix CVE-2014-8989.

I think this is both unnecessarily restrictive and that it doesn't fix
the bug.

You are going to have to work very hard to convince me this is
unnecessarily restrictive.

quoted

For example, I can exploit CVE-2014-8989 without ever
writing a uid map or a gid map.

Yes.  I realized just after I sent the patch that setgroups(0, NULL)
would still work without a mapping set.  That is a first glass grade a
oversight that resulted in a bug.  None of the other uid or gid changing
syscalls without a mapping set, and setgroups was just overlooked
because it was different.  Oops.

I will send an updated patch that stops setgroups from working without
a mapping set shortly.

quoted

IIUC, the only real issue is that user namespaces allow groups to be
dropped using setgroups that wouldn't otherwise be dropped.  Can we
get away with adding a per-user-ns flag that determines whether
setgroups can be used?

Being able to call setgroups is fundamental to login programs, and login
programs are one of the things user namespaces need to support.  So
adding an extra flag and an extra place where privilege is required
is just noise, and will wind up breaking every user of user namespaces.

Further being able to setup uid and gid mappings without privilege is
primarily a nice to have.  The original design did not have unprivileged
setting of uid and gid maps and if it proves insecure I goofed and the
feature isn't safe so it needs to be removed.

Being able to set a single-user uid map and gid map is very useful for
sandboxing.  This lets unprivileged users drop filesystem and network
access and still run most normal programs.  A surprising number of
normal unprivileged programs fail if run without a mapping.

This does mean that running a system with negative groups and users
delegated subordinate gids in /etc/subuid is a bad idea and system
administrators shouldn't do that as those negative groups won't prove
effective in stopping their users.  But this is all under system
administrator control so shrug.  There isn't a way to avoid that
fundamental conflict.

quoted

setgroups would be unusable until the gid_map has been written and
then it would become usable if and only if the parent userns could use
setgroups and the opener of gid_map was privileged.

That proposal sounds a lot more restrictive and a lot more of a pain
to use than what I have implemented in my patch.

quoted

If we wanted to allow finer-grained control, we could allow writing
control lines like:

options +setgroups

or

options -setgroups

in gid_map, or we could add user_ns_flags that can only be written
once and only before either uid_map or gid_map is written.

Definitely more complicated and I can't imagine a case where I need
a gid map without needing to call setgroups.

I do it all the time.  Unshare, set mappings (with no inner uid 0 at
all), set no_new_privs, drop caps, and go.

Can we try the intermediate approach?  If you set gid_map without
privilege and you have supplementary groups, then let the write to
gid_map succeed but prevent setgroups from ever working?  That should
only be a couple of lines of code longer than your patch, and it will
avoid breaking sandbox use cases.

If we want to get really fancy in the future, we could have a concept
of pinned groups.  That is, if you're in a userns and you're a member
of an unmapped group, then you can't drop that group.  (Actually, that
all by itself might be enough to fix this issue.)

--Andy

Eric

-- 
Andy Lutomirski
AMA Capital Management, LLC
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