userns idea: preventing SCM_CREDENTIALS from leaking out

[Eric W. Biederman]

"Serge E. Hallyn" <serge at hallyn.com> writes:

> Quoting Andy Lutomirski (luto at amacapital.net):
>> IIUC there are multiple ways to end up with a socket pair for which
>> one end is in a user namespace and the other is outside of it.  That
>> means that SCM_CREDENTIALS can be used by a process in a userns to
>> authenticate to a process outside.
>> 
>> This is all well and good (and, as far as I know, correct), but I'm
>
> And the cgroup manager I'm starting on depends on this.
>
>> not sure this is always the desired behavior.  In the context of a
>> tool like Docker, it might be useful to have several user namespaces
>> that have the *same* uids mapped.  Nonetheless, if one of those
>> namespaces is compromised, it probably shouldn't be permitted to
>> attack things outside the user namespace (or in the host, if any
>> interesting uids are mapped).
>> 
>> Would it make sense to have an option to allow a user namespace to opt
>> into different behavior so that its users show up as the invalid uid
>> as seen from outside (as least for SCM_CREDENTIALS and SO_PEERCRED)?
>> 
>> Implementing this might be awkward (ok, it might actively suck due to
>> a possible need for reference counting), but I'm wondering if it's a
>> good idea even in principle.
>
> Well, I'll grant you, if I have a single directory with a socket in
> it, and I make that the aufs or overlayfs underlay for two separate
> mounts, which each are in different containers, then you might have
> a problem here.
>
> Now maybe the answer to that is that the sockets should be created
> in tmpfss (/run, /tmp, etc) anyway.  But the more I think about it
> the more I, unfortunately, agree that this could be a problem.

I really hate the concept of mapping a uid in some contexts and not
others.  That seems very prone to go wrong. Given all of the possible
kinds of perumutations I can't imagine how we would get it correct.

MS_NOSUID and MS_RDONLY will help with some of the worst offenders.
But it will still be possible for the user namespace root to call
setuid(NNN); and create a process with that uid.  And if a unix domain
socket isn't the only means of interacting there will still be problems.

I will suggest that writing a uid mapping filesystem like overlayfs or
perhaps as a mount option of overlayfs is likely to be a more robuse
solution in general.  Certainly that is what I originally had on the
drawing board to solve this class of problem.

Eric