"Serge E. Hallyn" <serge@xxxxxxxxxx> writes: > Quoting Andy Lutomirski (luto@xxxxxxxxxxxxxx): >> 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 _______________________________________________ Containers mailing list Containers@xxxxxxxxxxxxxxxxxxxxxxxxxx https://lists.linuxfoundation.org/mailman/listinfo/containers
