Re: [PATCH 00/34] fs: idmapped mounts

["Serge E. Hallyn" <serge@hallyn.com>]

On Fri, Oct 30, 2020 at 01:01:57PM +0100, Christian Brauner wrote:
> On Thu, Oct 29, 2020 at 02:58:55PM -0700, Andy Lutomirski wrote:
> > 
> > 
> > > On Oct 28, 2020, at 5:35 PM, Christian Brauner <christian.brauner@ubuntu.com> wrote:
> > > 
> > > Hey everyone,
> > > 
> > > I vanished for a little while to focus on this work here so sorry for
> > > not being available by mail for a while.
> > > 
> > > Since quite a long time we have issues with sharing mounts between
> > > multiple unprivileged containers with different id mappings, sharing a
> > > rootfs between multiple containers with different id mappings, and also
> > > sharing regular directories and filesystems between users with different
> > > uids and gids. The latter use-cases have become even more important with
> > > the availability and adoption of systemd-homed (cf. [1]) to implement
> > > portable home directories.
> > > 
> > > The solutions we have tried and proposed so far include the introduction
> > > of fsid mappings, a tiny overlay based filesystem, and an approach to
> > > call override creds in the vfs. None of these solutions have covered all
> > > of the above use-cases.
> > > 
> > > The solution proposed here has it's origins in multiple discussions
> > > during Linux Plumbers 2017 during and after the end of the containers
> > > microconference.
> > > To the best of my knowledge this involved Aleksa, Stéphane, Eric, David,
> > > James, and myself. A variant of the solution proposed here has also been
> > > discussed, again to the best of my knowledge, after a Linux conference
> > > in St. Petersburg in Russia between Christoph, Tycho, and myself in 2017
> > > after Linux Plumbers.
> > > I've taken the time to finally implement a working version of this
> > > solution over the last weeks to the best of my abilities. Tycho has
> > > signed up for this sligthly crazy endeavour as well and he has helped
> > > with the conversion of the xattr codepaths.
> > > 
> > > The core idea is to make idmappings a property of struct vfsmount
> > > instead of tying it to a process being inside of a user namespace which
> > > has been the case for all other proposed approaches.
> > > It means that idmappings become a property of bind-mounts, i.e. each
> > > bind-mount can have a separate idmapping. This has the obvious advantage
> > > that idmapped mounts can be created inside of the initial user
> > > namespace, i.e. on the host itself instead of requiring the caller to be
> > > located inside of a user namespace. This enables such use-cases as e.g.
> > > making a usb stick available in multiple locations with different
> > > idmappings (see the vfat port that is part of this patch series).
> > > 
> > > The vfsmount struct gains a new struct user_namespace member. The
> > > idmapping of the user namespace becomes the idmapping of the mount. A
> > > caller that is either privileged with respect to the user namespace of
> > > the superblock of the underlying filesystem or a caller that is
> > > privileged with respect to the user namespace a mount has been idmapped
> > > with can create a new bind-mount and mark it with a user namespace.
> > 
> > So one way of thinking about this is that a user namespace that has an idmapped mount can, effectively, create or chown files with *any* on-disk uid or gid by doing it directly (if that uid exists in-namespace, which is likely for interesting ids like 0) or by creating a new userns with that id inside.
> > 
> > For a file system that is private to a container, this seems moderately safe, although this may depend on what exactly “private” means. We probably want a mechanism such that, if you are outside the namespace, a reference to a file with the namespace’s vfsmnt does not confer suid privilege.
> > 
> > Imagine the following attack: user creates a namespace with a root user and arranges to get an idmapped fs, e.g. by inserting an ext4 usb stick or using whatever container management tool does this.  Inside the namespace, the user creates a suid-root file.
> > 
> > Now, outside the namespace, the user has privilege over the namespace.  (I’m assuming there is some tool that will idmap things in a namespace owned by an unprivileged user, which seems likely.). So the user makes a new bind mount and if maps it to the init namespace. Game over.
> > 
> > So I think we need to have some control to mitigate this in a comprehensible way. A big hammer would be to require nosuid. A smaller hammer might be to say that you can’t create a new idmapped mount unless you have privilege over the userns that you want to use for the idmap and to say that a vfsmnt’s paths don’t do suid outside the idmap namespace.  We already do the latter for the vfsmnt’s mntns’s userns.
> 
> With this series, in order to create an idmapped mount the user must
> either be cap_sys_admin in the superblock of the underlying filesystem
> or if the mount is already idmapped and they want to create another
> idmapped mount from it they must have cap_sys_admin in the userns that
> the mount is currrently marked with. It is also not possible to change
> an idmapped mount once it has been idmapped, i.e. the user must create a
> new detached bind-mount first.

Yeah I spent quite some time last night trying to figure out the scenario
you were presenting, but I failed.  Andy, could you either rephrase or
give a more concrete end to end attack scenario?