[PATCH 08/11] fuse: Support fuse filesystems outside of init_user_ns

[Seth Forshee]

On Wed, Jan 17, 2018 at 11:59:06AM +0100, Alban Crequy wrote:
> [Adding Tejun, David, Tom for question about cuse]
> 
> On Fri, Dec 22, 2017 at 3:32 PM, Dongsu Park <dongsu at kinvolk.io> wrote:
> > From: Seth Forshee <seth.forshee at canonical.com>
> >
> > In order to support mounts from namespaces other than
> > init_user_ns, fuse must translate uids and gids to/from the
> > userns of the process servicing requests on /dev/fuse. This
> > patch does that, with a couple of restrictions on the namespace:
> >
> >  - The userns for the fuse connection is fixed to the namespace
> >    from which /dev/fuse is opened.
> >
> >  - The namespace must be the same as s_user_ns.
> >
> > These restrictions simplify the implementation by avoiding the
> > need to pass around userns references and by allowing fuse to
> > rely on the checks in inode_change_ok for ownership changes.
> > Either restriction could be relaxed in the future if needed.
> >
> > For cuse the namespace used for the connection is also simply
> > current_user_ns() at the time /dev/cuse is opened.
> 
> Was a use case discussed for using cuse in a new unprivileged userns?
> 
> I ran some tests yesterday with cusexmp [1] and I could add a new char
> device as an unprivileged user with:
> 
> $ unshare -U -r -m sh -c 'mount --bind /mnt/cuse /dev/cuse ; cusexmp
> --maj=99 --min=30 --name=foo
> 
> where /mnt/cuse is previously mknod'ed correctly and chmod'ed 777.
> Then, I could see the new device:
> 
> $ cat /proc/devices | grep foo
>  99 foo
> 
> On normal distros, we don't have a /mnt/cuse chmod'ed 777 but still it
> seems dangerous if the dev node can be provided otherwise and if we
> don't have a use case for it.
> 
> Thoughts?

I can't remember the specific reasons, but I had concluded that letting
unprivileged users use cuse within a user namespace isn't safe. But
having a cuse device node usable by regular users at all is equally
unsafe I suspect, so I don't think your example demonstrates any problem
specific to user namespaces. There shouldn't be any way to use a user
namespace to gain access permissions towards /dev/cuse, otherwise we
have bigger problems than cuse to worry about.

Seth