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

[Seth Forshee]

On Wed, Jan 17, 2018 at 07:56:59PM +0100, Alban Crequy wrote:
> On Wed, Jan 17, 2018 at 3:29 PM, Seth Forshee
> <seth.forshee@xxxxxxxxxxxxx> wrote:
> > 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@xxxxxxxxxx> wrote:
> >> > From: Seth Forshee <seth.forshee@xxxxxxxxxxxxx>
> >> >
> >> > 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,
> 
> This makes sense.
> 
> > 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.
> 
> From my tests, the patch seem safe but I don't fully understand why that is.
> 
> I am not trying to gain more permissions towards /dev/cuse but to
> create another cuse char file from within the unprivileged userns. I
> tested the scenario by patching the memfs userspace FUSE driver to
> generate the char device whenever the file is named "cuse" (turning
> the regular file into a char device with the cuse major/minor behind
> the scene):
> 
> $ unshare -U -r -m
> # memfs /mnt/memfs &
> # ls -l /mnt/memfs
> # echo -n > /mnt/memfs/cuse
> -bash: /mnt/memfs/cuse: Input/output error
> # ls -l /mnt/memfs/cuse
> crwxrwxrwx. 1 root root 10, 203 Jan 17 18:24 /mnt/memfs/cuse
> # cat /mnt/memfs/cuse
> cat: /mnt/memfs/cuse: Permission denied
> 
> But then, I could not use that char device, even though it seems to
> have the correct major/minor and permissions. The kernel FUSE code
> seems to call init_special_inode() to handle character devices. I
> don't understand why it seems to be safe.

Because for new mounts in non-init user namespaces alloc_super() sets
SB_I_NODEV flag in s_iflags, which disallows opening device nodes in
that filesystem.

Seth