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 _______________________________________________ Containers mailing list Containers@xxxxxxxxxxxxxxxxxxxxxxxxxx https://lists.linuxfoundation.org/mailman/listinfo/containers
