On Wed, Jan 17, 2018 at 8:31 PM, Seth Forshee <seth.forshee@xxxxxxxxxxxxx> wrote: > 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. I see. Thanks for the explanation!
