On Tue, Jul 19, 2016 at 07:44:11PM -0700, Sheng Yang wrote: > On Tue, Apr 26, 2016 at 12:36 PM, Seth Forshee > <seth.forshee@xxxxxxxxxxxxx> wrote: > > When the userspace process servicing fuse requests is running in > > a pid namespace then pids passed via the fuse fd are not being > > translated into that process' namespace. Translation is necessary > > for the pid to be useful to that process. > > > > Since no use case currently exists for changing namespaces all > > translations can be done relative to the pid namespace in use > > when fuse_conn_init() is called. For fuse this translates to > > mount time, and for cuse this is when /dev/cuse is opened. IO for > > this connection from another namespace will return errors. > > > > Requests from processes whose pid cannot be translated into the > > target namespace are not permitted, except for requests > > allocated via fuse_get_req_nofail_nopages. For no-fail requests > > in.h.pid will be 0 if the pid translation fails. > > Hi Seth, > > This patch caused a regression in our major container use case with > FUSE in Ubuntu 16.04, as patch was checked in as Ubuntu Sauce in > Ubuntu 4.4.0-6.21 kernel. > > The use case is: > 1. Create a Docker container. > 2. Inside the container, start the FUSE backend, and mounted fs. > 3. Following step 2 in the container, create a loopback device to map > a file in the mounted fuse to create a block device, which will be > available to the whole system. > > It works well before this commit. > > The use case is broken because no matter which namespace losetup runs, > the real request from loopback device seems always come from init ns, > thus it will be in different ns running fuse backend. So the request > will got denied, because the ns running fuse won't able to see the > things from higher level(level 0 in fact) pid namespace. > > I think since init pid ns has ability to access any process in the > system, it should able to access the fuse mounted by any pid namespace > process as well. > > What you think? It sounds like we need to remove the restriction on accessing the filesystem from a different pid namespace. I don't think this poses a security problem. However there's no pid mapping that is usable by the userspace fuse process, so what do we put in the fuse request? Probably the only candidates are 0 and 0xffffffff. So a question for the fuse developers - is one value or the other preferrable for fuse_in_header.pid when the pid cannot be mapped, and is this going to cause problems for any fuse filesystems? I suspect that few filesystems actually look at the pid anyway, and already for a filesystem mounted in a pid namespace the values being given to userspace won't be correct for the namespace of the fuse process. Seth -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
