[PATCH v4 18/21] fuse: Add support for pid namespaces

[Seth Forshee]

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 at canonical.com> 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