Re: [PATCH v2 3/3] virtiofsd: probe unshare(CLONE_FS) and print an error

[Vivek Goyal]

On Tue, Jul 28, 2020 at 03:12:54PM -0400, Daniel Walsh wrote:
> On 7/28/20 09:12, Vivek Goyal wrote:
> > On Tue, Jul 28, 2020 at 12:00:20PM +0200, Roman Mohr wrote:
> >> On Tue, Jul 28, 2020 at 3:07 AM misono.tomohiro@fujitsu.com <
> >> misono.tomohiro@fujitsu.com> wrote:
> >>
> >>>> Subject: [PATCH v2 3/3] virtiofsd: probe unshare(CLONE_FS) and print an
> >>> error
> >>>> An assertion failure is raised during request processing if
> >>>> unshare(CLONE_FS) fails. Implement a probe at startup so the problem can
> >>>> be detected right away.
> >>>>
> >>>> Unfortunately Docker/Moby does not include unshare in the seccomp.json
> >>>> list unless CAP_SYS_ADMIN is given. Other seccomp.json lists always
> >>>> include unshare (e.g. podman is unaffected):
> >>>>
> >>> https://raw.githubusercontent.com/seccomp/containers-golang/master/seccomp.json
> >>>> Use "docker run --security-opt seccomp=path/to/seccomp.json ..." if the
> >>>> default seccomp.json is missing unshare.
> >>> Hi, sorry for a bit late.
> >>>
> >>> unshare() was added to fix xattr problem:
> >>>
> >>> https://github.com/qemu/qemu/commit/bdfd66788349acc43cd3f1298718ad491663cfcc#
> >>> In theory we don't need to call unshare if xattr is disabled, but it is
> >>> hard to get to know
> >>> if xattr is enabled or disabled in fv_queue_worker(), right?
> >>>
> >>>
> >> In kubevirt we want to run virtiofsd in containers. We would already not
> >> have xattr support for e.g. overlayfs in the VM after this patch series (an
> >> acceptable con at least for us right now).
> >> If we can get rid of the unshare (and potentially of needing root) that
> >> would be great. We always assume that everything which we run in containers
> >> should work for cri-o and docker.
> > But cri-o and docker containers run as root, isn't it? (or atleast have
> > the capability to run as root). Havind said that, it will be nice to be able
> > to run virtiofsd without root. 
> >
> > There are few hurdles though.
> >
> > - For file creation, we switch uid/gid (seteuid/setegid) and that seems
> >   to require root. If we were to run unpriviliged, probably all files
> >   on host will have to be owned by unpriviliged user and guest visible
> >   uid/gid will have to be stored in xattrs. I think virtfs supports
> >   something similar.
> >
> > I am sure there are other restrictions but this probably is the biggest
> > one to overcome.
> >
> >  > 
> You should be able to run it within a user namespace with Namespaces
> capabilities.

User namespaces has always been a one TODO item. Few challenges are
how to manage uid/gid mapping for existing directories which will be
shared. We will have to pick a mapping range and then chown the
files accordingly. And chowning itself will require priviliges.

Now Stefan is adding support to run virtiofsd inside container. So
podman should be able virtiofsd inside a user namespace (And even
give CAP_SYS_ADMIN). That way we don't have to worry about setting
a user namespace and select a mapping etc. But persistent data
volumes will continue to be an issue with user namespace, right?

How are you dealing with it in podman. Containers running in 
user namespace and with volume mounts.

Vivek

> >> "Just" pointing docker to a different seccomp.json file is something which
> >> k8s users/admin in many cases can't do.
> > Or may be issue is that standard seccomp.json does not allow unshare()
> > and hence you are forced to use a non-standar seccomp.json.
> >
> > Vivek
> >
> >> Best Regards,
> >> Roman
> >>
> >>
> >>> So, it looks good to me.
> >>> Reviewed-by: Misono Tomohiro <misono.tomohiro@jp.fujitsu.com>
> >>>
> >>> Regards,
> >>> Misono
> >>>
> >>>> Cc: Misono Tomohiro <misono.tomohiro@jp.fujitsu.com>
> >>>> Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
> >>>> ---
> >>>>  tools/virtiofsd/fuse_virtio.c | 16 ++++++++++++++++
> >>>>  1 file changed, 16 insertions(+)
> >>>>
> >>>> diff --git a/tools/virtiofsd/fuse_virtio.c
> >>> b/tools/virtiofsd/fuse_virtio.c
> >>>> index 3b6d16a041..9e5537506c 100644
> >>>> --- a/tools/virtiofsd/fuse_virtio.c
> >>>> +++ b/tools/virtiofsd/fuse_virtio.c
> >>>> @@ -949,6 +949,22 @@ int virtio_session_mount(struct fuse_session *se)
> >>>>  {
> >>>>      int ret;
> >>>>
> >>>> +    /*
> >>>> +     * Test that unshare(CLONE_FS) works. fv_queue_worker() will need
> >>> it. It's
> >>>> +     * an unprivileged system call but some Docker/Moby versions are
> >>> known to
> >>>> +     * reject it via seccomp when CAP_SYS_ADMIN is not given.
> >>>> +     *
> >>>> +     * Note that the program is single-threaded here so this syscall
> >>> has no
> >>>> +     * visible effect and is safe to make.
> >>>> +     */
> >>>> +    ret = unshare(CLONE_FS);
> >>>> +    if (ret == -1 && errno == EPERM) {
> >>>> +        fuse_log(FUSE_LOG_ERR, "unshare(CLONE_FS) failed with EPERM. If
> >>> "
> >>>> +                "running in a container please check that the container
> >>> "
> >>>> +                "runtime seccomp policy allows unshare.\n");
> >>>> +        return -1;
> >>>> +    }
> >>>> +
> >>>>      ret = fv_create_listen_socket(se);
> >>>>      if (ret < 0) {
> >>>>          return ret;
> >>>> --
> >>>> 2.26.2
> >>>
>