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

[Stefan Hajnoczi]

On Fri, Jul 31, 2020 at 10:39:37AM +0200, Roman Mohr wrote:
> On Fri, Jul 31, 2020 at 10:26 AM Stefan Hajnoczi <stefanha@redhat.com>
> wrote:
> 
> > On Thu, Jul 30, 2020 at 06:21:34PM -0400, Daniel Walsh wrote:
> > > On 7/29/20 10:40, Stefan Hajnoczi wrote:
> > > > On Wed, Jul 29, 2020 at 09:59:01AM +0200, Roman Mohr wrote:
> > > >> On Tue, Jul 28, 2020 at 3:13 PM Vivek Goyal <vgoyal@redhat.com>
> > 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
> > > >> Yes they can run as root. I can tell you what we plan to do with the
> > > >> containerized virtiofsd: We run it as part of the user-owned pod (a
> > set of
> > > >> containers).
> > > >> One of our main goals at the moment is to run VMs in a user-owned pod
> > > >> without additional privileges.
> > > >> So that in case the user (VM-creator/owner) enters the pod or
> > something
> > > >> breaks out of the VM they are just in the unprivileged container
> > sandbox.
> > > >> As part of that we try to get also rid of running containers in the
> > > >> user-context with the root user.
> > > >>
> > > >> One possible scenario which I could think of as being desirable from a
> > > >> kubevirt perspective:
> > > >> We would run the VM in one container and have an unprivileged
> > > >> virtiofsd container in parallel.
> > > >> This container already has its own mount namespace and it is not that
> > > >> critical if something manages to enter this sandbox.
> > > >>
> > > >> But we are not as far yet as getting completely rid of root right now
> > in
> > > >> kubevirt, so if as a temporary step it needs root, the current
> > proposed
> > > >> changes would still be very useful for us.
> > > > What is the issue with root in user namespaces?
> > > >
> > > > I remember a few years ago it was seen as a major security issue but
> > > > don't remember if container runtimes were already using user namespaces
> > > > back then.
> > > >
> > > > I guess the goal might be simply to minimize Linux capabilities as much
> > > > as possible?
> > > >
> > > > virtiofsd could nominally run with an arbitrary uid/gid but it still
> > > > needs the Linux capabilities that allow it to change uid/gid and
> > > > override file system permission checks just like the root user. Not
> > sure
> > > > if there is any advantage to running with uid 1000 when you still have
> > > > these Linux capabilities.
> > > >
> > > > Stefan
> > >
> > > When you run in a user namespace, virtiofsd would only have
> > > setuid/setgid over the range of UIDs mapped into the user namespace.  So
> > > if UID=0 on the host is not mapped, then the container can not create
> > > real UID=0 files on disk.
> > >
> > > Similarly you can protect the user directories and any content by
> > > running the containers in a really high UID Mapping.
> >
> > Roman, do user namespaces address your concerns about uid 0 in
> > containers?
> >
> 
> They may eventually solve it. I would not let us hang up on this right now,
> since as said at least in kubevirt we can't get rid right now of root
> anyway.
> Even if it is at some point in the future save and supported on
> bleeding-edge managed k8s clusters to allow ordinary users to run with uid
> 0, from my perspective it is right now common to restrict namespaces with
> PodSecurityPolicies or SecurityContexts to not allow running pods as root
> for normal users.
> It is also common that a significant part of the community users run docker
> and/or run on managed k8s clusters where they can not influence if
> user-namespaces are enabled, if they can run pods as root, if the runtime
> points to a seccomp file they like or if the runtime they prefer is used.
> 
> But let me repeat again that we require root right now anyway and that we
> don't run the pods right now with the user privileges (but we should and we
> aim for that). Right now PSPs and SCCs restrict access to these pods by the
> users.
> So for our use case, at this exact moment root is acceptable, the unshare
> call is a little bit more problematic.

Okay, thanks for explaining.

Stefan

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