ioctl CAP_LINUX_IMMUTABLE is checked in the wrong namespace

[Serge Hallyn]

Quoting Andy Lutomirski (luto at amacapital.net):
> On Tue, Apr 29, 2014 at 5:21 PM, Serge Hallyn <serge.hallyn at ubuntu.com> wrote:
> > Quoting Andy Lutomirski (luto at amacapital.net):
> >> > It should be a nonissue so long as we make sure that a file owned by a
> >> > uid outside the scope of the container may not be changed even though
> >> > fs_owner_uid is set. Otherwise, it's just a matter of chmod +S on say
> >> > a shell and anyone who can see the fs from the host will be getting a
> >> > root shell (assuming said file is owned by the host's uid 0).
> >>
> >> I feel like that's too fragile.  I'd rather add a rule that one of
> >
> > yeah I don't wnat to rush something like that.  I'd rather stash
> > the userns of the task which did the mounting and check against
> > that.  Note that would make it worthless unless and until we allowed
> > mounting from non-init userns, but then we can only claim "our fs
> > superblock readers suck and therefore containers can't mount an fs"
> > so long before we start to feel some shame and audit them...
> >
> >> these filesystems always acts like it's nosuid unless you're inside a
> >> user namespace that matches fs_owner_uid.
> >>
> >> Maybe even that is too weird.  How about setuid, setgid, and fcaps
> >> only work on mounts that are in mount namespaces that are owned by the
> >> current user namespace or one of its parents?  IOW, a struct mount is
> >> only trusted if mnt->mnt_ns->user_ns == current user ns or one of its
> >> parents?
> >>
> >> Untrusted mounts would act like they are nosuid,nodev.  Someone can
> >> try to figure out a safe way to relax nodev at some point.
> 
> Do you like this variant?  We could add a way for global root to mount
> an fs on behalf of a userns.  I'd rather this be more explicit than
> just mounting it in a mount ns owned by the user namespace, though.

I'm missing something.  Which mnt are you talking about?  A user
can just clone a new userns and then clone(CLONE_NEWNS) to get a set
of mounts owned by himself...  We need to get a mnt (or a cred or
straight to a userns) tied to the first mount of the superblock, istm.

-serge