Re: [PATCH] fs: Treat non-ancestor-namespace mounts as MNT_NOSUID

["Serge E. Hallyn" <serge@hallyn.com>]

Quoting Andy Lutomirski (luto@amacapital.net):
> On Tue, Oct 14, 2014 at 3:14 PM, Serge E. Hallyn <serge@hallyn.com> wrote:
> > Quoting Serge E. Hallyn (serge@hallyn.com):
> >> Quoting Eric W. Biederman (ebiederm@xmission.com):
> >> > Andy Lutomirski <luto@amacapital.net> writes:
> >> >
> >> > > If a process gets access to a mount from a descendent or unrelated
> >> > > user namespace, that process should not be able to take advantage of
> >> > > setuid files or selinux entrypoints from that filesystem.
> >> > >
> >> > > This will make it safer to allow more complex filesystems to be
> >> > > mounted in non-root user namespaces.
> >> > >
> >> > > This does not remove the need for MNT_LOCK_NOSUID.  The setuid,
> >> > > setgid, and file capability bits can no longer be abused if code in
> >> > > a user namespace were to clear nosuid on an untrusted filesystem,
> >> > > but this patch, by itself, is insufficient to protect the system
> >> > > from abuse of files that, when execed, would increase MAC privilege.
> >> > >
> >> > > As a more concrete explanation, any task that can manipulate a
> >> > > vfsmount associated with a given user namespace already has
> >> > > capabilities in that namespace and all of its descendents.  If they
> >> > > can cause a malicious setuid, setgid, or file-caps executable to
> >> > > appear in that mount, then that executable will only allow them to
> >> > > elevate privileges in exactly the set of namespaces in which they
> >> > > are already privileges.
> >> > >
> >> > > On the other hand, if they can cause a malicious executable to
> >> > > appear with a dangerous MAC label, running it could change the
> >> > > caller's security context in a way that should not have been
> >> > > possible, even inside the namespace in which the task is confined.
> >> >
> >> > As presented this is complete and total nonsense.  Mount propgation
> >> > strongly weakens if not completely breaks the assumptions you are making
> >> > in this code.
> >> >
> >> > To write any generic code that knows anything we need to capture a user
> >> > namespace on struct super.
> >> >
> >> > Further I think all we really want is to filter out security labels from
> >> > unprivileged mounts.   uids/gids and the like should be completely fine
> >> > because of the uid mappings.
> >> >
> >> > Having been down the route of comparing uids as userns uid tuples I am
> >> > convinced that anything requires us to take the user namespace into
> >> > account on a routine basis in the core will simply be broken for someone
> >> > forgetting somewhere.  This looks like a design that has that kind of
> >> > susceptibility.
> >>
> >> The above paragraph is very compelling.  However Andy's patch is a step
> >> in the right direction from what we've got.  I think given what you say
> >> below and given Andy's rationale above, simply tweaking his patch to
> >> ignore the parent-userns loop, and return false if current_user_ns() !=
> >> mount_userns, should be right?  It'll prevent a child userns from
> >> setting a selinux/apparmor entrypoint or POSIX file capabilities on a
> >> file and having the parent userns trip over those.
> >
> > Ok, Andy's fn does the opposite, which will protect the parent userns,
> > which is good.
> >
> > I suspect simply insisting that the user_ns's be equal is still better.
> > It fits better with the idea that POSIX caps (and LSM entrypoints) are
> > orthogonal to DAC.  Kinda.
> 
> We could tighten it even further if we compared *mount* namespaces
> instead of user namespaces.  That would benefit Docker, non-userns-lxc
> and such, too (sigh).
> 
> Actually, I see to good reason to insist on userns equality but not on
> mountns equality.  If we're not going to trust executables in foreign
> namespaces, let's go all the way to distrust executables in all
> foreign namespaces, at least unless someone thinks of a reason this
> would break existing userspace.

I have no doubt there is code out there in production which ends up
executing /proc/pid/root/sbin/ifconfig etc.  Cause, you know, you really
wanna execute whatever garbage is there...  Breaking that might be a
good thing.

-serge