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

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

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.

-serge