Re: [patch 0/8] unprivileged mount syscall

Ram Pai
2007-04-16T08:18:29Z

On Fri, 2007-04-13 at 16:05 +0200, Miklos Szeredi wrote:
> > > Thinking a bit more about this, I'm quite sure most users wouldn't
> > > even want private namespaces. It would be enough to
> > >
> > > chroot /share/$USER
> > >
> > > and be done with it.
> > >
> > > Private namespaces are only good for keeping a bunch of mounts
> > > referenced by a group of processes. But my guess is, that the natural
> > > behavior for users is to see a persistent set of mounts.
> > >
> > > If for example they mount something on a remote machine, then log out
> > > from the ssh session and later log back in, they would want to see
> > > their previous mount still there.
> > >
> > > Miklos
> >
> > Agreed on desired behavior, but not on chroot sufficing. It actually
> > sounds like you want exactly what was outlined in the OLS paper.
> >
> > Users still need to be in a different mounts namespace from the admin
> > user so long as we consider the deluser and backup problems
>
> I don't think it matters, because /share/$USER duplicates a part or
> the whole of the user's namespace.
>
> So backup would have to be taught about /share anyway, and deluser
> operates on /home/$USER and not on /share/*, so there shouldn't be any
> problem.
>
> There's actually very little difference between rbind+chroot, and
> CLONE_NEWNS. In a private namespace:
>
> 1) when no more processes reference the namespace, the tree will be
> disbanded
>
> 2) the mount tree won't be accessible from outside the namespace
>
> Wanting a persistent namespace contradicts 1).
>
> Wanting a per-user (as opposed to per-session) namespace contradicts
> 2). The namespace _has_ to be accessible from outside, so that a new
> session can access/copy it.

As i mentioned in the previous mail, disbanding all the namespaces of a
user will not disband his mount tree, because a mirror of the mount tree
still continues to exist in /share/$USER in the admin namespace.

And a new user session can always use this copy to create a namespace
that looks identical to that which existed earlier.

>
> So both requirements point to the rbind/chroot solution.

Arn't there ways to escape chroot jails? Serge had pointed me to a URL
which showed chroots can be escaped. And if that is true than having all
user's private mount tree in the same namespace can be a security issue?

RP

>
> Miklos