[patch 0/8] unprivileged mount syscall

Ram Pai linuxram at us.ibm.com
Mon Apr 16 01:18:29 PDT 2007

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?


> Miklos

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