linux-kernel - Re: [patch 0/8] unprivileged mount syscall

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Message-Id: <E1HdBVc-0005pL-00@dorka.pomaz.szeredi.hu>
Date:	Sun, 15 Apr 2007 22:39:40 +0200
From:	Miklos Szeredi <miklos@...redi.hu>
To:	serue@...ibm.com
CC:	miklos@...redi.hu, serue@...ibm.com, linuxram@...ibm.com,
	akpm@...ux-foundation.org, linux-fsdevel@...r.kernel.org,
	containers@...ts.osdl.org, util-linux-ng@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [patch 0/8] unprivileged mount syscall

> > > 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.
> 
> In what I was thinking of, /share/$USER is bind mounted to
> ~$USER/share, so it would have to be done in a private namespace in
> order for deluser to not be tricked.

But /share/$USER is surely not bind mounted to ~$USER/share in the
_global_ namespace, is it?  I can't see any sense in that.

> > 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
> 
> But it *can* be, if properly set up.  That's part of the point of the
> example in the OLS paper.  When a user logs in, sshd clones a new
> namespace, then bind-mounts /share/$USER into ~$USER/share.  So assuming
> that /share/$USER was --make-shared'd, it and ~$USER are now in the
> same peer group, and any changes made by the user under ~$USER will
> be reflected back into /share/$USER.

I acknowledge, that it can be done.  My point was that it can be done
more simply _without_ using CLONE_NS.

> > Wanting a persistent namespace contradicts 1).
> 
> Not necessarily, see above.
> 
> > 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.
> 
> Again, I *think* you are wrong that private namespace contradicts this
> requirement.

I'm not saying there's any contradiction, I'm saying rbind+chroot is a
better fit.

I haven't yet heard a single reason why a per-session namespace with
parts shared per-user is better than just a per-user namespace.

Miklos
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