linux-kernel - Re: [rfc] fcntl: Add F_GETOWNER

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Date:	Tue, 27 Mar 2012 22:46:40 +0000
From:	"Serge E. Hallyn" <serge@...lyn.com>
To:	Cyrill Gorcunov <gorcunov@...nvz.org>
Cc:	"Serge E. Hallyn" <serge@...lyn.com>,
	Oleg Nesterov <oleg@...hat.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Pavel Emelyanov <xemul@...allels.com>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	"Serge E. Hallyn" <serge.hallyn@...onical.com>
Subject: Re: [rfc] fcntl: Add F_GETOWNER_UIDS option

Quoting Cyrill Gorcunov (gorcunov@...nvz.org):
> On Tue, Mar 27, 2012 at 10:29:23PM +0000, Serge E. Hallyn wrote:
> > Quoting Cyrill Gorcunov (gorcunov@...nvz.org):
> > > On Tue, Mar 27, 2012 at 05:25:34PM +0200, Oleg Nesterov wrote:
> > > > user_ns_map_uid() should translate uid_t from one namespace to another,
> > > > in this case the namespace is the same.
> > > > 
> > > > user_ns_map_uid(cred->user_ns, cred) must be the identical mapping,
> > > > no matter how we change the implementation.
> > > > 
> > > > What I think you need is
> > > > user_ns_map_uid(current_user_ns(), filp->f_owner.cred), the only
> > > > problem is that f_owner.cred doesn't exist.
> > > > 
> > > 
> > > Hmm, I was confused by likely() in user_ns_map_uid. But indeed, I think
> > > you're so right. Is there some reason why we can't carry f_owner.cred
> > > pointer?
> > 
> > We would need that for this, yes.  However, Eric is working on a new
> > patchset which changes the cross-userns uid mappings.  I think it's
> > worth simply leaving a comment that this will need to be addressed,
> > and leave in the unconverted uid.
> 
> Hi Serge, thanks for info. But if it will be unconverted uid, can't
> be there some security problem with that which I missed?

Noone is really using the user namespaces right now, but rather than
adding the cred (and refcounting concerns), my suggestion for now
would be to hardcode a check in modown() that current_user_ns() ==
&init_user_ns.

I *did* have a patch in the past which added the cred to fown, but
no idea where it is right now...

-serge
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