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

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Message-ID: <20120330123122.GB2024@moon>
Date:	Fri, 30 Mar 2012 16:31:22 +0400
From:	Cyrill Gorcunov <gorcunov@...nvz.org>
To:	"Serge E. Hallyn" <serge@...lyn.com>
Cc:	Serge Hallyn <serge.hallyn@...onical.com>,
	Oleg Nesterov <oleg@...hat.com>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Pavel Emelyanov <xemul@...allels.com>
Subject: Re: [rfc] fcntl: Add F_GETOWNER_UIDS option

On Thu, Mar 29, 2012 at 02:30:53AM +0000, Serge E. Hallyn wrote:
> Quoting Cyrill Gorcunov (gorcunov@...nvz.org):
> > On Wed, Mar 28, 2012 at 04:30:44PM -0500, Serge Hallyn wrote:
> > > Quoting Oleg Nesterov (oleg@...hat.com):
> > > > On 03/28, Serge E. Hallyn wrote:
> > > > >
> > > > > If you want to
> > > > > just add the struct cred to the f_owner and do proper uid conversion,
> > > > > I'll support that too.  (Just grab a ref to the cred in
> > > > > fs/fcntl.c:f_modown(), and drop the ref in fs/file_table.c:__fput() ).
> > > > 
> > > > In this case f_owner.*uid should go away, I guess.
> > > 
> > > Yup.
> > > 
> > > Which I guess is all the more reason *not* to do this unless we end up
> > > not going with Eric's userns mapping patchset (which is unlikely).
> > > 
> > > > And sigio_perm()
> > > > should be unified with kill_ok_by_cred() somehow (modulo
> > > > security_file_send_sigiotask).
> > > > 
> > > > Right?
> > > 
> > > Maybe, but other differences include current being the signal sender in
> > > one and recipient in the other, and CAP_KILL being relevent in only
> > > one.
> > 
> > Hi Serge, thanks a lot for comments! Replying to prev email --
> > I've skipped cred part intentionally, I guess we need to wait
> > until Eric's patches hit LKML (if I understand all right) then
> > I'll expand the patch. I'll think a bit more tomorrow, ok?
> 
> Sure.
> 
> Thinking about it, the cred being stored right now is the cred in the
> container.  That's what you want for checkpoint, right?  So if someone

Hi Serge, sorry for delay, the stored creds are the ones a task has
at checkpoint time (we parse /proc/pid/status), and the dumper/restorer
works with root privileges so they should be able to change creds to
the former values on restore procedure.

> with the privs to do it checkpoints a task in a child userns, and restarts
> that without doing so in a child user ns, he should be allowed to do so.

I think so. Basically we require both checkpointer and restorer
to have admin rights before they do c/r (it might be relaxed in
future probably) and actually I think we're more oriented to
achieve stable c/r from init-namespace first (once this accomplished
then c/r from inside nested namespaces could be considered).

> So what I'm saying is that it's not in-defensible to just not change
> anything in your original patch until we can discuss Eric's set.
> 

Yes, I wanna take a look on Eric's set first just to get right
"picture" of everything. And I wanted to find a minimal solution
with current kernel code base which could be extended in future.

That said I guess the current init-ns-only approach should do the
trick for a while. And (thanks for pointing) I need to add a test
if a caller which tries to obtain uids has enought credentials
for that (probably CAP_FOWNER), right?

> If we were to *not* go with Eric's set, then when using your proposed
> patch for debugging purposes, would we want to show a list of uids,
> starting with the uid in the reader's user namespace, up to the
> container being investigated?  So for instance if init_user_ns spawned
> userns1, and that spawned userns2, and root in userns1 is seeking this
> info for a f_owner in userns2, then he should see two userids, the one
> mapped into usern1, and the one in userns2.
> 
> In Eric's set, we may want to show only the kuid (since the mapped
> userid can be found other ways), or for convenience we may want to show
> both the kuid and the mapped uid.

I suspect operating with kuid's will be a way more easier.

	Cyrill
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