netdev - Re: [PATCH 2/2] net: Implement SO_PASSCGROUP to enable passing cgroup path

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Date:	Thu, 17 Apr 2014 13:47:01 -0400
From:	Simo Sorce <ssorce@...hat.com>
To:	Andy Lutomirski <luto@...capital.net>
Cc:	Vivek Goyal <vgoyal@...hat.com>,
	Daniel J Walsh <dwalsh@...hat.com>,
	David Miller <davem@...emloft.net>, Tejun Heo <tj@...nel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	lpoetter@...hat.com, cgroups@...r.kernel.org, kay@...hat.com,
	Network Development <netdev@...r.kernel.org>
Subject: Re: [PATCH 2/2] net: Implement SO_PASSCGROUP to enable passing
 cgroup path

On Thu, 2014-04-17 at 10:35 -0700, Andy Lutomirski wrote:
> On Thu, Apr 17, 2014 at 10:33 AM, Simo Sorce <ssorce@...hat.com> wrote:
> > On Thu, 2014-04-17 at 10:26 -0700, Andy Lutomirski wrote:
> >>
> >> Not really.  write(2) can't send SCM_CGROUP.  Callers of sendmsg(2)
> >> who supply SCM_CGROUP are explicitly indicating that they want their
> >> cgroup associated with that message.  Callers of write(2) and send(2)
> >> are simply indicating that they have some bytes that they want to
> >> shove into whatever's at the other end of the fd.
> >
> > But there is no attack vector that passes by tricking setuid binaries to
> > write to pre-opened file descriptors on sendmsg(), and for the other
> > cases (connected socket) journald can always cross check with
> > SO_PEERCGROUP, so why do we care again ?
> 
> Because the proposed code does not do what I described, at least as
> far I as I can tell.

You do realize that we have been speaking in hypothetical for a while
now ?

Even without doing the SO_PEERCRED, you are not going to fool the log,
as it gathers a ton of other info about the process, and cgroup is just
one of the infos used to classify the log.

There are also credentials, pid, and a lot of other things.
Even if a setuid binary could be tricked to send a message with an
"impostor" cgroup don't you think you'd see other things out of place ?
(wrong uid, wrong pid, etc...).

What I am telling you is that userspace has all the tools it needs to
not get fooled, as long as cgroup information retrieved via
SO_PASSCGROUP is not uniquely used to authenticate a peer process for
connected sockets.

Simo.

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