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

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Date:	Thu, 17 Apr 2014 14:50:23 -0400
From:	Vivek Goyal <vgoyal@...hat.com>
To:	Simo Sorce <ssorce@...hat.com>
Cc:	Andy Lutomirski <luto@...capital.net>,
	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, Apr 17, 2014 at 02:23:33PM -0400, Simo Sorce wrote:
> 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.
> 
> Ok let me backtrack, apparently if you explicitly use connect() on a
> datagram socket then you *can* write() (thanks to Vivek for checking
> this).
> 
> So you can trick something to write() to it but you can't do
> SO_PEERCGROUP on the other side, because it is not really a connected
> socket, the connection is only faked on the sender side by constructing
> sendmsg() messages with the original address passed into connect().
> 
> So given this unfortunate circumstance, requiring the client to
> explicitly pass cgroup data on unix datagram sockets may be an
> acceptable request IMO.
> 
> Perhaps this could be done with a sendmsg() header flag or simplified
> ancillary data even, rather than forcing the sender process to retrieve
> and construct the whole information which is already available in
> kernel.

So what would be the protocol here? When should somebody send an
SCM_CGROUP message using sendmsg()?

Thanks
Vivek
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