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

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Date:	Thu, 17 Apr 2014 11:33:58 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	Simo Sorce <ssorce@...hat.com>
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, Apr 17, 2014 at 11:23 AM, Simo Sorce <ssorce@...hat.com> 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.

It seems reasonable to me to have the sender give a blank SCM_CGROUP
and to let the kernel populate it.

I'm still a bit vague on what happens if the sender is in two
different cgroups.  Which one wins?  Presumably the unified hierarchy
one if one of them is in use, but I haven't kept track of all the
changes there.

--Andy
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