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

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Date:	Thu, 17 Apr 2014 11:05:01 -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 10:47 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.
>
> 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...).

Credentials and pid have much the same problem because SCM_CREDENTIALS
is screwed up.  That's not an excuse to screw up SCM_CGROUP in the
same way.

--Andy
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