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

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Date:	Thu, 17 Apr 2014 09:55:08 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	Simo Sorce <ssorce@...hat.com>
Cc:	Daniel J Walsh <dwalsh@...hat.com>,
	Vivek Goyal <vgoyal@...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 9:48 AM, Simo Sorce <ssorce@...hat.com> wrote:
> On Thu, 2014-04-17 at 09:37 -0700, Andy Lutomirski wrote:
>> On Thu, Apr 17, 2014 at 9:24 AM, Simo Sorce <ssorce@...hat.com> wrote:
>> > On Thu, 2014-04-17 at 09:11 -0700, Andy Lutomirski wrote:
>> >>
>> >> No.  The logging daemon thinks it wants to know who the writer is, but
>> >> the logging daemon is wrong.  It actually wants to know who composed a
>> >> log message destined to it.  The caller of write(2) may or may not be
>> >> the same entity.
>> >
>> > This works both ways, and doesn't really matter, you are *no* better off
>> > w/o this interface.
>> >
>> >> If this form of SO_PASSCGROUP somehow makes it into a pull request for
>> >> Linus, I will ask him not to pull it and/or to revert it.  I think
>> >> he'll agree that write(2) MUST NOT care who called it.
>> >
>> > And write() does not, there is no access control check being performed
>> > here. This call is the same as getting the pid of the process and
>> > crawling /proc with that information, just more efficient and race-free.
>>
>> Doing it using the pid of writer is wrong.  So is doing it with the
>> cgroup of the writer.  The fact that it's even possible to use the pid
>> of the caller of write(2) is a mistake, but that particular mistake
>> is, unfortunately, well-enshrined in history.
>>
>> >
>> > I repeat, it is *not* access control.
>> >
>>
>> Sure it is.
>>
>> Either correct attribution of logs doesn't matter, in which case it
>> makes little difference how you do it, or it does matter, in which
>> case it should be done right.
>
> Well journald can *also* get  SO_PEERCGROUP and log anomalies if the 2
> differ. That is if the log happens on a connected socket.
>
> If the log happens on a unix datagram* then SO_PEERCGROUP is not
> available because there is no connect(), however write() cannot be used
> either, only sendmsg() AFAIK, so the "setuid" binary attack does not
> apply.
>

Or you could only send SCM_CGROUP when the writer asks sendmsg to send
it, in which case this whole problem goes away.

--Andy
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