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

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Date:	Thu, 17 Apr 2014 11:04:08 -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 10:52 AM, Simo Sorce <ssorce@...hat.com> wrote:
> On Thu, 2014-04-17 at 10:26 -0700, Andy Lutomirski wrote:
>> On Thu, Apr 17, 2014 at 10:12 AM, Vivek Goyal <vgoyal@...hat.com> wrote:
>> > On Thu, Apr 17, 2014 at 09:55:08AM -0700, Andy Lutomirski wrote:
>> >> 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.
>> >
>> > Sending SCM_CGROUP explicitly is also sending cgroup info at write(2) time
>> > and if receiver uses that info for access control, it can be problematic.
>> >
>>
>> 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.
>
> So you are telling me that you want to change all code that writes to
> stderr to be changed to use sendmsg() instead of write() in order to get
> that information ?

No.  I'm telling you that I want whoever writes the logging code to
change *the logging code* to use sendmsg.

>
> If you are using datagram sockets then the sender explicitly has to use
> sendmsg() already and if a setuid binary can be convinced to send
> arbitrary data to an arbitrary datagram sokcet you have bigger problems
> in that binary, and said binary will send you whatever cgroup it is in.

Really?

--Andy
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