Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751554AbaDQSbg (ORCPT ); Thu, 17 Apr 2014 14:31:36 -0400 Received: from mx1.redhat.com ([209.132.183.28]:46438 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750918AbaDQSb3 (ORCPT ); Thu, 17 Apr 2014 14:31:29 -0400 Subject: Re: [PATCH 2/2] net: Implement SO_PASSCGROUP to enable passing cgroup path From: Simo Sorce To: Andy Lutomirski Cc: Vivek Goyal , Daniel J Walsh , David Miller , Tejun Heo , "linux-kernel@vger.kernel.org" , lpoetter@redhat.com, cgroups@vger.kernel.org, kay@redhat.com, Network Development In-Reply-To: References: <20140416180642.GG31074@redhat.com> <20140416185936.GJ31074@redhat.com> <534FF61B.4010901@redhat.com> <1397750674.2628.44.camel@willson.li.ssimo.org> <1397751853.2628.50.camel@willson.li.ssimo.org> <1397753323.2628.60.camel@willson.li.ssimo.org> <20140417171256.GB25334@redhat.com> <1397757169.2628.75.camel@willson.li.ssimo.org> Content-Type: text/plain; charset="UTF-8" Date: Thu, 17 Apr 2014 14:31:24 -0400 Message-ID: <1397759484.2628.93.camel@willson.li.ssimo.org> Mime-Version: 1.0 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, 2014-04-17 at 11:04 -0700, Andy Lutomirski wrote: > On Thu, Apr 17, 2014 at 10:52 AM, Simo Sorce wrote: > > On Thu, 2014-04-17 at 10:26 -0700, Andy Lutomirski wrote: > >> On Thu, Apr 17, 2014 at 10:12 AM, Vivek Goyal 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 wrote: > >> >> > On Thu, 2014-04-17 at 09:37 -0700, Andy Lutomirski wrote: > >> >> >> On Thu, Apr 17, 2014 at 9:24 AM, Simo Sorce 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? I want to retract my comment in light of the fact you can use connect() on a datagram socket and then kernel will "helpfully" then allow you to use write() on it, I forgot about this detail. For the logging q. above I wouldn't see any issue unless the journald people were planning on using connect() themselves to pie things like stderr over a datagram socket. If that were the case, well then ... OTOH, I still need to understand how you attack a setuid binary to fake a cgroup, it's not like uig/gid information that is changed by the simple fact of invoking the setuid binary, and this is a brand new contract, so it is not unreasonable to put in the security considerations that a setuid binary should take extracare to where they emit stdout/stderr message should they decide to change their cgroup ... Simo. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/