From: Simo Sorce Subject: Re: [PATCH 2/2] net: Implement SO_PASSCGROUP to enable passing cgroup path Date: Thu, 17 Apr 2014 13:33:45 -0400 Message-ID: <1397756025.2628.64.camel@willson.li.ssimo.org> 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> Mime-Version: 1.0 Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: Sender: cgroups-owner-u79uwXL29TY76Z2rM5mHXA@public.gmane.org List-ID: Content-Type: text/plain; charset="us-ascii" To: Andy Lutomirski Cc: Vivek Goyal , Daniel J Walsh , David Miller , Tejun Heo , "linux-kernel-u79uwXL29TY76Z2rM5mHXA@public.gmane.org" , lpoetter-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org, cgroups-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, kay-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org, Network Development 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. 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 ? Simo.