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

[Simo Sorce <ssorce@redhat.com>]

On Thu, 2014-04-17 at 09:37 -0700, Andy Lutomirski wrote:
> On Thu, Apr 17, 2014 at 9:24 AM, Simo Sorce <ssorce@redhat.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.

> Here's a real world example from my industry.  Suppose I used journald
> for logging on a production trading system.  The ability to write a
> log line that says "I just bought 100000 EUR/USD for
> such-and-such-price" attributed to a trading program is absolutely a
> security-sensitive operation and must be subject to access control.

Eh well if SCM_CREDNTIALS passed the euid you'd se a different user in
the logs from the one that is supposed to be writing the log ... but
that send the real uid instead oups ... but I think the point is moot
for logs, given the previous explanation.

> If Common Criteria doesn't say that audit logs need to be resistant to
> spoofing, then that's just one more reason that Common Criteria is
> broken.

IT does say a lot about audit logs, but journald is not classified as an
audit log under CC, and I am not sure it can ever be.

> I don't use journald for trading logs, and I'd be absolutely daft to
> use ordinary syslog, because ordinary syslog doesn't even pretend to
> be secure.

Right.

>   But if you're going to design something that claims to be
> secure, "well, I can't see how this issue would be exploited" is not
> good enough.

Did anyone claim the journal is secure to the level you claim it should
be. Regardless, systemd can be that secure if it uses also SO_PEERCGROUP
in the vulnerable case (when you have a connected socket).


Simo.


* I think this is the case that matters for journald