Subject: Re: [PATCH 2/2] net: Implement SO_PEERCGROUP

From: Vivek Goyal <[email-masked]>
Date: Thu, 13 Mar 2014 10:14:22 -0400

On Wed, Mar 12, 2014 at 02:12:33PM -0700, Andy Lutomirski wrote:
> On Wed, Mar 12, 2014 at 2:00 PM, Andy Lutomirski <[email-masked]> wrote:
> > On 03/12/2014 01:46 PM, Vivek Goyal wrote:
> >> Implement SO_PEERCGROUP along the lines of SO_PEERCRED. This returns the
> >> cgroup of first mounted hierarchy of the task. For the case of client,
> >> it represents the cgroup of client at the time of opening the connection.
> >> After that client cgroup might change.
> >
> > Even if people decide that sending cgroups over a unix socket is a good
> > idea, this API has my NAK in the strongest possible sense, for whatever
> > my NAK is worth.
> >
> > IMO SO_PEERCRED is a disaster.  Calling send(2) or write(2) should
> > *never* imply the use of a credential.  A program should always have to
> > *explicitly* request use of a credential.  What you want is SCM_CGROUP.
> >
> > (I've found privilege escalations before based on this observation, and
> > I suspect I'll find them again.)
> >
> >
> > Note that I think that you really want SCM_SOMETHING_ELSE and not
> > SCM_CGROUP, but I don't know what the use case is yet.
> 
> This might not be quite as awful as I thought.  At least you're
> looking up the cgroup at connection time instead of at send time.
> 
> OTOH, this is still racy -- the socket could easily outlive the cgroup
> that created it.

That's a good point. What guarantees that previous cgroup was not
reassigned to a different container.

What if a process A opens the connection with sssd. Process A passes the
file descriptor to a different process B in a differnt container.
Process A exits. Container gets removed from system and new one gets
launched which uses same cgroup as old one. Now process B sends a new
request and SSSD will serve it based on policy of newly launched
container.

This sounds very similar to pid race where socket/connection will outlive
the pid.

Thanks
Vivek