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Date: Wed, 6 Jan 2016 03:07:59 +0000
From: Serge Hallyn <serge.hallyn@...ntu.com>
To: oss-security@...ts.openwall.com
Cc: cve-assign@...re.org, john.johansen@...onical.com
Subject: Re: Re: CVE Request: Linux kernel: privilege
 escalation in user namespaces

Quoting Eric W. Biederman (ebiederm@...ssion.com):
> cve-assign@...re.org writes:
> 
> > Use CVE-2015-8709 for the issue fixed in the
> > https://lkml.org/lkml/2015/12/25/71 post.
> >
> > (This is not yet available at
> > http://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/log/kernel/ptrace.c
> > and http://marc.info/?l=linux-kernel&m=145118185526359 might be the
> > current end of the earlier discussion.)
> >
> > This issue has been covered in security advisories from one or more
> > Linux distributions, e.g.,
> >
> >>> http://www.ubuntu.com/usn/usn-2847-1
> >>> 
> >>> Jann Horn discovered a ptrace issue with user namespaces in the Linux
> >>> kernel. The namespace owner could potentially exploit this flaw by ptracing
> >>> a root owned process entering the user namespace to elevate its privileges
> >>> and potentially gain access outside of the namespace.
> >>> (http://bugs.launchpad.net/bugs/1527374)
> >
> >
> > There has been some discussion of whether the finding was a
> > vulnerability discovery, e.g.,
> >
> >>>> Date: Fri, 18 Dec 2015 00:07:19 +0100
> >>>> From: Jann Horn <jann@...jh.net>
> >>>> 
> >>>> I'm not sure whether this is CVE-worthy - the user_namespaces
> >>>> manpage says "the process has full privileges for operations
> >>>> inside the user namespace, but is unprivileged for operations
> >>>> outside the namespace". ptrace()ing a process in the
> >>>> namespace can reasonably be considered an "operation inside
> >>>> the user namespace" ...
> >>>> 
> >>>> In my opinion, this patch is somewhere between hardening and
> >>>> a security feature, but I wouldn't really call it a vuln fix.
> >
> >
> >>>>> Date: Thu, 17 Dec 2015 23:54:03 +0000
> >>>>> From: Serge Hallyn <serge.hallyn@...ntu.com>
> >>>>> 
> >>>>>> ptrace()ing a process in the
> >>>>>> namespace can reasonably be considered an "operation inside
> >>>>>> the user namespace"
> >>>>> 
> >>>>> Except by creating a file in the host namespace, you were, as
> >>>>> root in the container, able to escape your namespace, right?
> >
> > We feel that, more generally, the usn-2847-1 mention of "and
> > potentially gain access outside of the namespace" is a realistic
> > concern.
> 
> My mind is boggling at some of the logic involved here.
> 
> There is no potentially gaining access outside of the namespace when it
> is access to things that were put inside the namespace.
> 
> The discussion was about how to make it easier for userspace not to do
> stupid things, not how to fix a bug in the kernel.
> 
> The code we have been discussing most definitely does not make it safe
> for a arbitrary root owned processes to call setns and enter a user
> namespace with a hostile user namespace root.  You have to close file
> descriptors, unmap files and do I don't know what else.  Properly
> and safely dropping privileges is a challenging problem.
> 
> Calling bug because it is possible to use a kernel feature wrong feels
> completely inappropriate.

I could be wrong but think you are misunderstanding the cve.

IIRC the situation was:  if you setns(some-userns); setresgid(0,0);
setresuid(0,0); then between the setns and the setuids the container
can ptrace your task and do things using the host uids.  That's bad.

You can't stop the container from messing with you in general (by
ptracing later - though as you say we could set nodump, but I don't
think people would want htat), but it shouldn't be able to mess with the
host root uid.

-serge

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