[Devel] Re: [PATCH 1/2] namespaces: introduce sys_hijack (v10)

[Stephen Smalley]

On Tue, 2007-11-27 at 16:38 -0600, Serge E. Hallyn wrote:
> Quoting Stephen Smalley (sds at tycho.nsa.gov):
> > On Tue, 2007-11-27 at 10:11 -0600, Serge E. Hallyn wrote:
> > > Quoting Crispin Cowan (crispin at crispincowan.com):
> > > > Just the name "sys_hijack" makes me concerned.
> > > > 
> > > > This post describes a bunch of "what", but doesn't tell us about "why"
> > > > we would want this. What is it for?
> > > 
> > > Please see my response to Casey's email.
> > > 
> > > > And I second Casey's concern about careful management of the privilege
> > > > required to "hijack" a process.
> > > 
> > > Absolutely.  We're definately still in RFC territory.
> > > 
> > > Note that there are currently several proposed (but no upstream) ways to
> > > accomplish entering a namespace:
> > > 
> > > 	1. bind_ns() is a new pair of syscalls proposed by Cedric.  An
> > > 	nsproxy is given an integer id.  The id can be used to enter
> > > 	an nsproxy, basically a straight current->nsproxy = target_nsproxy;
> > > 
> > > 	2. I had previously posted a patchset on top of the nsproxy
> > > 	cgroup which allowed entering a nsproxy through the ns cgroup
> > > 	interface.
> > > 
> > > There are objections to both those patchsets because simply switching a
> > > task's nsproxy using a syscall or file write in the middle of running a
> > > binary is quite unsafe.  Eric Biederman had suggested using ptrace or
> > > something like it to accomplish the goal.
> > > 
> > > Just using ptrace is however not safe either.  You are inheriting *all*
> > > of the target's context, so it shouldn't be difficult for a nefarious
> > > container/vserver admin to trick the host admin into running something
> > > which gives the container/vserver admin full access to the host.
> > 
> > I don't follow the above - with ptrace, you are controlling a process
> > already within the container (hence in theory already limited to its
> > container), and it continues to execute within that container.  What's
> > the issue there?
> 
> Hmm, yeah, I may have overspoken - I'm not good at making up exploits
> but while I see it possible to confuse the host admin by setting bogus
> environment, I guess there may not be an actual exploit.
> 
> Still after the fork induced through ptrace, we'll have to execute a
> file out of the hijacked process' namespaces and path (unless we get
> *really* 'exotic').  With hijack, execution continues under the caller's
> control, which I do much prefer.
> 
> The remaining advantages of hijack over ptrace (beside "using ptrace for
> that is crufty") are
> 
> 	1. not subject to pid wraparound (when doing hijack_cgroup
> 	   or hijack_ns)
> 	2. ability to enter a namespace which has no active processes

So possibly I'm missing something, but the situation with hijack seems
more exploitable than ptrace to me - you've created a hybrid task with
one foot in current's world (open files, tty, connection to parent,
executable) and one foot in the target's world (namespaces, uid/gid)
which can then be leveraged by other tasks within the target's
world/container as a way of breaking out of the container.  No?

> These also highlight selinux issues.  In the case of hijacking an
> empty cgroup, there is no security context (because there is no task) so
> the context of 'current' will be used.  In the case of hijacking a
> populated cgroup, a task is chosen "at random" to be the hijack source.

Seems like you might be better off with a single operation for creating
a new task within a given namespace set / cgroup rather than trying to
handle multiple situations with different semantics / inheritance
behavior.  IOW, forget about hijacking a specific pid or picking a task
at random from a populated cgroup - just always initialize the state of
the newly created task in the same manner based solely on elements of
the caller's state and the cgroup's state.

> So there are two ways to look at deciding which context to use.  Since
> control continues in the original acting process' context, we might
> want the child to continue in its context.  However if the process
> creates any objects in the virtual server, we don't want them
> mislabeled, so we might want the task in the hijacked task's context.

I suspect that we want to continue in the parent's context, and then the
program can always use setfscreatecon() or exec a helper in a different
context if it wants to create files with contexts tailored to the
target.

> Sigh.  So here's why I thought I'd punt on selinux at least until I had
> a working selinux-enforced container/vserver  :)

-- 
Stephen Smalley
National Security Agency

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