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

[Mark Nelson]

Hi Paul and Eric,

Do you guys have any objections to dropping the hijack_pid() and
hijack_cgroup() parts of sys_hijack, leaving just hijack_ns() (see
below for discussion)?


Thanks!

Mark.

Serge E. Hallyn wrote:
> Quoting Stephen Smalley (sds at tycho.nsa.gov):
>> 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?
> 
> I *think* the things coming out of the new container are well enough
> chosen to prevent that.  I see where you're opening up to being killed
> by a task in the target container, though.  But apart from setting a
> PF_FLAG I'm not sure how to stop that anyway.
> 
> This actually reminds me that we need a valid uid in the target
> namespace in the HIJACK_NS case.  It's not a problem right now, but
> as I was just looking at fixing up kernel/signal.c in light of user
> namespaces, it is something to keep in mind.
> 
>>> 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 you're saying implement only the HIJACK_NS?
> 
> I'm fine with that.  Does anyone on the containers list object?
> 
>>> 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.
> 
> That sounds good to me...
> 
> So we're looking at:
> 
> 	1. drop HIJACK_PID and HIJACK_CGROUP
> 
> 	2. have selinux_task_alloc_security() always set task->security
> 	to current->security and allow the hijack case.
> 
> thanks,
> -serge
> 

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