[lxc-devel] CLONE_PARENT after setns(CLONE_NEWPID)

[Andy Lutomirski]

On Wed, Nov 6, 2013 at 2:50 PM, Eric W. Biederman <ebiederm at xmission.com> wrote:
> Oleg Nesterov <oleg at redhat.com> writes:
>
>> Hi Serge,
>>
>> On 11/06, Serge Hallyn wrote:
>>>
>>> Hi Oleg,
>>>
>>> commit 40a0d32d1eaffe6aac7324ca92604b6b3977eb0e :
>>> "fork: unify and tighten up CLONE_NEWUSER/CLONE_NEWPID checks"
>>> breaks lxc-attach in 3.12.  That code forks a child which does
>>> setns() and then does a clone(CLONE_PARENT).  That way the
>>> grandchild can be in the right namespaces (which the child was
>>> not) and be a child of the original task, which is the monitor.
>
> Serge that is a clever trick to get around the limitation that we can
> not change the pid namespace of our current process.  Given the
> challenging relaying of signals etc I can see why you would use this.
>
> At the same time it makes me a little sad to see new users of
> CLONE_PARENT.  With CLONE_THREAD in existence the original reasons for
> CLONE_PARENT are gone now.
>
> Having used bash as an init process I know it can handle unexpeted
> children.  However using CLONE_PARENT in this way still seems a little
> dodgy.  Or am I misunderstanding why you are using CLONE_PARENT?
>
> That trick sounds like it might be worth adding to nsenter in util-linux
> just to simplify the code.
>
>> Thanks...
>>
>> Yes, this is what 40a0d32d1ea explicitly tries to disallow.
>>
>>> Is there a real danger in allowing CLONE_PARENT
>>> when current->nsproxy->pidns_for_children is not our pidns,
>>> or was this done out of an "over-abundance of caution"?
>>
>> I am not sure... This all was based on the long discussion, and
>> it was decided that the CLONE_PARENT check should be consistent
>> wrt CLONE_NEWPID and pidns_for_children != task_active_pid_ns().
>>
>>> Can we
>>> safely revert that new extra check?
>>
>> Well, usually we do not break user-space, but I am not sure about
>> this case...
>>
>> Eric, Andy, what do you think?
>>
>> And if we allow CLONE_PARENT when ->pidns_for_children was changed,
>> should we also allow, say, CLONE_NEWPID && CLONE_PARENT ?
>
> The two fundamental things I know we can not allow are:
> - A shared signal queue aka CLONE_THREAD.  Because we compute the pid
>   and uid of the signal when we place it in the queue.
>
> - Changing the pid and by extention pid_namespace of an existing
>   process.
>
> From a parents perspective there is nothing special about the pid
> namespace, to deny CLONE_PARENT, because the parent simply won't know or
> care.
>
> From the childs perspective all that is special really are shared signal
> queues.
>
> User mode threading with CLONE_PARENT|CLONE_VM|CLONE_SIGHAND and tasks
> in different pid namespaces is almost certainly going to break because
> it is complicated.  But shared signal handlers can look at per thread
> information to know which pid namespace a process is in, so I don't know
> of any reason not to support CLONE_PARENT|CLONE_VM|CLONE_SIGHAND threads
> at the kernel level.  It would be absolutely stupid to implement but
> that is a different thing.
>
> So hmm.
>
> Because it can do no harm, and because it is a regression let's remove
> the CLONE_PARENT check and send it stable.
>
> diff --git a/kernel/fork.c b/kernel/fork.c
> index 086fe73..c447fbc 100644
> --- a/kernel/fork.c
> +++ b/kernel/fork.c
> @@ -1174,7 +1174,7 @@ static struct task_struct *copy_process(unsigned long clone_flags,
>          * do not allow it to share a thread group or signal handlers or
>          * parent with the forking task.
>          */
> -       if (clone_flags & (CLONE_SIGHAND | CLONE_PARENT)) {
> +       if (clone_flags & (CLONE_SIGHAND)) {
>                 if ((clone_flags & (CLONE_NEWUSER | CLONE_NEWPID)) ||
>                     (task_active_pid_ns(current) !=
>                                 current->nsproxy->pid_ns_for_children))
>

Acked-by: Andy Lutomirski <luto at amacapital.net>

>
> I don't know if there are shells that CLONE_PARENT can confuse but if
> there are lxcattach and nsenter using this functionality should be
> enough to slowly get that confusion fixed.
>
> Changing the CLONE_SIGHAND into CLONE_THREAD will need to happen in a
> separate patch.  It isn't stable material, and so far there is no
> compelling use case for it.
>
> Eric



-- 
Andy Lutomirski
AMA Capital Management, LLC