linux-kernel - Re: call_usermodehelper in containers

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Date:	Fri, 19 Feb 2016 12:08:04 +0900
From:	Kamezawa Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
To:	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Ian Kent <raven@...maw.net>
Cc:	Oleg Nesterov <oleg@...hat.com>,
	Stanislav Kinsbursky <skinsbursky@...allels.com>,
	Jeff Layton <jlayton@...hat.com>,
	Greg KH <gregkh@...uxfoundation.org>,
	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	linux-nfs@...r.kernel.org, devel@...nvz.org, bfields@...ldses.org,
	bharrosh@...asas.com,
	Linux Containers <containers@...ts.linux-foundation.org>
Subject: Re: call_usermodehelper in containers

On 2016/02/19 5:45, Eric W. Biederman wrote: 
> Personally I am a fan of the don't be clever and capture a kernel thread
> approach as it is very easy to see you what if any exploitation
> opportunities there are.  The justifications for something more clever
> is trickier.  Of course we do something that from this perspective would
> be considered ``clever'' today with kthreadd and user mode helpers.
> 

I read old discussion....let me allow clarification  to create a helper kernel thread 
to run usermodehelper with using kthreadd.

0) define a trigger to create an independent usermodehelper environment for a container.
   Option A) at creating some namespace (pid, uid, etc...)
   Option B) at creating a new nsproxy
   Option C).at a new systemcall is called or some sysctl, make_private_usermode_helper() or some,

  It's expected this should be triggered by init process of a container with some capability.
  And scope of the effect should be defined. pid namespace ? nsporxy ? or new namespace ?

1) create a helper thread.
   task = kthread_create(kthread_work_fn, ?, ?, "usermodehelper")
   switch task's nsproxy to current.(swtich_task_namespaces())
   switch task's cgroups to current (cgroup_attach_task_all())
   switch task's cred to current.
   copy task's capability from current
   (and any other ?)
   wake_up_process()

   And create a link between kthread_wq and container.

2) modify call_usermodehelper() to use kthread_worker
....

It seems the problem is which object container private user mode helper should be tied to.

Regards,
-Kame