call_usermodehelper in containers

Kamezawa Hiroyuki kamezawa.hiroyu at
Fri Feb 19 03:08:04 UTC 2016

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 ?)
   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.


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