call_usermodehelper in containers

Kamezawa Hiroyuki kamezawa.hiroyu at
Fri Feb 19 09:30:16 UTC 2016

On 2016/02/19 14:37, Ian Kent wrote:
> On Fri, 2016-02-19 at 12:08 +0900, Kamezawa Hiroyuki wrote:
>> 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.
> Not sure I quite understand this but I thought the difficulty with this
> approach previously (even though the approach was very much incomplete)
> was knowing that all the "moving parts" would not allow vulnerabilities.
Ok, that was discussed.

> And it looks like this would require a kernel thread for each instance.
> So for a thousand containers that each mount an NFS mount that means, at
> least, 1000 additional kernel threads. Might be able to sell that, if we
> were lucky, but from an system administration POV it's horrible.
I agree.

> There's also the question of existence (aka. lifetime) to deal with
> since the thread above needs to be created at a time other than the
> usermode helper callback.
> What happens for SIGKILL on a container?
It depends on how the helper kthread is tied to a container related object.
If kthread is linked with some namespace, we can kill it when a namespace
goes away.

So, with your opinion,
  - a helper thread should be spawned on demand
  - the lifetime of it should be clear. It will be good to have as same life time as the container.

I wonder there is no solution for "moving part" problem other than calling
do_fork() or copy_process() with container's init process context if we do all in the kernel.
Is that possible ?


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