linux-kernel - Re: call_usermodehelper in containers

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Date:	Thu, 18 Feb 2016 12:43:31 +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 2016/02/18 11:57, Eric W. Biederman wrote:
> 
> Ccing The containers list because a related discussion is happening there
> and somehow this thread has never made it there.
> 
> Ian Kent <raven@...maw.net> writes:
> 
>> On Mon, 2013-11-18 at 18:28 +0100, Oleg Nesterov wrote:
>>> On 11/15, Eric W. Biederman wrote:
>>>>
>>>> I don't understand that one.  Having a preforked thread with the
>>>> proper
>>>> environment that can act like kthreadd in terms of spawning user
>>>> mode
>>>> helpers works and is simple.
>>
>> Forgive me replying to such an old thread but ...
>>
>> After realizing workqueues can't be used to pre-create threads to run
>> usermode helpers I've returned to look at this.
> 
> If someone can wind up with a good implementation I will be happy.
> 
>>> Can't we ask ->child_reaper to create the non-daemonized kernel thread
>>> with the "right" ->nsproxy, ->fs, etc?
>>
>> Eric, do you think this approach would be sufficient too?
>>
>> Probably wouldn't be quite right for user namespaces but should provide
>> what's needed for other cases?
>>
>> It certainly has the advantage of not having to maintain a plague of
>> processes waiting around to execute helpers.
> 
> That certainly sounds attractive.  Especially for the case of everyone
> who wants to set a core pattern in a container.
> 
> I am fuzzy on all of the details right now, but what I do remember is
> that in the kernel the user mode helper concepts when they attempted to
> scrub a processes environment were quite error prone until we managed to
> get kthreadd(pid 2) on the scene which always had a clean environment.
> 
> If we are going to tie this kind of thing to the pid namespace I
> recommend simplying denying it if you are in a user namespace without
> an approrpriate pid namespace.  AKA simply not allowing thigns to be setup
> if current->pid_ns->user_ns != current->user_ns.
> 
Can't be handled by simple capability like CAP_SYS_USERMODEHELPER ?

User_ns check seems not to allow core-dump-cather in host will not work if user_ns is used.

Thanks,
-Kame