[Devel] call_usermodehelper in containers

[Ian Kent]

On Thu, 2016-02-18 at 12:43 +0900, Kamezawa Hiroyuki wrote:
> 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 at themaw.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.

I don't think so but I'm not sure.

The approach I was talking about assumes the init process of the caller
(say within a container, corresponding to ->child_reaper) is an
appropriate template for umh thread execution.

But I don't think that covers the case where unshare has created
different namespaces, like a mount namespace for example.

The current workqueue sub system can't be used to pre-create a thread to
be used for umh execution so, either is needs changes or yet another
mechanism needs to be implemented.

For uses other than core dumping capturing a reference to the struct pid
of the environment init process and using that as an execution template
should be sufficient and takes care of environment existence problems
with some extra checks, not to mention eliminating the need for a
potentially huge number of kernel threads needing to be created to
provide execution templates.

Where to store this and how to access it when needed is another problem.

Not sure a usermode helper capability is the right thing either as I
thought one important use of user namespaces was to allow unprivileged
users to perform operations they otherwise can't.

Maybe a CAP_SYS_USERNSCOREDUMP or similar would be sensible ....

Still an appropriate execution template would be needed and IIUC we
can't trust getting that from within a user created namespace
environment.

> 
> Thanks,
> -Kame
>