call_usermodehelper in containers

Ian Kent raven at themaw.net
Sat Feb 20 03:28:41 UTC 2016


On Fri, 2016-02-19 at 18:30 +0900, Kamezawa Hiroyuki wrote:
> 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?
> > 

First understand that the fork and workqueue code is not something I've
needed to look at in the past so it's still quite new to me even now.

> 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.

I don't know how to do that so without knowing any better I assume it
could be difficult and complicated but, of course, I don't know.

> 
> 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.

This was always what I believed to be the best way to do it but ...

Not sure you've seen the other threads on this by me so let me provide
some history.

I started out posting a series (totally untested, an RFC only) in the
hope of finding a way to do this.

After a few iterations that lead to the conclusion that a kernel thread
would need to be created to provide context for subsequent helper
execution (for every distinct context), much the same as we have here,
and that the init process of the required context would probably be
sufficient for this, required as the environment of the thread
requesting helper execution itself could be used subvert execution.

I ended up accepting that even if I could work out what needed to be
captured and work out what needed to be done to switch to the
namspace(s) and other bits that would be high maintenance as it would be
fairly complicated and subsystems may be added or changed over time.

Also I had assumed a singlethread workqueue would create a single thread
for helper execution which was wrong.

After realizing what I had was far from what's needed I went back and
started reviewing the previous threads.

That lead me to following a link Oleg had posted to this thread where I
finally saw his suggestion about using ->child_reaper as the execution
template.

That really got my attention because of its simplicity and that's why I
want to give that a try now and see where it leads. However user
namespaces do sound like a problem even with this.

Having finally got a simple test scenario I see now that the palaces I
use to capture the information used to run the helper is also wrong but
that's less important than getting an execution method that works, is
safe, and is as simple as it can be. 

> 
> 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.

Not sure I understand this but I believe that ultimately there will be
the equivalent of a fork (perhaps two) and exec (we need to exec the
helper anyway) no matter how this is done.

For example, IIUC, a fork must be done to change pid namespace but a
template like the container init process would already have that pid
namespace in cases other than possibly user namespaces.

I hope I understood what you were asking and haven't needlessly rambled
on,  ;)

Ian


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