call_usermodehelper in containers

Ian Kent raven at themaw.net
Tue Feb 23 02:55:30 UTC 2016


On Fri, 2016-02-19 at 13:14 +0800, Ian Kent wrote:
> On Thu, 2016-02-18 at 14:45 -0600, Eric W. Biederman wrote:
> > Ian Kent <raven at themaw.net> writes:
> > 
> > > On Thu, 2016-02-18 at 14:36 +0800, Ian Kent wrote:
> > > > 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 ?
> > 
> > I wasn't talking about a capability I was talking about how to
> > identify
> > where the user mode helper lives.
> > 
> > > > > User_ns check seems not to allow core-dump-cather in host will
> > > > > not
> > > > > work if user_ns is used.
> > 
> > The bottom line is all of this approaches non-sense if user
> > namespaces
> > are not used.  If you just have a pid namespace or a mount namespace
> > (or
> > perhaps both) and your fire off a new fangled user mode helper you
> > get
> > a
> > deep problem.  The user space process started to handle your core
> > dump
> > or
> > your nfs callback will have a full set of capabilities (because it
> > is
> > still in the root user namespace).  With a full set of capabilities
> > and perhaps a little luck there is no containment.
> > 
> > The imperfect solution that currently exists for the core dump
> > helper
> > is to provide enough information to the user space application that
> > it can query and find out the context of the core dumping
> > application
> > and keep everything in that application sandbox if it so desires.
> > I expect something similar could be done for other user mode helper
> > style callbacks.
> > 
> > To make starting the user space application other than how we do
> > today
> > needs a good argument that you are you can allow a lesser privileged
> > process set things up and that it can be exploited to gain
> > privielge.
> > 
> > > > 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.
> > > 
> > > Perhaps, if a struct cred could be captured at some appropriate
> > > time
> > > that could be used to cater for user namespaces.
> > > 
> > > Eric, do you think that would be possible to do without allowing
> > > users
> > > to circumvent security?
> > 
> > The general problem with capturing less than a full process is that
> > we always mess it up and forget to capture something important.
> > 
> > In a lot of ways this is a very simpilar problem to setting up an at
> > job
> > or a cron job.  You build a script you test it then you tell at to
> > run
> > it at a certain time and it fails, because your working environment
> > did
> > not include something important that was in your actuall
> > environment.
> > 
> > Unfortunately in this case the failures we are talking about are
> > container escapes and privilege escalation, so we do need to tread
> > carefully.
> > 
> > We might be able to safely define the context as the context of the
> > currently running init process (Which we can identifiy with a struct
> > pid).  Justifying that looks a little trickier but doable.
> 
> Right, that seems like a fairly straight forward thing to implement
> based on Olegs' example patch.
> 
> I'll put together a series based on that approach.
> 
> Keep in mind that the patches in my previous posts for sub-system
> usage
> are definitely wrong but I can use them (and they will be only an
> initial example of how to use the mechanism) to verify that contained
> execution happens. They will need to change.
> 
> I was thinking that also capturing a struct cred (although I need to
> look more at the relationship between the process cred, and the
> nsproxy
> locations) at a particular time combined with a double fork and exec
> could allow inclusion of user namespace.
> 
> Perhaps at only one level deep, ie. only allowing the first user
> namesapec created from init or from container and not user namespaces
> created from within a user namespace (if I can work out how to
> identify
> that case).

You know, wrt. the mechanism Oleg suggested, I've been wondering if it's
even necessary to capture process template information for execution.

Isn't the main issue the execution of unknown arbitrary objects getting
access to a privileged context?

Then perhaps it is sufficient to require registration of an SHA hash (of
some sort) for these objects by a suitably privileged process and only
allow helper execution of valid objects.

If that is sufficient then helper execution from within a container or
user namespace could just use the callers environment itself.

What else do we need to be wary of, any thoughts Eric?

Ian


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