[Devel] call_usermodehelper in containers

[Eric W. Biederman]

Stanislav Kinsbursky <skinsbursky at parallels.com> writes:

> 12.11.2013 17:30, Jeff Layton пишет:
>> On Tue, 12 Nov 2013 17:02:36 +0400
>> Stanislav Kinsbursky <skinsbursky at parallels.com> wrote:
>>
>>> 12.11.2013 15:12, Jeff Layton пишет:
>>>> On Mon, 11 Nov 2013 16:47:03 -0800
>>>> Greg KH <gregkh at linuxfoundation.org> wrote:
>>>>
>>>>> On Mon, Nov 11, 2013 at 07:18:25AM -0500, Jeff Layton wrote:
>>>>>> We have a bit of a problem wrt to upcalls that use call_usermodehelper
>>>>>> with containers and I'd like to bring this to some sort of resolution...
>>>>>>
>>>>>> A particularly problematic case (though there are others) is the
>>>>>> nfsdcltrack upcall. It basically uses call_usermodehelper to run a
>>>>>> program in userland to track some information on stable storage for
>>>>>> nfsd.
>>>>>
>>>>> I thought the discussion at the kernel summit about this issue was:
>>>>> 	- don't do this.
>>>>> 	- don't do it.
>>>>> 	- if you really need to do this, fix nfsd
>>>>>
>>>>
>>>> Sorry, I couldn't make the kernel summit so I missed that discussion. I
>>>> guess LWN didn't cover it?
>>>>
>>>> In any case, I guess then that we'll either have to come up with some
>>>> way to fix nfsd here, or simply ensure that nfsd can never be started
>>>> unless root in the container has a full set of a full set of
>>>> capabilities.
>>>>
>>>> One sort of Rube Goldberg possibility to fix nfsd is:
>>>>
>>>> - when we start nfsd in a container, fork off an extra kernel thread
>>>>     that just sits idle. That thread would need to be a descendant of the
>>>>     userland process that started nfsd, so we'd need to create it with
>>>>     kernel_thread().
>>>>
>>>> - Have the kernel just start up the UMH program in the init_ns mount
>>>>     namespace as it currently does, but also pass the pid of the idle
>>>>     kernel thread to the UMH upcall.
>>>>
>>>> - The program will then use /proc/<pid>/root and /proc/<pid>/ns/* to set
>>>>     itself up for doing things properly.
>>>>
>>>> Note that with this mechanism we can't actually run a different binary
>>>> per container, but that's probably fine for most purposes.
>>>>
>>>
>>> Hmmm... Why we can't? We can go a bit further with userspace idea.
>>>
>>> We use UMH some very limited number of user programs. For 2, actually:
>>> 1) /sbin/nfs_cache_getent
>>> 2) /sbin/nfsdcltrack
>>>
>>
>> No, the kernel uses them for a lot more than that. Pretty much all of
>> the keys API upcalls use it. See all of the callers of
>> call_usermodehelper. All of them are running user binaries out of the
>> kernel, and almost all of them are certainly broken wrt containers.
>>
>>> If we convert them into proxies, which use /proc/<pid>/root and /proc/<pid>/ns/*, this will allow us to lookup the right binary.
>>> The only limitation here is presence of this "proxy" binaries on "host".
>>>
>>
>> Suppose I spawn my own container as a user, using all of this spiffy
>> new user namespace stuff. Then I make the kernel use
>> call_usermodehelper to call the upcall in the init_ns, and then trick
>> it into running my new "escape_from_namespace" program with "real" root
>> privileges.
>>
>> I don't think we can reasonably assume that having the kernel exec an
>> arbitrary binary inside of a container is safe. Doing so inside of the
>> init_ns is marginally more safe, but only marginally so...
>>
>>> And we don't need any significant changes in kernel.
>>>
>>> BTW, Jeff, could you remind me, please, why exactly we need to use UMH to run the binary?
>>> What are this capabilities, which force us to do so?
>>>
>>
>> Nothing _forces_ us to do so, but upcalls are very difficult to handle,
>> and UMH has a lot of advantages over a long-running daemon launched by
>> userland.
>>
>> Originally, I created the nfsdcltrack upcall as a running daemon called
>> nfsdcld, and the kernel used rpc_pipefs to communicate with it.
>>
>> Everyone hated it because no one likes to have to run daemons for
>> infrequently used upcalls. It's a pain for users to ensure that it's
>> running and it's a pain to handle when it isn't. So, I was encouraged
>> to turn that instead into a UMH upcall.
>>
>> But leaving that aside, this problem is a lot larger than just nfsd. We
>> have a *lot* of UMH upcalls in the kernel, so this problem is more
>> general than just "fixing" nfsd's.
>>
>
> Ok. So we are talking about generic approach to UMH support in a container (and/or namespace).
>
> Actually, as far as I can see, there are more that one aspect, which is not supported.
> One one them is executing of the right binary. Another one is
> capabilities (and maybe there are more, like user namespaces), but I
> don't really care about them for now.
> Executing the right binary, actually, is not about namespaces at all. This is about lookup implementation in VFS (do_execve_common).



> Would be great to unshare FS for forked UHM kthread and swap it to
> desired root. This will solve the problem with proper lookup. However,
> as far as I understand, this approach is not welcome by the community.

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.  The only downside I can see is that there
is extra overhead.

Beyond that though for the user mode helpers spawned to populate
security keys it is not clear which context they should be run in,
even if we do have kernel threads.

> This problem, probably, can be solved by constructing full binary path
> (i.e. not in a container, but in kernel thread root context) in UMH
> "init" callack. However, this will help only is the dentry is
> accessible from "init" root. Which is usually no true in case on mount
> namespaces, if I understand them right.

You are correct it can not be assumed that what is visible in one mount
namespace is visible in another.  And of course in addition to picking
the correct binary to run you have to set up a proper environment for
that binary to run in.  It may be that it's configuration file is only
avaiable at the expected location in the proper mount namespace, even
if the binary is available in all of the mount namespaces.

Eric