Re: [RFC PATCH] fs: call_usermodehelper_root helper introduced

From: Jeff Layton
Date: Thu May 23 2013 - 07:56:36 EST

On Thu, 23 May 2013 15:38:17 +0400
Stanislav Kinsbursky <skinsbursky@xxxxxxxxxxxxx> wrote:

> 23.05.2013 15:31, Jeff Layton ÐÐÑÐÑ:
> > On Thu, 23 May 2013 14:35:53 +0400
> > Stanislav Kinsbursky <skinsbursky@xxxxxxxxxxxxx> wrote:
> >
> >> 23.05.2013 14:00, Eric W. Biederman ÐÐÑÐÑ:
> >>> Stanislav Kinsbursky <skinsbursky@xxxxxxxxxxxxx> writes:
> >>>
> >>>> 22.05.2013 21:33, Eric W. Biederman ÐÐÑÐÑ:
> >>>>> Stanislav Kinsbursky <skinsbursky@xxxxxxxxxxxxx> writes:
> >>>>>
> >>>>>> Usermode helper executes all binaries in global "init" root context. This
> >>>>>> doesn't allow to call a binary from other root context (for example in a
> >>>>>> container).
> >>>>>> Currently, both containerized NFS client and NFS server requires an ability to
> >>>>>> execute a binary in a container's root context. Root swap can be done in
> >>>>>> "init" callback, passed by UMH caller.
> >>>>>> But since we have 2 callers already (and more of them are expected to appear
> >>>>>> in future) and because set_fs_root() in not exported, it looks reasonable to
> >>>>>> add one more generic UMH helper to generic fs code.
> >>>>>> Root path reference must be hold by the caller, since it will be put on UMH
> >>>>>> thread exit.
> >>>>>
> >>>>> Awesome. With this patch as an uprivilieged user I get to pick which
> >>>>> binary the kernel will execute. At least if nfs and nfsd ever runs in a
> >>>>> user namespace (something that looks like only matter of time).
> >>>>>
> >>>>
> >>>> Not really. Only by using a kernel module to call the UMH.
> >>>> And an unprivileged can't load a module as far a I know.
> >>>> I.e. NFSd, for example, will use unprivileged user's root to perform this call.
> >>>
> >>> To help me understand the context which instances of call user mode
> >>> helper are you expecting to use this facility?
> >>>
> >>
> >> Ok. Here is how the NFSd uses UMH:
> >> UMH is used on NFSd service to start user-space client tracker daemon
> >> ("/sbin/nfsdcltarck"), which is used to store some per-client locks data on
> >> persistent storage.
> >>
> >>>>> I think this is a seriously bad idea.
> >>>>>
> >>>>> Why can't we do this in userspace with setns as we do with the core dump
> >>>>> helper?
> >>>>>
> >>>>
> >>>> Could you, please, clarify, how setns can help here?
> >>>
> >>> setns can change the mount namespace, and chroot can change to root
> >>> directory in the specified mount namespace. Essentially you can enter
> >>> into a containers complete context (pid, mnt, root, etc) comming from
> >>> the outside.
> >>>
> >>
> >> So, you are actually suggesting to move the binary start from the kernel to user-space.
> >> IOW, you are suggesting to do not using UMH at all.
> >> Am I right?
> >> I don't know the reasons, why it was done by using UMH and not in userspace.
> >> Could you clarify this, Jeff?
> >>
> >
> > nfsdcltrack is a "one-shot" program for managing and querying the nfsd
> > client tracking database. When knfsd needs to query or modify the
> > db, it uses the UMH infrastructure to call this program that does
> > what's requested and then exits.
> >
> > So, I'm not sure I really understand your question. It wasn't done in
> > userspace since the whole purpose of this program is to handle upcalls
> > from the kernel.
> >
> The question is what was the reason to start this binary from kernel by UMH?

Manipulating and querying the client tracking database is an infrequent
event, so having a continuously running daemon is wasteful and means
that the admin has to ensure that it's running. A UMH upcall is much
simpler and generally "just works" if the program is present.

> I.e. why it can't be started by some user-space process before or after NFSd start?
> I don't familiar with this client tracking facility and that's the only reason why I'm asking.

This program is not a daemon that runs continuously. It's only called
when the kernel needs to manipulate the database. Are you asking
whether we could turn this into a continuously running daemon? If so
then the answer is "yes", but that's not really a good idea either.

In fact, we had that with the nfsdcld program, but no one liked it
(including me) for the reasons I detailed above.

Jeff Layton <jlayton@xxxxxxxxxx>
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