Re: [RFC PATCH] fs: call_usermodehelper_root helper introduced

From: J. Bruce Fields
Date: Thu May 23 2013 - 15:06:45 EST

On Wed, May 22, 2013 at 08:37:23PM -0700, Eric W. Biederman wrote:
> "J. Bruce Fields" <bfields@xxxxxxxxxxxx> writes:
> > On Wed, May 22, 2013 at 11:35:56AM -0700, Eric W. Biederman wrote:
> >> ebiederm@xxxxxxxxxxxx (Eric W. Biederman) writes:
> >>
> >> > I am missing a lot of context here and capturing the context of a
> >> > process at time time we mount the filesystem and reconstituing it in
> >> > call user mode helper seems like something we could do.
> >
> > So it's not enough just to ensure that the user namespace is set
> > correctly? (To the namespace of the mount process in the nfs case, or
> > of the process that starts nfsd in the nfsd case.)
> No. By just setting the user namespace, you gain the ability to create
> processes outside of your curremt rlimits, and cgroups, and pid
> namespace, etc.
> With the wrong set of namespaces you will talk to the wrong processes,
> or utilize the wrong resources.
> Outside of your current cgroup you gain the ability to use more
> resources than you were limited to.
> Having thought about it what I would propose is to fork a process from
> the context of the mounting process (or possibly from the context of the
> process that writes to the sysctl that sets the program to spawn) and
> have that process be a daemon that becomes responsible for spawning user
> mode helpers with context. Either that or whe need the user mode helper
> userspace infrastructure to become namespace aware and call setns.
> >> If we want to do something like this the only sane thing I can see is to
> >> have a per container version of kthread call it uthread. That the user
> >> mode helper code would use to launch a new process.
> >>
> >> Anything else and I expect we will be tearing our hair out for the rest
> >> of our lives with weird corner cases or unexpected semantics.
> >
> > Could you give examples of weird corner cases or unexpected semantics?
> I gave a couple and it is the classic problem with suid executables
> where a lot of unexpected things are inherited from the environment, and
> so things become a never ending race. We replaced daemonize in the
> kernel with just forking processes from kthreadd for this very reason.
> There was always something else that needed to be reset to make a
> process a proper kernel thread.

Got it, thanks for the explanation.

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