linux-kernel - Re: [RFC PATCH] fs: call_usermodehelper

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Date:	Thu, 23 May 2013 15:06:02 -0400
From:	"J. Bruce Fields" <bfields@...ldses.org>
To:	"Eric W. Biederman" <ebiederm@...ssion.com>
Cc:	Stanislav Kinsbursky <skinsbursky@...allels.com>,
	viro@...iv.linux.org.uk, serge.hallyn@...onical.com,
	jlayton@...hat.com, lucas.demarchi@...fusion.mobi,
	rusty@...tcorp.com.au, linux-kernel@...r.kernel.org,
	oleg@...hat.com, bharrosh@...asas.com,
	linux-fsdevel@...r.kernel.org, akpm@...ux-foundation.org,
	devel@...nvz.org
Subject: Re: [RFC PATCH] fs: call_usermodehelper_root helper introduced

On Wed, May 22, 2013 at 08:37:23PM -0700, Eric W. Biederman wrote:
> "J. Bruce Fields" <bfields@...ldses.org> writes:
> 
> > On Wed, May 22, 2013 at 11:35:56AM -0700, Eric W. Biederman wrote:
> >> ebiederm@...ssion.com (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.

--b.
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