linux-kernel - Re: [PATCH 3/3] fuse: Allow mounts from user namespaces

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Date:	Mon, 21 Jul 2014 09:34:42 -0500
From:	Seth Forshee <seth.forshee@...onical.com>
To:	Miklos Szeredi <miklos@...redi.hu>
Cc:	Kernel Mailing List <linux-kernel@...r.kernel.org>,
	fuse-devel <fuse-devel@...ts.sourceforge.net>,
	lxc-devel@...ts.linuxcontainers.org,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Serge Hallyn <serge.hallyn@...ntu.com>,
	"Michael H. Warfield" <mhw@...tsend.com>,
	Seth Forshee <seth.forshee@...onical.com>
Subject: Re: [PATCH 3/3] fuse: Allow mounts from user namespaces

On Mon, Jul 21, 2014 at 03:09:14PM +0200, Miklos Szeredi wrote:
> On Mon, Jul 21, 2014 at 2:47 PM, Seth Forshee
> <seth.forshee@...onical.com> wrote:
> > On Fri, Jul 18, 2014 at 05:33:23PM +0200, Miklos Szeredi wrote:
> >> On Mon, Jul 14, 2014 at 9:18 PM, Seth Forshee
> >> <seth.forshee@...onical.com> wrote:
> >> > Update fuse to allow mounts from user namespaces. During mount
> >> > current_user_ns() is stashed away,
> >>
> >> Same thing here.  While practically this may work, it's theoretically
> >> wrong, and possibly may go wrong in special situations.   In fuse
> >> there's no official "server process", so storing information, like
> >> namespace, about one is going to be wrong.
> >
> > What you're suggesting would probably work fine when dealing with pids.
> > It's not going to work though for the checks I've added in
> > fuse_allow_current_process() that the process is in the mount owner's
> > user ns, and without those checks or something similar I don't think
> > it's safe to permit allow_other for user ns mounts.
> 
> You can add that check in fuse_dev_do_read() as well.  If the
> fsuid/fsgid doesn't exist in the "server's" namespace, then set
> req->out.h.error and call request_end().

Okay, that seems like it should work.

> > Can you elaborate on what special situations might violate these
> > assumptions or otherwise cause problems?
> 
> What's preventing a fuse fs implementation from handling FUSE_INIT in
> one process and then handling the rest in a different process
> (possibly in a different namespace)?

Nothing, but I'm having a hard time imagining why that would ever be
useful. The user/group ids passed in the mount options would have to be
mapped into that namespace, otherwise all requests will just fail in the
check you suggest above. The only thing I can think of would be if
someone wanted to proxy mounts trough a process in a more privileged
context, but then the main point of these patches is to make that
unnecessary.

But I also think your approach should work just as well as mine for the
use cases that do make sense to me, so I'll go ahead and give it a try.

Thanks,
Seth
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