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

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Date:	Mon, 21 Jul 2014 15:09:14 +0200
From:	Miklos Szeredi <miklos@...redi.hu>
To:	Miklos Szeredi <miklos@...redi.hu>,
	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>
Subject: Re: [PATCH 3/3] fuse: Allow mounts from user namespaces

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().

> 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)?

Thanks,
Miklos
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