Re: For review: user_namespace(7) man page

From: Andy Lutomirski <luto@amacapital.net>
To: "Eric W. Biederman" <ebiederm@xmission.com>
Cc: "Michael Kerrisk (man-pages)" <mtk.manpages@gmail.com>,
	lkml <linux-kernel@vger.kernel.org>,
	"linux-man@vger.kernel.org" <linux-man@vger.kernel.org>,
	Linux Containers <containers@lists.linux-foundation.org>,
	richard -rw- weinberger <richard.weinberger@gmail.com>,
	"Serge E. Hallyn" <serge@hallyn.com>
Subject: Re: For review: user_namespace(7) man page
Date: Tue, 9 Sep 2014 12:39:44 -0700	[thread overview]
Message-ID: <CALCETrVhBz7YT9yw+ZSpW+3cM85iYRXXnLY1cRPVhb04NN=_Pw@mail.gmail.com> (raw)
In-Reply-To: <CALCETrV4WizRXD9JuwibUBbQE9hhNrRDJ3cYyXdhd=OfPziF5g@mail.gmail.com>

On Tue, Sep 9, 2014 at 12:26 PM, Andy Lutomirski <luto@amacapital.net> wrote:
> On Tue, Sep 9, 2014 at 9:05 AM, Eric W. Biederman <ebiederm@xmission.com> wrote:
>>
>> We may also want to discuss the specific restrictions on chroot.
>>
>> The text about chroot at least gives people a strong hint that the
>> chroot rules are affected by user namespaces.
>>
>> The restrictions that we have settled on to avoid chroot being a problem
>> are the creator of a user namespace must not be chrooted in their
>> current mount namespace, and the creator of the user namespace must not
>> be threaded.
>>
>> Andy can you check me on this it looks like unshare is currently buggy
>> in that it will allow a threaded application to create a user namespace.
>
> I think it's this code in unshare:
>
>     /*
>      * If unsharing a user namespace must also unshare the thread.
>      */
>     if (unshare_flags & CLONE_NEWUSER)
>         unshare_flags |= CLONE_THREAD | CLONE_FS;
>
> I suppose that this should be documented.
>
> CLONE_FS prevents the chroot from leaking out of the namespace.  (But
> see the other thread that I'm about to start...)

Never mind.  I thought I saw a bug, but I was wrong.

--Andy