Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67;
Subject: Re: [PATCH 03/34] teach move_mount(2) to work with OPEN_TREE_CLONE
 [ver #12]
To:     David Howells <dhowells@redhat.com>, viro@zeniv.linux.org.uk
Cc:     torvalds@linux-foundation.org, ebiederm@xmission.com,
        linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org,
        mszeredi@redhat.com
References: <16207.1539249451@warthog.procyon.org.uk>
 <5c6f3d62-4cec-2aea-4693-62928611c526@gmail.com>
 <153754740781.17872.7869536526927736855.stgit@warthog.procyon.org.uk>
 <153754743491.17872.12115848333103740766.stgit@warthog.procyon.org.uk>
 <de050902-f3d0-b2db-2627-983a36c87b3c@gmail.com>
 <862e36a2-2a6f-4e26-3228-8cab4b4cf230@gmail.com>
 <6533.1539260096@warthog.procyon.org.uk>
From:   Alan Jenkins <alan.christopher.jenkins@gmail.com>
Message-ID: <92bfc752-100b-619d-f1c9-b1ea93244ab4@gmail.com>
Date:   Thu, 11 Oct 2018 13:23:35 +0100
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101
 Thunderbird/60.0
MIME-Version: 1.0
In-Reply-To: <6533.1539260096@warthog.procyon.org.uk>
Content-Type: text/plain; charset=utf-8; format=flowed
Content-Transfer-Encoding: 8bit
Content-Language: en-US
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

On 11/10/2018 13:14, David Howells wrote:
> David Howells <dhowells@redhat.com> wrote:
>
>> The reason that you can do this with open_tree()/move_mount() is that it
>> allows you to create a mount tree (OPEN_TREE_CLONE) that has no namespace
>> assignment, pass it through the namespace switch and then attach it inside the
>> child namespace.  The cross-namespace checks in do_move_mount() are bypassed
>> because the root of the newly-cloned mount tree doesn't have one.
> It's worse than that.  The apparently disconnected tree given you by
> open_tree(OPEN_TREE_CLONE) is still subject to modification by outside
> forces.  All it takes is one shared object within that tree.
>
> So I do wonder if it's possible to form a ring, even in an upstream kernel, by
> using the propagation mechanism to push through an nsfs mount into itself,
> possibly with a layer of indirection (ie. having two mutually-referential
> namespaces).
>
> David

Upstream does cover the mount propagation case, by simply never 
propagating mounts of mount NS files.  See commit 4ce5d2b1a8fd "vfs: 
Don't copy mount bind mounts of /proc/<pid>/ns/mnt between namespaces" / 
https://unix.stackexchange.com/questions/473717/what-code-prevents-mount-namespace-loops-in-a-more-complex-case-involving-mount-propagation