From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-0.8 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_PASS autolearn=ham autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 6AEDAC32788 for ; Thu, 11 Oct 2018 09:17:36 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 2DA5120835 for ; Thu, 11 Oct 2018 09:17:36 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 2DA5120835 Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=redhat.com Authentication-Results: mail.kernel.org; spf=none smtp.mailfrom=linux-kernel-owner@vger.kernel.org Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1728200AbeJKQn5 convert rfc822-to-8bit (ORCPT ); Thu, 11 Oct 2018 12:43:57 -0400 Received: from mx1.redhat.com ([209.132.183.28]:56644 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726223AbeJKQn5 (ORCPT ); Thu, 11 Oct 2018 12:43:57 -0400 Received: from smtp.corp.redhat.com (int-mx01.intmail.prod.int.phx2.redhat.com [10.5.11.11]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mx1.redhat.com (Postfix) with ESMTPS id 1D378308338F; Thu, 11 Oct 2018 09:17:34 +0000 (UTC) Received: from warthog.procyon.org.uk (ovpn-120-149.rdu2.redhat.com [10.10.120.149]) by smtp.corp.redhat.com (Postfix) with ESMTP id 6719F60472; Thu, 11 Oct 2018 09:17:32 +0000 (UTC) Organization: Red Hat UK Ltd. Registered Address: Red Hat UK Ltd, Amberley Place, 107-111 Peascod Street, Windsor, Berkshire, SI4 1TE, United Kingdom. Registered in England and Wales under Company Registration No. 3798903 From: David Howells In-Reply-To: <5c6f3d62-4cec-2aea-4693-62928611c526@gmail.com> References: <5c6f3d62-4cec-2aea-4693-62928611c526@gmail.com> <153754740781.17872.7869536526927736855.stgit@warthog.procyon.org.uk> <153754743491.17872.12115848333103740766.stgit@warthog.procyon.org.uk> <862e36a2-2a6f-4e26-3228-8cab4b4cf230@gmail.com> To: Alan Jenkins Cc: dhowells@redhat.com, viro@zeniv.linux.org.uk, torvalds@linux-foundation.org, ebiederm@xmission.com, linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org, mszeredi@redhat.com Subject: Re: [PATCH 03/34] teach move_mount(2) to work with OPEN_TREE_CLONE [ver #12] MIME-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-ID: <16206.1539249451.1@warthog.procyon.org.uk> Content-Transfer-Encoding: 8BIT Date: Thu, 11 Oct 2018 10:17:31 +0100 Message-ID: <16207.1539249451@warthog.procyon.org.uk> X-Scanned-By: MIMEDefang 2.79 on 10.5.11.11 X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.5.110.44]); Thu, 11 Oct 2018 09:17:34 +0000 (UTC) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Alan Jenkins wrote: > # unshare --mount=private_mnt/child_ns --propagation=shared ls -l /proc/self/ns/mnt I think the problem is that the mount of the nsfs object done by unshare here pins the new mount namespace - but doesn't add the namespace's contents into the mount tree, so the mount struct cycle-detection code is bypassed. I think it's fine for all other namespaces, just not the mount namespace. It looks like this bug might theoretically exist upstream also, though I don't think there's any way to actually effect it given that mount() doesn't take a dirfd argument. 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. Unfortunately, just searching the newly-cloned mount tree for a conflicting nsfs mount doesn't help because the potential loop could be hidden several levels deep. I think the simplest solution is to either reject a request for open_tree(OPEN_TREE_CLONE) if there are any nsfs objects in the source tree, or to just not copy said objects. David --- Test script: mount -t tmpfs none /a mount --make-shared /a cd /a mkdir private_mnt mount -t tmpfs xxx private_mnt mount --make-private private_mnt touch private_mnt/child_ns unshare --mount=private_mnt/child_ns --propagation=shared \ ls -l /proc/self/ns/mnt findmnt ~/open_tree 3