linux-kernel - Re: [PATCH] [RFC] mnt: add ability to clone mntns starting with the current root

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Message-ID: <87siizla5p.fsf@x220.int.ebiederm.org>
Date:	Tue, 07 Oct 2014 16:42:10 -0700
From:	ebiederm@...ssion.com (Eric W. Biederman)
To:	Andy Lutomirski <luto@...capital.net>
Cc:	Serge Hallyn <serge.hallyn@...ntu.com>,
	Al Viro <viro@...iv.linux.org.uk>,
	Andrey Vagin <avagin@...nvz.org>,
	Linux FS Devel <linux-fsdevel@...r.kernel.org>,
	"linux-kernel\@vger.kernel.org" <linux-kernel@...r.kernel.org>,
	Linux API <linux-api@...r.kernel.org>,
	Andrey Vagin <avagin@...il.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Cyrill Gorcunov <gorcunov@...nvz.org>,
	Pavel Emelyanov <xemul@...allels.com>,
	Serge Hallyn <serge.hallyn@...onical.com>,
	Rob Landley <rob@...dley.net>
Subject: Re: [PATCH] [RFC] mnt: add ability to clone mntns starting with the current root

Andy Lutomirski <luto@...capital.net> writes:

> On Tue, Oct 7, 2014 at 3:42 PM, Eric W. Biederman <ebiederm@...ssion.com> wrote:
>> Andy Lutomirski <luto@...capital.net> writes:
>>
>>> On Tue, Oct 7, 2014 at 2:42 PM, Eric W. Biederman <ebiederm@...ssion.com> wrote:
>>>>
>>>> I am squinting and looking this way and that but while I can imagine
>>>> someone more clever than I can think up some unique property of rootfs
>>>> that makes it a little more exploitable than just mounting a ramfs,
>>>> but since you have to be root to exploit those properties I think the
>>>> game is pretty much lost.
>>>
>>> Yes.  rootfs might not be empty, it might have totally insane
>>> permissions, and it's globally shared, which makes it into a wonderful
>>> channel to pass things around that shouldn't be passed around.
>>
>> But if only root with proc mounted can reach it...  I don't know.
>
> It doesn't have to be global root.  It could be userns root.
>
>> There might be a case for setting MNT_LOCKED when we overmount "/"
>> as root but I don't yet see it.
>>
>>> Can non-root do this?  You'd need to be in a userns with a "/" that
>>> isn't MNT_LOCKED.  Can this happen on any normal setup?
>>>
>>> FWIW, I think we should unconditionally MNT_LOCKED the root on userns
>>> unshare, even if it's the only mount.
>>
>> To the best of my knowledge MNT_LOCKED is set uncondintially on userns
>> unshare.
>
> Only if list_empty(&old->mnt_expire), whatever that means, I think.

An autofs or nfs automounted mount.  Can those ever become root?

Eric
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