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

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Date:	Tue, 7 Oct 2014 16:44:45 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	"Eric W. Biederman" <ebiederm@...ssion.com>
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@...r.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

On Tue, Oct 7, 2014 at 4:42 PM, Eric W. Biederman <ebiederm@...ssion.com> wrote:
> 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?

Dunno.

I thought that this code was what set MNT_LOCKED for things that
should be locked:

    /* Don't allow unprivileged users to reveal what is under a mount */
    if ((flag & CL_UNPRIVILEGED) && list_empty(&old->mnt_expire))
        mnt->mnt.mnt_flags |= MNT_LOCKED;

Now I'm confused.  Shouldn't that be checking for submounts?  Is that
what it's doing?

Anyway, I think that this should unconditionally set MNT_LOCKED on the
thing that mounted on rootfs.

--Andy
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