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

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Message-ID: <87vbnvif9e.fsf@x220.int.ebiederm.org>
Date:	Tue, 07 Oct 2014 17:20:13 -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 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:

It does.

>     /* 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?

As it copies each mount (mostly submounts) it sets MNT_LOCKED.

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

As I read the code mnt_set_expiry is only for nfs, cifs, and afs
submounts (and perhaps proc should use them). So as they are generated
mnt_expiry should never start on the root of filesystem of the mount tree.

Furthermore mnt_expiry is cleared when a mount is moved, and when
it is bind mounted, or propagated.

pivot_root does look as though it may be missing the proper mnt_expiry
handling list_del_init(&old->mnt_expire), but pivot_root does have
proper MNT_LOCKED handling.

Looking that test was slightly off and it should be:
	if ((flag & CL_UNPRIVILEGED) &&
	    (!(flag & CL_EXPIRE) || list_empty(&old->mnt_expire))
		mnt->mnt.mnt_flags |= MNT_LOCKED;

So on that note we might clear CL_EXPIRE when CL_UNPRIVILEGED is set
in copy_tree but I don't see that as being really needed.

Eric
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