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 17:25:34 -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 5:20 PM, Eric W. Biederman <ebiederm@...ssion.com> wrote:
> 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.

Oh.  They're *all* MNT_LOCKED.  Duh.

>
>> 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.

pivot_root is quite broken, as noted in my other email.  It's just not
broken like this, I think. :)

>
> 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



-- 
Andy Lutomirski
AMA Capital Management, LLC
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