[REVIEW][PATCH 1/2] userns: Better restrictions on when proc and sysfs can be mounted

[Andy Lutomirski]

On Thu, Nov 14, 2013 at 3:10 AM, Gao feng <gaofeng at cn.fujitsu.com> wrote:
> On 11/13/2013 03:26 PM, Gao feng wrote:
>> On 11/09/2013 01:42 PM, Eric W. Biederman wrote:
>>> Right now I would rather not have the empty directory exception than
>>> remove this code.
>>>
>>> The test is a little trickier to write than it might otherwise be
>>> because /proc and /sys tend to be slightly imperfect filesystems.
>>>
>>> I think the only way to really test that is to call readdir on the
>>> directory itself :(  I don't like that thought.
>>>
>>> I don't know what I was thinking when I wrote that test but I definitely
>>> goofed up.  Grr!
>>>
>>> I can certainly filter out any directory with nlink > 2.  That would be
>>> an easy partial step forward.
>>>
>>> The real question though is how do I detect directories it is safe to
>>> mount on where there will not be files in them.  I can't call iterate
>>> with the namespace_lock held so things are a bit tricky.
>>>
>>
>> I know this problem is not easy to be resolved. why not let the user
>> make the decision?  maybe we can introduce a new mount option MS_LOCK,
>> if user wants to use mount to hide something, he should use mount with
>> option MS_LOCK. so the unpriviged user can't umount this filesystem and
>> fail to mount the filesystem if one of it's child mount is mounted with
>> MS_LOCK option otherwise he use MS_REC too.
>>
>
> Something like this.
>
> From 437f33ea366623c7a9d557b2e84cae424876a44f Mon Sep 17 00:00:00 2001
> From: Gao feng <gaofeng at cn.fujitsu.com>
> Date: Wed, 13 Nov 2013 16:06:46 +0800
> Subject: [PATCH] userns: introduce new mount option MS_LOCK
>
> After commit 5ff9d8a65ce80efb509ce4e8051394e9ed2cd942
> vfs: Lock in place mounts from more privileged users,
> in userns, the mounts of child mntns which copied from
> parent mntns is locked and user has no rights to umount/move
> them, it's too strict.
>
> The core purpose of above commit is trying to prevent
> unprivileged user from accessing files hidden by mount.
> This patch introduces a new mount option MS_LOCK, this
> gives user the capable to mount filesystem as the type
> of lock if he wants to use mount to hide something.
>

This is bad -- if something was secure in old kernels, it needs to
stay secure.  If you had MS_NOT_A_LOCK, that would be okay, but it
might not solve your problem.

--Andy