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

[Gao feng <gaofeng@cn.fujitsu.com>]

On 11/15/2013 12:54 AM, Andy Lutomirski wrote:
> On Thu, Nov 14, 2013 at 3:10 AM, Gao feng <gaofeng@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@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.
> 

what you mean old kernels here? I saw patch "vfs: Lock in place mounts from more privileged users"
is merged into upstream in linux 3.12-rc1, this is not very old. I think there
are not many userspace processes rely on this feature.

If user think host needs to be secure, he should use MS_LOCK to mount filesystem.
we can't make decison for user.

Thanks