Re: [PATCH] userns: allow privileged user to operate locked mount

[ebiederm@xmission.com (Eric W. Biederman)]

Gao feng <gaofeng@cn.fujitsu.com> writes:

> On 11/15/2013 07:50 AM, Eric W. Biederman wrote:
>> Gao feng <gaofeng@cn.fujitsu.com> writes:
>> 
>>> Privileged user should have rights to mount/umount/move
>>> these even locked mount.
>> 
>> Hmm. This is pretty much a can't happen case, as the only exist in mount
>> namespaces where the global root isn't the root.  How are you getting
>> into this situation?  Using setns() ?
>> 
>
> Before, priviged user can use setns to set his mount namespace to the
> container's mount namespace, and change container's mount directly.
> this patch just gives back host the control of container.

Having thought about this patch a little more I really don't like it.

There are other ways for a privileged user to get around the limitations
when the mount namespace is being created or the mounts are being
propagated.

This approach would require more then a signgle bit of accounting to
work in the nested user namespace case.

The lock says one or several mounts are mounted as a unit and need to
stay that way.

If there are real advantages to splitting things up I might be persuaded
to change my mind.  But right now it looks like you are introducing
extra complexity for a very corner edge case that we don't want to
encourage people to use.

Eric