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

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Date:	Thu, 14 Nov 2013 17:38:48 -0800
From:	ebiederm@...ssion.com (Eric W. Biederman)
To:	Gao feng <gaofeng@...fujitsu.com>
Cc:	containers@...ts.linux-foundation.org,
	linux-fsdevel@...r.kernel.org, serge@...lyn.com,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] userns: allow privileged user to operate locked mount

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

> On 11/15/2013 07:50 AM, Eric W. Biederman wrote:
>> Gao feng <gaofeng@...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
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