Re: [PATCH RFC] allow some kernel filesystems to be mounted in a user namespace

[Andy Lutomirski <luto@amacapital.net>]

On 07/16/2013 12:50 PM, Serge E. Hallyn wrote:
> Quoting Al Viro (viro@ZenIV.linux.org.uk):
>> On Tue, Jul 16, 2013 at 02:29:20PM -0500, Serge Hallyn wrote:
>>> All the files will be owned by host root, so there's no security
>>> concern in allowing this.
>>
>> Files owned by root != very bad things can't be done by non-root.
>> Especially for debugfs, which is very much a "don't even think about
>> mounting that on a production box" thing...
> 
> I would prefer it not be mounted.  But near as I can tell there
> should be no regression security-wise whether an unprivileged
> user on the host has access to it, or whether a user in a
> non-init user ns is allowed to mount it.  (Obviously I could very
> well be wrong)

I would argue that either (a) debugfs denies everything to non-root, so
mounting it in a (rootless) userns is useless or (b) it doesn't, in
which case it's dangerous.

In neither case does it make sense to me to allow the mount.

--Andy