Re: [RFC PATCH 2/3] netdev: kernel-only IFF_HIDDEN netdevice

[Siwei Liu <loseweigh@gmail.com>]

(put discussions back on list which got accidentally removed)

On Tue, Apr 3, 2018 at 4:08 PM, Stephen Hemminger
<stephen@networkplumber.org> wrote:
> On Tue, 3 Apr 2018 12:04:38 -0700
> Siwei Liu <loseweigh@gmail.com> wrote:
>
>> On Tue, Apr 3, 2018 at 10:35 AM, Stephen Hemminger
>> <stephen@networkplumber.org> wrote:
>> > On Sun,  1 Apr 2018 05:13:09 -0400
>> > Si-Wei Liu <si-wei.liu@oracle.com> wrote:
>> >
>> >> Hidden netdevice is not visible to userspace such that
>> >> typical network utilites e.g. ip, ifconfig and et al,
>> >> cannot sense its existence or configure it. Internally
>> >> hidden netdev may associate with an upper level netdev
>> >> that userspace has access to. Although userspace cannot
>> >> manipulate the lower netdev directly, user may control
>> >> or configure the underlying hidden device through the
>> >> upper-level netdev. For identification purpose, the
>> >> kobject for hidden netdev still presents in the sysfs
>> >> hierarchy, however, no uevent message will be generated
>> >> when the sysfs entry is created, modified or destroyed.
>> >>
>> >> For that end, a separate namescope needs to be carved
>> >> out for IFF_HIDDEN netdevs. As of now netdev name that
>> >> starts with colon i.e. ':' is invalid in userspace,
>> >> since socket ioctls such as SIOCGIFCONF use ':' as the
>> >> separator for ifname. The absence of namescope started
>> >> with ':' can rightly be used as the namescope for
>> >> the kernel-only IFF_HIDDEN netdevs.
>> >>
>> >> Signed-off-by: Si-Wei Liu <si-wei.liu@oracle.com>
>> >> ---
>> >
>> > I understand the use case. I proposed using . as a prefix before
>> > but that ran into resistance. Using colon seems worse.
>>
>> Using dot (.) can't be good because it would cause namespace collision
>> and thus breaking apps when you hide the device. Imagine a user really
>> wants to add a link with the same name as the one hidden and it starts
>> with a dot. It would fail, and users don't know its just because the
>> name starts with dot. IMHO users should be agnostic of (the namespace
>> of) hidden device at all if what they pick is a valid name.
>>
>> ":" is an invalid prefix to userspace, there's no such problem if
>> being used to construct the namescope for hidden devices.
>>
>> However, technically, just as what I alluded to in the reply earlier,
>> it might really be consistent to put this under a separeate namespace
>> instead than fiddling with name prefix. But I am just not sure if that
>> is a big hammer and would like to earn enough feedback and attention
>> before going that way too quickly.
>>
>>
>> >
>> > Rather than playing with names and all the issues that can cause,
>> > why not make it an attribute flag of the device in netlink.
>>
>> Atrribute flag doesn't help. It's a matter of namespace.
>>
>> Regards,
>> -Siwei
>
> In Vyatta, we used names like ".isatap" for devices that would clutter up
> the user experience. They are naturally not visible by simple scans of
> /sys/class/net, and there was a patch to ignore them in iproute2.
> It was a hack which worked but not really worth upstreaming.
>
> The question is if this a security feature then it needs to be more

I don't expect the namespace to be a security aspect of feature, but
rather a way to make old userspace unmodified  to work with a new
feature. And, we're going to add API to expose the netdev info for the
invisible IFF_AUTO_MANAGED links anyway. We don't need to make it
secure and all hidden under the dark to be honest.

> robust than just name prefix. Plus it took years to handle network
> namespaces everywhere; this kind of flag would start same problems.
>
> Network namespaces work but have the problem namespaces only weakly
> support hierarchy and nesting. I prefer the namespace approach
> because it fits better and has less impact.

Great, thanks!

-Siwei