Re: [RFC] mac80211: add support for split-MAC implementations

[Johan Almbladh <ja@anyfi.net>]

On Fri, Aug 9, 2013 at 3:07 PM, Johannes Berg <johannes@sipsolutions.net> wrote:
>
> On Thu, 2013-08-08 at 15:21 +0200, Johan Almbladh wrote:
> > This patch enables power save processing for encrypted frames even if the
> > encryption key is not set. This is a requirement when implementing split-MAC
> > systems like Anyfi.net [1] and CAPWAP [2] on mac80211 using monitor frame
> > injection and reception.
>
> I have no idea what these are, nor do I actually want to care much...

Anyfi.net is a dynamic split-mac system where the security part of the
802.11 stack is located in your home router and the realtime part is
handled by any router or AP that happens to be near your current
location. The two parts are connected dynamically via a UDP tunnel
that carries raw encrypted 802.11 frames, forming a complete 802.11
stack that provides your home Wi-Fi wherever you are. In a community
Wi-Fi deployment, the users get secure Wi-Fi access with automatic
sign-on via their home Wi-Fi network which is simply available
everywhere.

Should you find the concept interesting there is quite extensive
technical documentation at http://anyfi.net/documentation#architecture
:-)


> You presumably use Felix's active monitor mode?

I use a monitor socket in a userspace daemon. The daemon receives
encrypted 802.11 frames with radiotap encapsulation on this monitor
socket. It also injects encrypted 802.11 frames with radiotap
encapsulation by transmitting them on the same socket. I believe this
is the way hostapd used to handle transmission of management and EAPOL
frames before they switched to nl80211.


> > The mac80211 RX handlers are reordered slightly so
> > that the power save handler is invoked before the decryption handler.
> >
> > The patch is minimal in the sense that it provides the required functionality
> > with a minimal change, but I am open to suggestions if this change is too
> > intrusive. Please let me know what you think.
>
> I think you should ask yourself if this makes sense in the normal wifi
> context... :-)

You are right about that, but I think this little feature can be added
without affecting the normal operation :-) To be honest, mac80211 has
all the interfaces required for any split-mac implementation, thanks
to the mac80211/hostapd partitioning. The *only* thing missing is the
ability to handle AP power save processing without handling the
encryption...


> It actually seems like it *does* make sense, so it should have an
> appropriate description for that, but I'm a bit worried about IBSS in
> sta_process.

The patch enables power save processing even if there is no unicast
key set, but *also* if key is set but the decryption failed. This is
what I meant with "intrusive". The IBSS updating in sta_process will
also run in this case, but the STA is still required to be authorized.

It's possible to narrow it down to only affect the case where no
encryption key is set:

* Keep the RX handlers in their original order
* Don't drop frames where rx->key is NULL in ieee80211_rx_h_decrypt.
Instead, mark the frame with a flag
* Drop any marked frames at the end of ieee80211_rx_h_sta_process with
RX_DROP_MONITOR

I can prepare a new patch if you prefer this solution.

> Also I've tried to keep the code in the file sequential, so this patch
> should be moving ieee80211_rx_h_decrypt() itself as well.

I'll make sure to put them in the right order.

Johan