Re: [PATCH net-next v2 6/9] net: macsec: hardware offloading infrastructure

Igor Russkikh
2019-08-21T09:20:18Z

>
> Talking about packet numbers, can you describe how PN exhaustion is
> handled? I couldn't find much about packet numbers at all in the
> driver patches (I hope the hw doesn't wrap around from 2^32-1 to 0 on
> the same SA). At some point userspace needs to know that we're
> getting close to 2^32 and that it's time to re-key. Since the whole
> TX path of the software implementation is bypassed, it looks like the
> PN (as far as drivers/net/macsec.c is concerned) never increases, so
> userspace can't know when to negotiate a new SA.

I think there should be driver specific implementation of this functionality.
As an example, our macsec HW issues an interrupt towards the host to indicate
PN threshold has reached and it's time for userspace to change the keys.

In contrast, current SW macsec implementation just stops this SA/secy.

> I don't see how this implementation handles non-macsec traffic (on TX,
> that would be packets sent directly through the real interface, for
> example by wpa_supplicant - on RX, incoming MKA traffic for
> wpa_supplicant). Unless I missed something, incoming MKA traffic will
> end up on the macsec interface as well as the lower interface (not
> entirely critical, as long as wpa_supplicant can grab it on the lower
> device, but not consistent with the software implementation). How does
> the driver distinguish traffic that should pass through unmodified
> from traffic that the HW needs to encapsulate and encrypt?

I can comment on our HW engine - where it has special bypass rules
for configured ethertypes. This way macsec engine skips encryption on TX and
passes in RX unencrypted for the selected control packets.

But thats true, realdev driver is hard to distinguish encrypted/unencrypted
packets. In case realdev should make a decision where to put RX packet,
it only may do some heuristic (since after macsec decription all the
macsec related info is dropped. Thats true at least for our HW implementation).

> If you look at IPsec offloading, the networking stack builds up the
> ESP header, and passes the unencrypted data down to the driver. I'm
> wondering if the same would be possible with MACsec offloading: the
> macsec virtual interface adds the header (and maybe a dummy ICV), and
> then the HW does the encryption. In case of HW that needs to add the
> sectag itself, the driver would first strip the headers that the stack
> created. On receive, the driver would recreate a sectag and the macsec
> interface would just skip all verification (decrypt, PN).

I don't think this way is good, as driver have to do per packet header mangling.
That'll harm linerate performance heavily.

Regards,
Igor