Re: mac80211 drops packet with old IV after rekeying

[Johannes Berg <johannes@sipsolutions.net>]

On Sun, 2015-05-17 at 22:49 +0300, Emmanuel Grumbach wrote:

> Yeah - ok. But how come we *already* set the pointer to the new key
> while the HW is still successfully decrypting with the old key. This
> is the point I can' figure out. I'd expect the transmitting side to
> stop using the old key prior to sending the EAPOL (which #triggers the
> set key pointer line). So those 2 lines don't make sense to me:
> 
> >  # set key pointer to new key
> >  * data RX in HW, decrypt w/ old key, PN=999
> 
> After all, the Rx path is serialized all the way through from the air
> to mac80211. The only thing I can think about is that the sending side
> is still using the old key *after* it already sent its EAPOL frames.

Not sure, isn't the key only installed on EAPOL acknowledgement or so?
With PTK rekeying, I'm not really sure what the timing is, and there's
not really any way it can be correct (without extended key ID support.)

> Then, by pure change, we can still decrypt them in HW because the HW
> hasn't been updated yet (these frames are successfully decrypted
> because of race basically) and then, these frames come up to mac80211
> *after* the EAPOL but with the old key.
> This is what the submitter says:
> 
> "
> The encryption key indeed changes immediately after the last packet of
> the handshake, but the Initialization Vector is still counting up
> against the old value.
> "
> 
> So maybe that's the real issue?

You're thinking there's a transmitter issue instead? I can't see how
that could happen, especially that way around ...

Actually, yes, this *could* happen, in exactly the same way as on the
receiver, with hardware crypto that does PN generation in software. Not
on iwlwifi, because we copy the key material into the TX command, but on
any hardware that uses hw_key_idx to identify the key, and could replace
the key:

 * mac80211: encrypt TX frame - assign PN, ((tx_info
*)skb->cb)->hw_key_idx = 7
 * mac80211: replace key with new key
   * driver: remove old key from hw accel (hw_key_idx 7)
   * driver: insert new key to hw accel (reusing hw_key_idx 7)

johannes