Re: [PATCH v2] mac80211: Fix wlan freezes under load at rekey

[Johannes Berg <johannes@sipsolutions.net>]

On Tue, 2018-05-15 at 12:22 +0200, Alexander Wetzel wrote:
> Rekeying a pairwise key with encryption offload and only keyid 0 has two
> potential races which can freeze the wlan conection till rekeyed again:
> 
>  1) For incomming packets:
>     If the local STA installs the key prior to the remote STA we still
>     have the old key active in the hardware for a short time after
>     mac80211 switched to the new key.
>     The card can still hand over packets decoded with the old key to
>     mac80211, bumping the new PN (IV) value to an incorrect high number and
>     tricking the local replay detection to drop all packets really sent
>     with the new key.
> 
>  2) For outgoing packets:
>     If mac80211 is providing the PN (IV) and hands over the cleartext
>     packets for encryption to the hardware immediately prior to a key
>     change the driver/card may process the queued packets after
>     switching to the new key.
>     This will immediatelly bump the PN (IV) value on the remote STA to
>     an incorrect high number, also freezing the connection.
> 
> Both issues can be prevented by first replacing the key in the HW and
> makeing sure no aggregation sessions are running during the rekey.

Getting back to this, am I understanding correctly that in the latter
(outgoing) case this would cause 

Also, I think you should probably describe better why the aggregation
session stuff is needed. I'm already thinking there times about it again
...


> +               ieee80211_sta_tear_down_BA_sessions(
> +                   sta, AGG_STOP_LOCAL_REQUEST);

minor indentation issue here

> +       ieee80211_flush_queues(key->local, key->sdata, false);
> +       ieee80211_key_disable_hw_accel(key);

I'm not sure all drivers implement drv_flush() [correctly], what happens
if they don't? I guess some packets end up being transmitted in clear
text or a dummy key, unless the hardware/firmware knows about this and
drops them?

Perhaps that means we should make this whole thing opt-in, and leave it
up to driver authors to first validate that they handle the flushing
correctly?


Regarding the code: the whole dance you do with ieee80211_key_link() and
ieee80211_key_replace() seems to be a little pointless because you still
add the key to debugfs and then free it, on errors that is?

johannes