Re: Mac80211 : Wpa rekeying issue

[Matthias May <matthias.may@neratec.com>]

On 05/01/16 16:50, Johannes Berg wrote:
> On Tue, 2016-01-05 at 11:54 +0100, Matthias May wrote:
>>
>> Not safe as in "access to stuff which has to be locked", or not safe
>> as
>> in "a CCMP replay attack is possible"?
>> When changing this we argumented that since we are not really
>> connected
>> yet, a CCMP replay attack doesn't really make sense.
>>
>
> It's a bit more complicated than my first look suggested, it seems.
>
> However, I'm not sure what effect your patch is supposed to have.
> You're skipping CCMP replay checking and update when not authorized
> yet, at which point the station isn't receiving frames anyway (though
> they'd be checked for all this, they'd later be discarded).
>
> Once it becomes authorized, you do the checks. However, it never
> becomes unauthorized again, even for rekeying, so for the PTK rekeying
> issue at hand it's pretty much a no-op?
>
afaik it solves the issue that when a STA roames from AP1 to AP2, and 
key material is installed at different times.
We observed encrypted frames which had a wrong CCMP counter. If the STA 
updates it's counter with these frames then depending on the wrong CCMP 
value received, up to a few hundred frames were dropped.
Not exactly the same as rekeying but the effect are pretty similar.

> johannes
>
> PS: the comment in your patch is also wrong:
>
>> +               /* If we are a station update the ccmp counter only when we are
>> +                * authorised. For all other modes always update. */
>> +               if (!rx->sta ||
>> +                   (rx->sta && test_sta_flag(rx->sta, WLAN_STA_AUTHORIZED)) ) {
>
> There's no check for "if we are a station" here.
>
Yeah this doesn't make sense.
Also the check on !rx-sta seems superfluous since it's already checked a 
few lines above.

Regards
Matthias