[rt2x00-users] [rt2800pci (AP) - ath9k] 802.11w: broken aggregation handling?

Andreas Hartmann andihartmann at 01019freenet.de
Tue May 8 14:16:38 EDT 2012


Jouni Malinen wrote:
> On Tue, May 08, 2012 at 08:28:33AM +0200, Andreas Hartmann wrote:
>>> The main requirements:
>>> - support CCMP encryption/decryption of unicast robust management frames
>>>   (subset of Action frames, Deauthentication, Disassociation)
>>
>> I tested WPA-EAP-SHA256 with group key ccmp.
> 
> The key point here is to test whether at least one of those management
> frames gets encrypted and decrypted correctly. Deauthentication frames
> may be easiest for that purpose and you can request the station to
> disconnect to test AP's capability of receiving the frame and then use
> hostapd_cli deauthenticate <addr> command on the AP to request the
> station to be disconnected.

Deauth works fine as long as both ralink devices (AP and STA) use
nowhwcrypt (or both are using hwcrypt - but this does not work with
ath9k STA e.g.).

If one of both runs hwencryption, it doesn't work any more - exactly the
same as with BA session requests.

>> The IGTK is a single key (shared key). There are a maximum of 4 shared
>> keys designated in rt2x00mac.c for each BSS for use with hw encryption.
> 
> BIP uses key indexes 4 and 5 (i.e., the 5th and 6th index).. Anyway,
> this would most likely be handled in software so the main point here is
> to prevent hardware from doing anything additional to the frames.
> 
>> Since rt2800usb is working fine with nohwcrypt=1 (but not with
>> encryption done in hw), I assume, that there is no differentiation
>> between the encryption / decryption of different frame types. If hw
>> encryption is turned on, all types of frames are encrypted / decrypted
>> by hardware and vice versa.
> 
> BIP is kind of special since it doesn't actually even set the Protected
> field in the frame header which may make this easier.. The frames are
> not actually encrypted, i.e., BIP protects only authenticity of the
> frames.

Thanks for this explanation!

>> I'm not sure how BIP is secured if hw encryption is enabled. BIP is
>> implemented in mac80211 as part of decryption. Is BIP done during hw
>> encryption, too? Or is it done by mac80211 w/ enabled hw encryption, too?
> 
> With most drivers, yes, I would expect mac80211 to handle both TX and RX
> side. The driver should just return -EOPNOTSUPP in the set_key() handler
> for WLAN_CIPHER_SUITE_AES_CMAC to leave BIP for mac80211 and CCMP for
> hwardware.

So, this is lower priority for me at the moment :-).

>> This means for rt2x00: to get 11w working with hw encryption enabled,
>> there needs to be some differentiation for the encryption of management
>> frames: if management frame, let mac80211 do the job - all other frames
>> should be encrypted by hw.
>> Correct?
> 
> Well, if you are saying that the issues shows up with unicast robust
> management frames, too, and not just BIP (group addressed robust
> management frames), 

Yes. My primary problem at the moment is the handling of the unicast
robust management frames.

> then you are in problems.. 

yes - that's why I am here :-)

> With good luck, you could
> be able to make the hardware not mess up with unicast robust management
> frames and handle them in mac80211. This may be easier for TX,

How do I exactly recognize this situation? The unicast robust management
frames aren't decrypted with WLAN_CIPHER_SUITE_AES_CMAC. Could they be
given back to mac80211 because of the fact they are management frames?

> but RX
> can be a bit complex.

This seems to be my problem here, too. Sending the deauth from AP
(nohwcrypt=1) can't be handled by STA with hwcrypt enabled.

> It may go to the point of having to use the
> driver to workaround the mess that hardware did (i.e., re-encrypted the
> frame in incorrect way to get back to the correctly encrypted form and
> then sent that to mac80211)..  All this depends on the exact behavior of
> the hardware with a frame that was designed after the hardware was, so
> good luck figuring that out ;-).

Thank you!


Regards,
Andreas


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