Re: NL80211_SCAN_FLAG_RANDOM_ADDR ?

[Denis Kenzior <denkenz@gmail.com>]

Hi Ben,

On 04/11/2019 06:20 PM, Ben Greear wrote:
> On 4/11/19 4:19 PM, Ben Greear wrote:
>> On 4/11/19 3:30 PM, Denis Kenzior wrote:
>>> Hi,
>>>
>>> I've been poking around at how this flag is used and I noticed this 
>>> check in net/wireless/nl80211.c:
>>>
>>> nl80211_check_scan_flags()
>>>
>>>          if (*flags & NL80211_SCAN_FLAG_RANDOM_ADDR) {
>>>                  int err;
>>>
>>>                  if (!(wiphy->features & randomness_flag) ||
>>>                      (wdev && wdev->current_bss))
>>>                          return -EOPNOTSUPP;
>>>
>>>
>>> The above disallows the use of RANDOM_ADDR for scans while connected. 
>>> The nl80211.h uapi header seems to concur:
>>>
>>>   "@NL80211_FEATURE_SCAN_RANDOM_MAC_ADDR: This device/driver supports 
>>> using a random MAC address during scan (if the device is unassociated);"
>>>
>>> However, if I create a P2P Device (in addition to the default STA 
>>> device), the kernel happily lets me scan on the wdev while the STA 
>>> interface is connected.
>>>
>>> sudo iw phy0 interface add p2p type __p2pdev
>>> sudo iw wdev 0x2 p2p start
>>> sudo iw wdev 0x2 scan randomize
>>>
>>> So the immediate question I have is, should the RANDOM_ADDR flag 
>>> indeed be limited to unassociated STA interfaces?  It would seem the 
>>> hardware is capable randomizing even when connected? Please educate 
>>> me :)
>>
>> You can be sure that each driver/hardware has its own bugs and 
>> limitations related to this.
>>
>> Ath10k wave 1 and wave 2 that I am aware of would ignore and/or not 
>> ACK probe responses
>> sent back to an MAC address that is not that of the station itself.  
>> And changing the mac of a station
>> would require complete re-association AFAIK.  That is likely just one 
>> of the many issues.

Yes, I understand that some hardware would not support this.  But the 
question is does this check belong at the nl80211 layer (e.g. no 
hardware can do this) vs somewhere at the driver layer + additional 
feature bit as needed.

> 
> I should add:  If you really want to scan in this manner, you could just 
> create a new station vdev with
> random addr and have it do the scanning, then delete it when done?  The 
> original station will continue on
> its way unmolested.
> 

So you mean something like:
sudo iw phy0 interface add sta2 type station
sudo iw dev sta2 scan randomize
command failed: Network is down (-100)
sudo ifconfig sta2 up
SIOCSIFFLAGS: Device or resource busy

I guess I'm running into this:

	valid interface combinations:
		 * #{ managed } <= 1, #{ AP, P2P-client, P2P-GO } <= 1, #{ P2P-device 
} <= 1,
		   total <= 3, #channels <= 2

Or did you mean something else?

Regards,
-Denis