[PATCH] wifi: mac80211: Fix ieee80211_convert_to_unicast() logic

[Felix Fietkau]

On 16.08.24 16:30, Jeff Johnson wrote:
> On 8/16/2024 4:31 AM, Toke Høiland-Jørgensen wrote:
>> Jeff Johnson <quic_jjohnson at quicinc.com> writes:
>> 
>>> The current logic in ieee80211_convert_to_unicast() uses skb_clone()
>>> to obtain an skb for each individual destination of a multicast
>>> frame, and then updates the destination address in the cloned skb's
>>> data buffer before placing that skb on the provided queue.
>>>
>>> This logic is flawed since skb_clone() shares the same data buffer
>>> with the original and the cloned skb, and hence each time the
>>> destination address is updated, it overwrites the previous destination
>>> address in this shared buffer. As a result, due to the special handing
>>> of the first valid destination, all of the skbs will eventually be
>>> sent to that first destination.
>> 
>> Did you actually observe this happen in practice? ieee80211_change_da()
>> does an skb_ensure_writable() check on the Ethernet header before
>> writing it, so AFAICT it does not, in fact, overwrite the data of the
>> original frame.
> 
> I'm proxying this change for our internal team, and they have observed that
> unicast frames are not being sent to the separate STAs.
> 
> In response to your reply I went through the code again and it seems the
> manner in which this functionality fails isn't as it was described to me.
> 
> Instead this functionality fails because we'd fail on the first
> ieee80211_change_da() call and hence goto multicast and where only the
> original skb would be queued and transmitted as a multicast frame
> 
> So the original logic is still faulty, only the actual faulty behavior is not
> being described correctly: instead of sending multiple unicast frames to the
> same STA we'd send a single multicast frame.

While I agree with switching over to skb_copy (or maybe pskb_copy to 
reduce realloc on fragmented skbs), it's still not clear to me why 
ieee80211_change_da fails. It should detect that the packet was cloned, 
letting pskb_expand_head reallocate the head of the skb.

Please run some more tests to figure out the exact reason for the 
failure. There might be another hidden bug lurking there, which would 
get papered over by this change.

- Felix