wifi: mac80211: Discard Beacon frames to non-broadcast address

Summary

In the Linux kernel, the following vulnerability has been resolved: wifi: mac80211: Discard Beacon frames to non-broadcast address Beacon frames are required to be sent to the broadcast address, see IEEE Std 802.11-2020, 11.1.3.1 ("The Address 1 field of the Beacon .. frame shall be set to the broadcast address"). A unicast Beacon frame might be used as a targeted attack to get one of the associated STAs to do something (e.g., using CSA to move it to another channel). As such, it is better have strict filtering for this on the received side and discard all Beacon frames that are sent to an unexpected address. This is even more important for cases where beacon protection is used. The current implementation in mac80211 is correctly discarding unicast Beacon frames if the Protected Frame bit in the Frame Control field is set to 0. However, if that bit is set to 1, the logic used for checking for configured BIGTK(s) does not actually work. If the driver does not have logic for dropping unicast Beacon frames with Protected Frame bit 1, these frames would be accepted in mac80211 processing as valid Beacon frames even though they are not protected. This would allow beacon protection to be bypassed. While the logic for checking beacon protection could be extended to cover this corner case, a more generic check for discard all Beacon frames based on A1=unicast address covers this without needing additional changes. Address all these issues by dropping received Beacon frames if they are sent to a non-broadcast address.