cfg80211: validate SSID/MBSSID element ordering assumption

cfg80211: validate SSID/MBSSID element ordering assumption

BugLink: https://bugs.launchpad.net/bugs/1848039
commit 242b0931c1918c56cd1dc5563fd250a3c39b996d upstream.

The code copying the data assumes that the SSID element is
before the MBSSID element, but since the data is untrusted
from the AP, this cannot be guaranteed.

Validate that this is indeed the case and ignore the MBSSID
otherwise, to avoid having to deal with both cases for the
copy of data that should be between them.

Cc: stable@vger.kernel.org
Fixes: 0b8fb8235be8 ("cfg80211: Parsing of Multiple BSSID information in scanning")
Link: https://lore.kernel.org/r/1569009255-I1673911f5eae02964e21bdc11b2bf58e5e207e59@changeid
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Paolo Pisati <paolo.pisati@canonical.com>
Signed-off-by: Kleber Sacilotto de Souza <kleber.souza@canonical.com>