wifi: cfg80211/mac80211: reject bad MBSSID elements

Per spec, the maximum value for the MaxBSSID ('n') indicator is 8,
and the minimum is 1 since a multiple BSSID set with just one BSSID
doesn't make sense (the # of BSSIDs is limited by 2^n).

Limit this in the parsing in both cfg80211 and mac80211, rejecting
any elements with an invalid value.

This fixes potentially bad shifts in the processing of these inside
the cfg80211_gen_new_bssid() function later.

I found this during the investigation of CVE-2022-41674 fixed by the
previous patch.

Fixes: 0b8fb8235b ("cfg80211: Parsing of Multiple BSSID information in scanning")
Fixes: 78ac51f815 ("mac80211: support multi-bssid")
Reviewed-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>

net/mac80211/util.c

@@ -1445,6 +1445,8 @@ static size_t ieee802_11_find_bssid_profile(const u8 *start, size_t len,
 	for_each_element_id(elem, WLAN_EID_MULTIPLE_BSSID, start, len) {
 		if (elem->datalen < 2)
 			continue;
+		if (elem->data[0] < 1 || elem->data[0] > 8)
+			continue;
 
 		for_each_element(sub, elem->data + 1, elem->datalen - 1) {
 			u8 new_bssid[ETH_ALEN];

net/wireless/scan.c

@@ -2143,6 +2143,8 @@ static void cfg80211_parse_mbssid_data(struct wiphy *wiphy,
 	for_each_element_id(elem, WLAN_EID_MULTIPLE_BSSID, ie, ielen) {
 		if (elem->datalen < 4)
 			continue;
+		if (elem->data[0] < 1 || (int)elem->data[0] > 8)
+			continue;
 		for_each_element(sub, elem->data + 1, elem->datalen - 1) {
 			u8 profile_len;