ieee80211: add helper to check HE capability element size

This element has a very dynamic structure, create a small helper
function to validate its size. We're currently checking it in
mac80211 in a conversion function, but that's actually slightly
buggy.

Link: https://lore.kernel.org/r/20220214172920.750bee9eaf37.Ie18359bd38143b7dc949078f10752413e6d36854@changeidSigned-off-by: Johannes Berg <johannes.berg@intel.com>

include/linux/ieee80211.h

@@ -9,7 +9,7 @@
  * Copyright (c) 2006, Michael Wu <flamingice@sourmilk.net>
  * Copyright (c) 2013 - 2014 Intel Mobile Communications GmbH
  * Copyright (c) 2016 - 2017 Intel Deutschland GmbH
- * Copyright (c) 2018 - 2021 Intel Corporation
+ * Copyright (c) 2018 - 2022 Intel Corporation
  */
 
 #ifndef LINUX_IEEE80211_H
@@ -2338,6 +2338,29 @@ ieee80211_he_ppe_size(u8 ppe_thres_hdr, const u8 *phy_cap_info)
 	return n;
 }
 
+static inline bool ieee80211_he_capa_size_ok(const u8 *data, u8 len)
+{
+	const struct ieee80211_he_cap_elem *he_cap_ie_elem = (const void *)data;
+	u8 needed = sizeof(*he_cap_ie_elem);
+
+	if (len < needed)
+		return false;
+
+	needed += ieee80211_he_mcs_nss_size(he_cap_ie_elem);
+	if (len < needed)
+		return false;
+
+	if (he_cap_ie_elem->phy_cap_info[6] &
+			IEEE80211_HE_PHY_CAP6_PPE_THRESHOLD_PRESENT) {
+		if (len < needed + 1)
+			return false;
+		needed += ieee80211_he_ppe_size(data[needed],
+						he_cap_ie_elem->phy_cap_info);
+	}
+
+	return len >= needed;
+}
+
 /* HE Operation defines */
 #define IEEE80211_HE_OPERATION_DFLT_PE_DURATION_MASK		0x00000007
 #define IEEE80211_HE_OPERATION_TWT_REQUIRED			0x00000008