wifi: cfg80211/mac80211: correctly parse S1G beacon optional elements

S1G beacons are not traditional beacons but a type of extension frame.
Extension frames contain the frame control and duration fields, followed
by zero or more optional fields before the frame body. These optional
fields are distinct from the variable length elements.

The presence of optional fields is indicated in the frame control field.
To correctly locate the elements offset, the frame control must be parsed
to identify which optional fields are present. Currently, mac80211 parses
S1G beacons based on fixed assumptions about the frame layout, without
inspecting the frame control field. This can result in incorrect offsets
to the "variable" portion of the frame.

Properly parse S1G beacon frames by using the field lengths defined in
IEEE 802.11-2024, section 9.3.4.3, ensuring that the elements offset is
calculated accurately.

Fixes: 9eaffe5078ca ("cfg80211: convert S1G beacon to scan results")
Fixes: cd418ba63f0c ("mac80211: convert S1G beacon to scan results")
Signed-off-by: Lachlan Hodges <[e-mail]>
Link: https://patch.msgid.link/[e-mail]
Signed-off-by: Johannes Berg <[e-mail]>

include/linux/ieee80211.h

···
 
 /* bits unique to S1G beacon */
 #define IEEE80211_S1G_BCN_NEXT_TBTT	0x100
+#define IEEE80211_S1G_BCN_CSSID		0x200
+#define IEEE80211_S1G_BCN_ANO		0x400
 
 /* see 802.11ah-2016 9.9 NDP CMAC frames */
 #define IEEE80211_S1G_1MHZ_NDP_BITS	25
···
 #define IEEE80211_NDP_2M_PREQ_RTYPE_S                  36
 
 #define IEEE80211_ANO_NETTYPE_WILD              15
-
-/* bits unique to S1G beacon */
-#define IEEE80211_S1G_BCN_NEXT_TBTT    0x100
 
 /* control extension - for IEEE80211_FTYPE_CTL | IEEE80211_STYPE_CTL_EXT */
 #define IEEE80211_CTL_EXT_POLL		0x2000
···
 	return (fc & cpu_to_le16(IEEE80211_FCTL_FTYPE |
 				 IEEE80211_FCTL_STYPE)) ==
 	       cpu_to_le16(IEEE80211_FTYPE_EXT | IEEE80211_STYPE_S1G_BEACON);
+}
+
+/**
+ * ieee80211_s1g_has_next_tbtt - check if IEEE80211_S1G_BCN_NEXT_TBTT
+ * @fc: frame control bytes in little-endian byteorder
+ * Return: whether or not the frame contains the variable-length
+ *	next TBTT field
+ */
+static inline bool ieee80211_s1g_has_next_tbtt(__le16 fc)
+{
+	return ieee80211_is_s1g_beacon(fc) &&
+		(fc & cpu_to_le16(IEEE80211_S1G_BCN_NEXT_TBTT));
+}
+
+/**
+ * ieee80211_s1g_has_ano - check if IEEE80211_S1G_BCN_ANO
+ * @fc: frame control bytes in little-endian byteorder
+ * Return: whether or not the frame contains the variable-length
+ *	ANO field
+ */
+static inline bool ieee80211_s1g_has_ano(__le16 fc)
+{
+	return ieee80211_is_s1g_beacon(fc) &&
+		(fc & cpu_to_le16(IEEE80211_S1G_BCN_ANO));
+}
+
+/**
+ * ieee80211_s1g_has_cssid - check if IEEE80211_S1G_BCN_CSSID
+ * @fc: frame control bytes in little-endian byteorder
+ * Return: whether or not the frame contains the variable-length
+ *	compressed SSID field
+ */
+static inline bool ieee80211_s1g_has_cssid(__le16 fc)
+{
+	return ieee80211_is_s1g_beacon(fc) &&
+		(fc & cpu_to_le16(IEEE80211_S1G_BCN_CSSID));
 }
 
 /**
···
 			u8 change_seq;
 			u8 variable[0];
 		} __packed s1g_beacon;
-		struct {
-			u8 sa[ETH_ALEN];
-			__le32 timestamp;
-			u8 change_seq;
-			u8 next_tbtt[3];
-			u8 variable[0];
-		} __packed s1g_short_beacon;
 	} u;
 } __packed __aligned(2);
+
+/**
+ * ieee80211_s1g_optional_len - determine length of optional S1G beacon fields
+ * @fc: frame control bytes in little-endian byteorder
+ * Return: total length in bytes of the optional fixed-length fields
+ *
+ * S1G beacons may contain up to three optional fixed-length fields that
+ * precede the variable-length elements. Whether these fields are present
+ * is indicated by flags in the frame control field.
+ *
+ * From IEEE 802.11-2024 section 9.3.4.3:
+ *  - Next TBTT field may be 0 or 3 bytes
+ *  - Short SSID field may be 0 or 4 bytes
+ *  - Access Network Options (ANO) field may be 0 or 1 byte
+ */
+static inline size_t
+ieee80211_s1g_optional_len(__le16 fc)
+{
+	size_t len = 0;
+
+	if (ieee80211_s1g_has_next_tbtt(fc))
+		len += 3;
+
+	if (ieee80211_s1g_has_cssid(fc))
+		len += 4;
+
+	if (ieee80211_s1g_has_ano(fc))
+		len += 1;
+
+	return len;
+}
 
 #define IEEE80211_TWT_CONTROL_NDP			BIT(0)
 #define IEEE80211_TWT_CONTROL_RESP_MODE			BIT(1)

net/mac80211/mlme.c

···
 	bssid = ieee80211_get_bssid(hdr, len, sdata->vif.type);
 	if (ieee80211_is_s1g_beacon(mgmt->frame_control)) {
 		struct ieee80211_ext *ext = (void *) mgmt;
-
-		if (ieee80211_is_s1g_short_beacon(ext->frame_control))
-			variable = ext->u.s1g_short_beacon.variable;
-		else
-			variable = ext->u.s1g_beacon.variable;
+		variable = ext->u.s1g_beacon.variable +
+			   ieee80211_s1g_optional_len(ext->frame_control);
 	}
 
 	baselen = (u8 *) variable - (u8 *) mgmt;

net/mac80211/scan.c

···
 	struct ieee80211_mgmt *mgmt = (void *)skb->data;
 	struct ieee80211_bss *bss;
 	struct ieee80211_channel *channel;
+	struct ieee80211_ext *ext;
 	size_t min_hdr_len = offsetof(struct ieee80211_mgmt,
 				      u.probe_resp.variable);
 
···
 		return;
 
 	if (ieee80211_is_s1g_beacon(mgmt->frame_control)) {
-		if (ieee80211_is_s1g_short_beacon(mgmt->frame_control))
-			min_hdr_len = offsetof(struct ieee80211_ext,
-					       u.s1g_short_beacon.variable);
-		else
-			min_hdr_len = offsetof(struct ieee80211_ext,
-					       u.s1g_beacon);
+		ext = (struct ieee80211_ext *)mgmt;
+		min_hdr_len =
+			offsetof(struct ieee80211_ext, u.s1g_beacon.variable) +
+			ieee80211_s1g_optional_len(ext->frame_control);
 	}
 
 	if (skb->len < min_hdr_len)

net/wireless/scan.c

···
 	const u8 *ie;
 	size_t ielen;
 	u64 tsf;
+	size_t s1g_optional_len;
 
 	if (WARN_ON(!mgmt))
 		return NULL;
···
 
 	if (ieee80211_is_s1g_beacon(mgmt->frame_control)) {
 		ext = (void *) mgmt;
-		if (ieee80211_is_s1g_short_beacon(mgmt->frame_control))
-			min_hdr_len = offsetof(struct ieee80211_ext,
-					       u.s1g_short_beacon.variable);
-		else
-			min_hdr_len = offsetof(struct ieee80211_ext,
-					       u.s1g_beacon.variable);
+		s1g_optional_len =
+			ieee80211_s1g_optional_len(ext->frame_control);
+		min_hdr_len =
+			offsetof(struct ieee80211_ext, u.s1g_beacon.variable) +
+			s1g_optional_len;
 	} else {
 		/* same for beacons */
 		min_hdr_len = offsetof(struct ieee80211_mgmt,
···
 		const struct ieee80211_s1g_bcn_compat_ie *compat;
 		const struct element *elem;
 
-		if (ieee80211_is_s1g_short_beacon(mgmt->frame_control))
-			ie = ext->u.s1g_short_beacon.variable;
-		else
-			ie = ext->u.s1g_beacon.variable;
-
+		ie = ext->u.s1g_beacon.variable + s1g_optional_len;
 		elem = cfg80211_find_elem(WLAN_EID_S1G_BCN_COMPAT, ie, ielen);
 		if (!elem)
 			return NULL;