wifi: mt76: mt7996: fix out-of-bounds link array access in mt7996_tx()

When mac80211 leaves the link unspecified, mt7996_tx() substitutes the
primary link id of the station or vif. That value is
IEEE80211_LINK_UNSPECIFIED (0xf) until the first link has been added,
and it is then used unchecked to index vif->link_conf[],
mvif->mt76.link[] and sta->link[], all of which hold
IEEE80211_MLD_MAX_NUM_LINKS (15) entries.

Clamp the primary link id to the default link before using it, and use
the clamped value for the link_sta fallback as well.

Fixes: 1609b01 ("wifi: mt76: mt7996: Overwrite unspecified link_id in mt7996_tx()")
Link: https://patch.msgid.link/20260801145334.1166751-10-nbd@nbd.name
Signed-off-by: Felix Fietkau <nbd@nbd.name>

Author: nbd168

drivers/net/wireless/mediatek/mt76/mt7996/main.c

@@ -1515,20 +1515,26 @@ static void mt7996_tx(struct ieee80211_hw *hw,
 	struct ieee80211_vif *vif = info->control.vif;
 	struct mt7996_vif *mvif = vif ? (void *)vif->drv_priv : NULL;
 	struct mt76_wcid *wcid = &dev->mt76.global_wcid;
+	u8 deflink_id = IEEE80211_LINK_UNSPECIFIED;
 	u8 link_id = u32_get_bits(info->control.flags,
 				  IEEE80211_TX_CTRL_MLO_LINK);
 
 	rcu_read_lock();
 
+	if (msta)
+		deflink_id = msta->deflink_id;
+	else if (mvif)
+		deflink_id = mvif->mt76.deflink_id;
+
+	/* the primary link is unset until the first link has been added */
+	if (deflink_id >= IEEE80211_MLD_MAX_NUM_LINKS)
+		deflink_id = 0;
+
 	/* Use primary link_id if the value from mac80211 is set to
 	 * IEEE80211_LINK_UNSPECIFIED.
 	 */
-	if (link_id == IEEE80211_LINK_UNSPECIFIED) {
-		if (msta)
-			link_id = msta->deflink_id;
-		else if (mvif)
-			link_id = mvif->mt76.deflink_id;
-	}
+	if (link_id == IEEE80211_LINK_UNSPECIFIED)
+		link_id = deflink_id;
 
 	if (vif && ieee80211_vif_is_mld(vif)) {
 		struct ieee80211_bss_conf *link_conf;
@@ -1538,7 +1544,7 @@ static void mt7996_tx(struct ieee80211_hw *hw,
 
 			link_sta = rcu_dereference(sta->link[link_id]);
 			if (!link_sta)
-				link_sta = rcu_dereference(sta->link[msta->deflink_id]);
+				link_sta = rcu_dereference(sta->link[deflink_id]);
 
 			if (link_sta) {
 				memcpy(hdr->addr1, link_sta->addr, ETH_ALEN);