wifi: mac80211: use RCU-safe iteration in ieee80211_csa_finish

The ieee80211_csa_finish() function currently uses for_each_sdata_link()
to iterate over links of sdata. However, this macro internally uses
wiphy_dereference(), which expects the wiphy->mtx lock to be held.
When ieee80211_csa_finish() is invoked under an RCU read-side critical
section (e.g., under rcu_read_lock()), this leads to a warning from the
RCU debugging framework.

  WARNING: suspicious RCU usage
  net/mac80211/cfg.c:3830 suspicious rcu_dereference_protected() usage!

This warning is triggered because wiphy_dereference() is not safe to use
without holding the wiphy mutex, and it is being used in an RCU context
without the required locking.

Fix this by introducing and using a new macro, for_each_sdata_link_rcu(),
which performs RCU-safe iteration over sdata links using
list_for_each_entry_rcu() and rcu_dereference(). This ensures that the
link pointers are accessed safely under RCU and eliminates the warning.

Fixes: f600832 ("wifi: mac80211: restructure tx profile retrieval for MLO MBSSID")
Signed-off-by: Maharaja Kennadyrajan <[email protected]>
Link: https://patch.msgid.link/[email protected]
[unindent like the non-RCU macro]
Signed-off-by: Johannes Berg <[email protected]>

Author: Maharaja Kennadyrajan

net/mac80211/cfg.c

@@ -3841,7 +3841,7 @@ void ieee80211_csa_finish(struct ieee80211_vif *vif, unsigned int link_id)
 		 */
 		struct ieee80211_link_data *iter;
 
-		for_each_sdata_link(local, iter) {
+		for_each_sdata_link_rcu(local, iter) {
 			if (iter->sdata == sdata ||
 			    rcu_access_pointer(iter->conf->tx_bss_conf) != tx_bss_conf)
 				continue;

net/mac80211/ieee80211_i.h

@@ -1226,6 +1226,21 @@ struct ieee80211_sub_if_data *vif_to_sdata(struct ieee80211_vif *p)
 	if ((_link = wiphy_dereference((_local)->hw.wiphy,		\
 				       ___sdata->link[___link_id])))
 
+/*
+ * for_each_sdata_link_rcu() must be used under RCU read lock.
+ */
+#define for_each_sdata_link_rcu(_local, _link)						\
+	/* outer loop just to define the variables ... */				\
+	for (struct ieee80211_sub_if_data *___sdata = NULL;				\
+	     !___sdata;									\
+	     ___sdata = (void *)~0 /* always stop */)					\
+	list_for_each_entry_rcu(___sdata, &(_local)->interfaces, list)			\
+	if (ieee80211_sdata_running(___sdata))						\
+	for (int ___link_id = 0;							\
+	     ___link_id < ARRAY_SIZE((___sdata)->link);					\
+	     ___link_id++)								\
+	if ((_link = rcu_dereference((___sdata)->link[___link_id])))
+
 #define for_each_link_data(sdata, __link)					\
 	struct ieee80211_sub_if_data *__sdata = sdata;				\
 	for (int __link_id = 0;							\