nl80211: use RCU to read regdom in reg get/dump

Use RCU here to read the regdomain, this will allow us
to remove the RTNL locking from the setter.

Note in nl80211_get_reg_do() we still need the RTNL to
do the wiphy lookup.

Link: https://lore.kernel.org/r/20220214101820.5d4acbcf2a46.Ibfc91980439862125e983d9adeebaba73fe38e2d@changeidSigned-off-by: Johannes Berg <johannes.berg@intel.com>

net/wireless/nl80211.c

@@ -7948,6 +7948,7 @@ static int nl80211_get_reg_do(struct sk_buff *skb, struct genl_info *info)
 	struct cfg80211_registered_device *rdev;
 	struct wiphy *wiphy = NULL;
 	struct sk_buff *msg;
+	int err = -EMSGSIZE;
 	void *hdr;
 
 	msg = nlmsg_new(NLMSG_DEFAULT_SIZE, GFP_KERNEL);
@@ -7966,34 +7967,35 @@ static int nl80211_get_reg_do(struct sk_buff *skb, struct genl_info *info)
 
 		rdev = cfg80211_get_dev_from_info(genl_info_net(info), info);
 		if (IS_ERR(rdev)) {
-			nlmsg_free(msg);
-			rtnl_unlock();
-			return PTR_ERR(rdev);
+			err = PTR_ERR(rdev);
+			goto nla_put_failure;
 		}
 
 		wiphy = &rdev->wiphy;
 		self_managed = wiphy->regulatory_flags &
 			       REGULATORY_WIPHY_SELF_MANAGED;
+
+		rcu_read_lock();
+
 		regdom = get_wiphy_regdom(wiphy);
 
 		/* a self-managed-reg device must have a private regdom */
 		if (WARN_ON(!regdom && self_managed)) {
-			nlmsg_free(msg);
-			rtnl_unlock();
-			return -EINVAL;
+			err = -EINVAL;
+			goto nla_put_failure_rcu;
 		}
 
 		if (regdom &&
 		    nla_put_u32(msg, NL80211_ATTR_WIPHY, get_wiphy_idx(wiphy)))
-			goto nla_put_failure;
+			goto nla_put_failure_rcu;
+	} else {
+		rcu_read_lock();
 	}
 
 	if (!wiphy && reg_last_request_cell_base() &&
 	    nla_put_u32(msg, NL80211_ATTR_USER_REG_HINT_TYPE,
 			NL80211_USER_REG_HINT_CELL_BASE))
-		goto nla_put_failure;
-
-	rcu_read_lock();
+		goto nla_put_failure_rcu;
 
 	if (!regdom)
 		regdom = rcu_dereference(cfg80211_regdomain);
@@ -8013,7 +8015,7 @@ static int nl80211_get_reg_do(struct sk_buff *skb, struct genl_info *info)
 	rtnl_unlock();
 put_failure:
 	nlmsg_free(msg);
-	return -EMSGSIZE;
+	return err;
 }
 
 static int nl80211_send_regdom(struct sk_buff *msg, struct netlink_callback *cb,
@@ -8059,19 +8061,19 @@ static int nl80211_get_reg_dump(struct sk_buff *skb,
 	struct cfg80211_registered_device *rdev;
 	int err, reg_idx, start = cb->args[2];
 
-	rtnl_lock();
+	rcu_read_lock();
 
 	if (cfg80211_regdomain && start == 0) {
 		err = nl80211_send_regdom(skb, cb, cb->nlh->nlmsg_seq,
 					  NLM_F_MULTI, NULL,
-					  rtnl_dereference(cfg80211_regdomain));
+					  rcu_dereference(cfg80211_regdomain));
 		if (err < 0)
 			goto out_err;
 	}
 
 	/* the global regdom is idx 0 */
 	reg_idx = 1;
-	list_for_each_entry(rdev, &cfg80211_rdev_list, list) {
+	list_for_each_entry_rcu(rdev, &cfg80211_rdev_list, list) {
 		regdom = get_wiphy_regdom(&rdev->wiphy);
 		if (!regdom)
 			continue;
@@ -8090,7 +8092,7 @@ static int nl80211_get_reg_dump(struct sk_buff *skb,
 	cb->args[2] = reg_idx;
 	err = skb->len;
 out_err:
-	rtnl_unlock();
+	rcu_read_unlock();
 	return err;
 }