genetlink: hold RCU in genlmsg_mcast()

While running net selftests with CONFIG_PROVE_RCU_LIST=y I saw
one lockdep splat [1].

genlmsg_mcast() uses for_each_net_rcu(), and must therefore hold RCU.

Instead of letting all callers guard genlmsg_multicast_allns()
with a rcu_read_lock()/rcu_read_unlock() pair, do it in genlmsg_mcast().

This also means the @flags parameter is useless, we need to always use
GFP_ATOMIC.

[1]
[10882.424136] =============================
[10882.424166] WARNING: suspicious RCU usage
[10882.424309] 6.12.0-rc2-virtme #1156 Not tainted
[10882.424400] -----------------------------
[10882.424423] net/netlink/genetlink.c:1940 RCU-list traversed in non-reader section!!
[10882.424469]
other info that might help us debug this:

[10882.424500]
rcu_scheduler_active = 2, debug_locks = 1
[10882.424744] 2 locks held by ip/15677:
[10882.424791] #0: ffffffffb6b491b0 (cb_lock){++++}-{3:3}, at: genl_rcv (net/netlink/genetlink.c:1219)
[10882.426334] #1: ffffffffb6b49248 (genl_mutex){+.+.}-{3:3}, at: genl_rcv_msg (net/netlink/genetlink.c:61 net/netlink/genetlink.c:57 net/netlink/genetlink.c:1209)
[10882.426465]
stack backtrace:
[10882.426805] CPU: 14 UID: 0 PID: 15677 Comm: ip Not tainted 6.12.0-rc2-virtme #1156
[10882.426919] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.16.3-debian-1.16.3-2 04/01/2014
[10882.427046] Call Trace:
[10882.427131] <TASK>
[10882.427244] dump_stack_lvl (lib/dump_stack.c:123)
[10882.427335] lockdep_rcu_suspicious (kernel/locking/lockdep.c:6822)
[10882.427387] genlmsg_multicast_allns (net/netlink/genetlink.c:1940 (discriminator 7) net/netlink/genetlink.c:1977 (discriminator 7))
[10882.427436] l2tp_tunnel_notify.constprop.0 (net/l2tp/l2tp_netlink.c:119) l2tp_netlink
[10882.427683] l2tp_nl_cmd_tunnel_create (net/l2tp/l2tp_netlink.c:253) l2tp_netlink
[10882.427748] genl_family_rcv_msg_doit (net/netlink/genetlink.c:1115)
[10882.427834] genl_rcv_msg (net/netlink/genetlink.c:1195 net/netlink/genetlink.c:1210)
[10882.427877] ? __pfx_l2tp_nl_cmd_tunnel_create (net/l2tp/l2tp_netlink.c:186) l2tp_netlink
[10882.427927] ? __pfx_genl_rcv_msg (net/netlink/genetlink.c:1201)
[10882.427959] netlink_rcv_skb (net/netlink/af_netlink.c:2551)
[10882.428069] genl_rcv (net/netlink/genetlink.c:1220)
[10882.428095] netlink_unicast (net/netlink/af_netlink.c:1332 net/netlink/af_netlink.c:1357)
[10882.428140] netlink_sendmsg (net/netlink/af_netlink.c:1901)
[10882.428210] ____sys_sendmsg (net/socket.c:729 (discriminator 1) net/socket.c:744 (discriminator 1) net/socket.c:2607 (discriminator 1))

Fixes: 33f72e6f0c67 ("l2tp : multicast notification to the registered listeners")
Signed-off-by: Eric Dumazet <[e-mail]>
Cc: James Chapman <[e-mail]>
Cc: Tom Parkin <[e-mail]>
Cc: Johannes Berg <[e-mail]>
Link: https://patch.msgid.link/[e-mail]
Signed-off-by: Jakub Kicinski <[e-mail]>

drivers/target/target_core_user.c

···
 	}
 
 	ret = genlmsg_multicast_allns(&tcmu_genl_family, skb, 0,
-				      TCMU_MCGRP_CONFIG, GFP_KERNEL);
+				      TCMU_MCGRP_CONFIG);
 
 	/* Wait during an add as the listener may not be up yet */
 	if (ret == 0 ||

include/net/genetlink.h

···
  * @skb: netlink message as socket buffer
  * @portid: own netlink portid to avoid sending to yourself
  * @group: offset of multicast group in groups array
- * @flags: allocation flags
  *
  * This function must hold the RTNL or rcu_read_lock().
  */
 int genlmsg_multicast_allns(const struct genl_family *family,
 			    struct sk_buff *skb, u32 portid,
-			    unsigned int group, gfp_t flags);
+			    unsigned int group);
 
 /**
  * genlmsg_unicast - unicast a netlink message

net/l2tp/l2tp_netlink.c

···
 				  NLM_F_ACK, tunnel, cmd);
 
 	if (ret >= 0) {
-		ret = genlmsg_multicast_allns(family, msg, 0, 0, GFP_ATOMIC);
+		ret = genlmsg_multicast_allns(family, msg, 0, 0);
 		/* We don't care if no one is listening */
 		if (ret == -ESRCH)
 			ret = 0;
···
 				   NLM_F_ACK, session, cmd);
 
 	if (ret >= 0) {
-		ret = genlmsg_multicast_allns(family, msg, 0, 0, GFP_ATOMIC);
+		ret = genlmsg_multicast_allns(family, msg, 0, 0);
 		/* We don't care if no one is listening */
 		if (ret == -ESRCH)
 			ret = 0;

net/netlink/genetlink.c

···
 	if (IS_ERR(msg))
 		return PTR_ERR(msg);
 
-	if (!family->netnsok) {
+	if (!family->netnsok)
 		genlmsg_multicast_netns(&genl_ctrl, &init_net, msg, 0,
 					0, GFP_KERNEL);
-	} else {
-		rcu_read_lock();
-		genlmsg_multicast_allns(&genl_ctrl, msg, 0,
-					0, GFP_ATOMIC);
-		rcu_read_unlock();
-	}
+	else
+		genlmsg_multicast_allns(&genl_ctrl, msg, 0, 0);
 
 	return 0;
 }
···
 
 core_initcall(genl_init);
 
-static int genlmsg_mcast(struct sk_buff *skb, u32 portid, unsigned long group,
-			 gfp_t flags)
+static int genlmsg_mcast(struct sk_buff *skb, u32 portid, unsigned long group)
 {
 	struct sk_buff *tmp;
 	struct net *net, *prev = NULL;
 	bool delivered = false;
 	int err;
 
+	rcu_read_lock();
 	for_each_net_rcu(net) {
 		if (prev) {
-			tmp = skb_clone(skb, flags);
+			tmp = skb_clone(skb, GFP_ATOMIC);
 			if (!tmp) {
 				err = -ENOMEM;
 				goto error;
 			}
 			err = nlmsg_multicast(prev->genl_sock, tmp,
-					      portid, group, flags);
+					      portid, group, GFP_ATOMIC);
 			if (!err)
 				delivered = true;
 			else if (err != -ESRCH)
···
 
 		prev = net;
 	}
+	err = nlmsg_multicast(prev->genl_sock, skb, portid, group, GFP_ATOMIC);
 
-	err = nlmsg_multicast(prev->genl_sock, skb, portid, group, flags);
+	rcu_read_unlock();
+
 	if (!err)
 		delivered = true;
 	else if (err != -ESRCH)
 		return err;
 	return delivered ? 0 : -ESRCH;
  error:
+	rcu_read_unlock();
+
 	kfree_skb(skb);
 	return err;
 }
 
 int genlmsg_multicast_allns(const struct genl_family *family,
 			    struct sk_buff *skb, u32 portid,
-			    unsigned int group, gfp_t flags)
+			    unsigned int group)
 {
 	if (WARN_ON_ONCE(group >= family->n_mcgrps))
 		return -EINVAL;
 
 	group = family->mcgrp_offset + group;
-	return genlmsg_mcast(skb, portid, group, flags);
+	return genlmsg_mcast(skb, portid, group);
 }
 EXPORT_SYMBOL(genlmsg_multicast_allns);
 

net/wireless/nl80211.c

···
 
 	genlmsg_end(msg, hdr);
 
-	rcu_read_lock();
 	genlmsg_multicast_allns(&nl80211_fam, msg, 0,
-				NL80211_MCGRP_REGULATORY, GFP_ATOMIC);
-	rcu_read_unlock();
+				NL80211_MCGRP_REGULATORY);
 
 	return;
 
···
 
 	genlmsg_end(msg, hdr);
 
-	rcu_read_lock();
 	genlmsg_multicast_allns(&nl80211_fam, msg, 0,
-				NL80211_MCGRP_REGULATORY, GFP_ATOMIC);
-	rcu_read_unlock();
+				NL80211_MCGRP_REGULATORY);
 
 	return;