ipv4: do not abuse GFP_ATOMIC in inet_netconf_notify_devconf()

BugLink: https://bugs.launchpad.net/bugs/1878232

commit fa17806c upstream.

inet_forward_change() runs with RTNL held.
We are allowed to sleep if required.

If we use __in_dev_get_rtnl() instead of __in_dev_get_rcu(),
we no longer have to use GFP_ATOMIC allocations in
inet_netconf_notify_devconf(), meaning we are less likely to miss
notifications under memory pressure, and wont touch precious memory
reserves either and risk dropping incoming packets.

inet_netconf_get_devconf() can also use GFP_KERNEL allocation.

Fixes: edc9e748 ("rtnl/ipv4: use netconf msg to advertise forwarding status")
Fixes: 9e551110 ("rtnl/ipv4: add support of RTM_GETNETCONF")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Nicolas Dichtel <nicolas.dichtel@6wind.com>
Acked-by: Nicolas Dichtel <nicolas.dichtel@6wind.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Ian May <ian.may@canonical.com>
Signed-off-by: Kleber Sacilotto de Souza <kleber.souza@canonical.com>

net/ipv4/devinet.c

@@ -1814,7 +1814,7 @@ void inet_netconf_notify_devconf(struct net *net, int type, int ifindex,
 	struct sk_buff *skb;
 	int err = -ENOBUFS;
 
-	skb = nlmsg_new(inet_netconf_msgsize_devconf(type), GFP_ATOMIC);
+	skb = nlmsg_new(inet_netconf_msgsize_devconf(type), GFP_KERNEL);
 	if (!skb)
 		goto errout;
 
@@ -1826,7 +1826,7 @@ void inet_netconf_notify_devconf(struct net *net, int type, int ifindex,
 		kfree_skb(skb);
 		goto errout;
 	}
-	rtnl_notify(skb, net, 0, RTNLGRP_IPV4_NETCONF, NULL, GFP_ATOMIC);
+	rtnl_notify(skb, net, 0, RTNLGRP_IPV4_NETCONF, NULL, GFP_KERNEL);
 	return;
 errout:
 	if (err < 0)
@@ -1883,7 +1883,7 @@ static int inet_netconf_get_devconf(struct sk_buff *in_skb,
 	}
 
 	err = -ENOBUFS;
-	skb = nlmsg_new(inet_netconf_msgsize_devconf(-1), GFP_ATOMIC);
+	skb = nlmsg_new(inet_netconf_msgsize_devconf(-1), GFP_KERNEL);
 	if (!skb)
 		goto errout;
 
@@ -2007,16 +2007,16 @@ static void inet_forward_change(struct net *net)
 
 	for_each_netdev(net, dev) {
 		struct in_device *in_dev;
+
 		if (on)
 			dev_disable_lro(dev);
-		rcu_read_lock();
-		in_dev = __in_dev_get_rcu(dev);
+
+		in_dev = __in_dev_get_rtnl(dev);
 		if (in_dev) {
 			IN_DEV_CONF_SET(in_dev, FORWARDING, on);
 			inet_netconf_notify_devconf(net, NETCONFA_FORWARDING,
 						    dev->ifindex, &in_dev->cnf);
 		}
-		rcu_read_unlock();
 	}
 }