net: Fix data-races around sysctl_devconf_inherit_init_net.

While reading sysctl_devconf_inherit_init_net, it can be changed
concurrently.  Thus, we need to add READ_ONCE() to its readers.

Fixes: 856c395c ("net: introduce a knob to control whether to inherit devconf config")
Signed-off-by: NKuniyuki Iwashima <kuniyu@amazon.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

include/linux/netdevice.h

@@ -650,6 +650,15 @@ static inline bool net_has_fallback_tunnels(const struct net *net)
 #endif
 }
 
+static inline int net_inherit_devconf(void)
+{
+#if IS_ENABLED(CONFIG_SYSCTL)
+	return READ_ONCE(sysctl_devconf_inherit_init_net);
+#else
+	return 0;
+#endif
+}
+
 static inline int netdev_queue_numa_node_read(const struct netdev_queue *q)
 {
 #if defined(CONFIG_XPS) && defined(CONFIG_NUMA)

net/ipv4/devinet.c

@@ -2682,23 +2682,27 @@ static __net_init int devinet_init_net(struct net *net)
 #endif
 
 	if (!net_eq(net, &init_net)) {
-		if (IS_ENABLED(CONFIG_SYSCTL) &&
-		    sysctl_devconf_inherit_init_net == 3) {
+		switch (net_inherit_devconf()) {
+		case 3:
 			/* copy from the current netns */
 			memcpy(all, current->nsproxy->net_ns->ipv4.devconf_all,
 			       sizeof(ipv4_devconf));
 			memcpy(dflt,
 			       current->nsproxy->net_ns->ipv4.devconf_dflt,
 			       sizeof(ipv4_devconf_dflt));
-		} else if (!IS_ENABLED(CONFIG_SYSCTL) ||
-			   sysctl_devconf_inherit_init_net != 2) {
-			/* inherit == 0 or 1: copy from init_net */
+			break;
+		case 0:
+		case 1:
+			/* copy from init_net */
 			memcpy(all, init_net.ipv4.devconf_all,
 			       sizeof(ipv4_devconf));
 			memcpy(dflt, init_net.ipv4.devconf_dflt,
 			       sizeof(ipv4_devconf_dflt));
+			break;
+		case 2:
+			/* use compiled values */
+			break;
 		}
-		/* else inherit == 2: use compiled values */
 	}
 
 #ifdef CONFIG_SYSCTL

net/ipv6/addrconf.c

@@ -7162,9 +7162,8 @@ static int __net_init addrconf_init_net(struct net *net)
 	if (!dflt)
 		goto err_alloc_dflt;
 
-	if (IS_ENABLED(CONFIG_SYSCTL) &&
-	    !net_eq(net, &init_net)) {
-		switch (sysctl_devconf_inherit_init_net) {
+	if (!net_eq(net, &init_net)) {
+		switch (net_inherit_devconf()) {
 		case 1:  /* copy from init_net */
 			memcpy(all, init_net.ipv6.devconf_all,
 			       sizeof(ipv6_devconf));