net: ipv4: add capability check for net administration

Root in init user namespace can modify /proc/sys/net/ipv4/ip_forward
without CAP_NET_ADMIN, this doesn't follow the principle of
capabilities. For example, let's take a look at netdev_store(),
root can't modify netdev attribute without CAP_NET_ADMIN.
So let's keep the consistency of permission check logic.
Reported-by: Zeal Robot <zealci@zte.com.cn>
Signed-off-by: Yang Yang <yang.yang29@zte.com.cn>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/devinet.c

@@ -2424,11 +2424,15 @@ static int devinet_sysctl_forward(struct ctl_table *ctl, int write,
 	int *valp = ctl->data;
 	int val = *valp;
 	loff_t pos = *ppos;
-	int ret = proc_dointvec(ctl, write, buffer, lenp, ppos);
+	struct net *net = ctl->extra2;
+	int ret;
 
-	if (write && *valp != val) {
-		struct net *net = ctl->extra2;
+	if (write && !ns_capable(net->user_ns, CAP_NET_ADMIN))
+		return -EPERM;
+
+	ret = proc_dointvec(ctl, write, buffer, lenp, ppos);
 
+	if (write && *valp != val) {
 		if (valp != &IPV4_DEVCONF_DFLT(net, FORWARDING)) {
 			if (!rtnl_trylock()) {
 				/* Restore the original values before restarting */