icmp: Fix a data-race around sysctl_icmp_echo_ignore_broadcasts.

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

Fixes: 1da177e4 ("Linux-2.6.12-rc2")
Signed-off-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/icmp.c

@@ -1249,7 +1249,7 @@ int icmp_rcv(struct sk_buff *skb)
 		 */
 		if ((icmph->type == ICMP_ECHO ||
 		     icmph->type == ICMP_TIMESTAMP) &&
-		    net->ipv4.sysctl_icmp_echo_ignore_broadcasts) {
+		    READ_ONCE(net->ipv4.sysctl_icmp_echo_ignore_broadcasts)) {
 			reason = SKB_DROP_REASON_INVALID_PROTO;
 			goto error;
 		}

net/ipv4/sysctl_net_ipv4.c

@@ -617,6 +617,8 @@ static struct ctl_table ipv4_net_table[] = {
 		.maxlen		= sizeof(u8),
 		.mode		= 0644,
 		.proc_handler	= proc_dou8vec_minmax,
+		.extra1		= SYSCTL_ZERO,
+		.extra2		= SYSCTL_ONE
 	},
 	{
 		.procname	= "icmp_ignore_bogus_error_responses",