[INET]: local port range robustness

Expansion of original idea from Denis V. Lunev <den@openvz.org>

Add robustness and locking to the local_port_range sysctl.
1. Enforce that low < high when setting.
2. Use seqlock to ensure atomic update.

The locking might seem like overkill, but there are
cases where sysadmin might want to change value in the
middle of a DoS attack.

Signed-off-by: Stephen Hemminger <shemminger@linux-foundation.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 6 insertions, 5 deletions

diff --git a/net/sctp/socket.c b/net/sctp/socket.c
index 7cd58ef..9c6a4b5 100644
--- a/net/sctp/socket.c
+++ b/net/sctp/socket.c
@@ -5315,11 +5315,12 @@ static long sctp_get_port_local(struct sock *sk, union sctp_addr *addr)
 
 	if (snum == 0) {
 		/* Search for an available port. */
-		unsigned int low = sysctl_local_port_range[0];
-		unsigned int high = sysctl_local_port_range[1];
-		unsigned int remaining = (high - low) + 1;
-		unsigned int rover = net_random() % remaining + low;
-		int index;
+		int low, high, remaining, index;
+		unsigned int rover;
+
+		inet_get_local_port_range(&low, &high);
+		remaining = (high - low) + 1;
+		rover = net_random() % remaining + low;
 
 		do {
 			rover++;