tcp: make challenge acks less predictable

[deliverable/linux.git] / net / ipv4 / tcp_input.c

diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
index d6c8f4cd080001a527f7c137021cc6a3f3604344..91868bb1781864cdf316ca5efc6be1d70d967407 100644 (file)
--- a/net/ipv4/tcp_input.c
+++ b/net/ipv4/tcp_input.c
@@ -87,7 +87,7 @@ int sysctl_tcp_adv_win_scale __read_mostly = 1;
 EXPORT_SYMBOL(sysctl_tcp_adv_win_scale);
 
 /* rfc5961 challenge ack rate limiting */
-int sysctl_tcp_challenge_ack_limit = 100;
+int sysctl_tcp_challenge_ack_limit = 1000;
 
 int sysctl_tcp_stdurg __read_mostly;
 int sysctl_tcp_rfc1337 __read_mostly;
@@ -3458,7 +3458,7 @@ static void tcp_send_challenge_ack(struct sock *sk, const struct sk_buff *skb)
        static u32 challenge_timestamp;
        static unsigned int challenge_count;
        struct tcp_sock *tp = tcp_sk(sk);
-       u32 now;
+       u32 count, now;
 
        /* First check our per-socket dupack rate limit. */
        if (tcp_oow_rate_limited(sock_net(sk), skb,
@@ -3466,13 +3466,18 @@ static void tcp_send_challenge_ack(struct sock *sk, const struct sk_buff *skb)
                                 &tp->last_oow_ack_time))
                return;
 
-       /* Then check the check host-wide RFC 5961 rate limit. */
+       /* Then check host-wide RFC 5961 rate limit. */
        now = jiffies / HZ;
        if (now != challenge_timestamp) {
+               u32 half = (sysctl_tcp_challenge_ack_limit + 1) >> 1;
+
                challenge_timestamp = now;
-               challenge_count = 0;
+               WRITE_ONCE(challenge_count, half +
+                          prandom_u32_max(sysctl_tcp_challenge_ack_limit));
        }
-       if (++challenge_count <= sysctl_tcp_challenge_ack_limit) {
+       count = READ_ONCE(challenge_count);
+       if (count > 0) {
+               WRITE_ONCE(challenge_count, count - 1);
                NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPCHALLENGEACK);
                tcp_send_ack(sk);
        }