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authorEric Dumazet <edumazet@google.com>2016-07-10 10:04:02 +0200
committerGreg Kroah-Hartman <gregkh@linuxfoundation.org>2016-08-16 09:29:03 +0200
commit860c53258e634c54f70252c352bae7bac30724a9 (patch)
tree84f72363c08b41e3e3cb906954f427ca174cc7be
parentd92f45a046e909bfb8b292b2f7d56ccbedf48d55 (diff)
tcp: make challenge acks less predictable
[ Upstream commit 75ff39ccc1bd5d3c455b6822ab09e533c551f758 ] Yue Cao claims that current host rate limiting of challenge ACKS (RFC 5961) could leak enough information to allow a patient attacker to hijack TCP sessions. He will soon provide details in an academic paper. This patch increases the default limit from 100 to 1000, and adds some randomization so that the attacker can no longer hijack sessions without spending a considerable amount of probes. Based on initial analysis and patch from Linus. Note that we also have per socket rate limiting, so it is tempting to remove the host limit in the future. v2: randomize the count of challenge acks per second, not the period. Fixes: 282f23c6ee34 ("tcp: implement RFC 5961 3.2") Reported-by: Yue Cao <ycao009@ucr.edu> Signed-off-by: Eric Dumazet <edumazet@google.com> Suggested-by: Linus Torvalds <torvalds@linux-foundation.org> Cc: Yuchung Cheng <ycheng@google.com> Cc: Neal Cardwell <ncardwell@google.com> Acked-by: Neal Cardwell <ncardwell@google.com> Acked-by: Yuchung Cheng <ycheng@google.com> Signed-off-by: David S. Miller <davem@davemloft.net>
-rw-r--r--net/ipv4/tcp_input.c12
1 files changed, 9 insertions, 3 deletions
diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
index 5b10c59ba8a9..90f9d00a3fbc 100644
--- 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;
@@ -3293,12 +3293,18 @@ static void tcp_send_challenge_ack(struct sock *sk)
static u32 challenge_timestamp;
static unsigned int challenge_count;
u32 now = jiffies / HZ;
+ u32 count;
if (now != challenge_timestamp) {
+ u32 half = (sysctl_tcp_challenge_ack_limit + 1) >> 1;
+
challenge_timestamp = now;
- challenge_count = 0;
+ challenge_count = half +
+ prandom_u32_max(sysctl_tcp_challenge_ack_limit);
}
- if (++challenge_count <= sysctl_tcp_challenge_ack_limit) {
+ count = challenge_count;
+ if (count > 0) {
+ challenge_count = count - 1;
NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_TCPCHALLENGEACK);
tcp_send_ack(sk);
}