From: Eric Dumazet Date: Sun, 10 Jul 2016 08:04:02 +0000 (+0200) Subject: tcp: make challenge acks less predictable X-Git-Tag: v3.2.82~2 X-Git-Url: https://git.openpandora.org/cgi-bin/gitweb.cgi?p=pandora-kernel.git;a=commitdiff_plain;h=07ab6b6274057612a056b5db754d9bcbd319d291;ds=sidebyside tcp: make challenge acks less predictable commit 75ff39ccc1bd5d3c455b6822ab09e533c551f758 upstream. 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 Signed-off-by: Eric Dumazet Suggested-by: Linus Torvalds Cc: Yuchung Cheng Cc: Neal Cardwell Acked-by: Neal Cardwell Acked-by: Yuchung Cheng Signed-off-by: David S. Miller [bwh: Backported to 3.2: - Adjust context - Use ACCESS_ONCE() instead of {READ,WRITE}_ONCE() - Open-code prandom_u32_max()] Signed-off-by: Ben Hutchings --- diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c index 039e40f33860..eb7a8fba6f4f 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; @@ -3715,13 +3715,20 @@ static void tcp_send_challenge_ack(struct sock *sk) /* unprotected vars, we dont care of overwrites */ static u32 challenge_timestamp; static unsigned int challenge_count; - u32 now = jiffies / HZ; + u32 count, now = jiffies / HZ; if (now != challenge_timestamp) { + u32 half = (sysctl_tcp_challenge_ack_limit + 1) >> 1; + challenge_timestamp = now; - challenge_count = 0; - } - if (++challenge_count <= sysctl_tcp_challenge_ack_limit) { + ACCESS_ONCE(challenge_count) = + half + (u32)( + ((u64) random32() * sysctl_tcp_challenge_ack_limit) + >> 32); + } + count = ACCESS_ONCE(challenge_count); + if (count > 0) { + ACCESS_ONCE(challenge_count) = count - 1; NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_TCPCHALLENGEACK); tcp_send_ack(sk); }