3.2.82-rc1 review patch. If anyone has any objections, please let me know. ------------------ From: Eric Dumazet <edumazet@xxxxxxxxxx> 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 <ycao009@xxxxxxx> Signed-off-by: Eric Dumazet <edumazet@xxxxxxxxxx> Suggested-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> Cc: Yuchung Cheng <ycheng@xxxxxxxxxx> Cc: Neal Cardwell <ncardwell@xxxxxxxxxx> Acked-by: Neal Cardwell <ncardwell@xxxxxxxxxx> Acked-by: Yuchung Cheng <ycheng@xxxxxxxxxx> Signed-off-by: David S. Miller <davem@xxxxxxxxxxxxx> [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 <ben@xxxxxxxxxxxxxxx> --- net/ipv4/tcp_input.c | 17 ++++++++++++----- 1 file changed, 12 insertions(+), 5 deletions(-) --- a/net/ipv4/tcp_input.c +++ b/net/ipv4/tcp_input.c @@ -87,7 +87,7 @@ int sysctl_tcp_adv_win_scale __read_most 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(struc /* 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); } -- To unsubscribe from this list: send the line "unsubscribe stable" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html