From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755209AbcIEIkT (ORCPT ); Mon, 5 Sep 2016 04:40:19 -0400 Received: from smtp-out10.electric.net ([185.38.180.41]:58880 "EHLO smtp-out10.electric.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753280AbcIEIkQ (ORCPT ); Mon, 5 Sep 2016 04:40:16 -0400 X-Greylist: delayed 985 seconds by postgrey-1.27 at vger.kernel.org; Mon, 05 Sep 2016 04:40:16 EDT From: Sona Sarmadi To: , , , CC: Subject: [PATCH] tcp: make challenge acks less predictable Date: Mon, 5 Sep 2016 10:23:43 +0200 Message-ID: <1473063823-25430-1-git-send-email-sona.sarmadi@enea.com> X-Mailer: git-send-email 1.9.1 MIME-Version: 1.0 Content-Type: text/plain X-Outbound-IP: 192.36.1.72 X-Env-From: sona.sarmadi@enea.com X-Proto: esmtps X-Revdns: mx-3.enea.com X-HELO: mx-3.enea.com X-TLS: TLSv1:AES128-SHA:128 X-Authenticated_ID: X-PolicySMART: 6551647 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org From: Eric Dumazet [ 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 CVE-2016-5696. [backport of 3.14 commit 860c53258e634c54f70252c352bae7bac30724a9] 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 Signed-off-by: Sona Sarmadi --- diff -Nurp a/include/linux/random.h b/include/linux/random.h --- a/include/linux/random.h 2016-08-31 14:01:17.311438431 +0200 +++ b/include/linux/random.h 2016-09-02 11:45:26.275741639 +0200 @@ -33,6 +33,23 @@ void prandom_seed(u32 seed); u32 prandom_u32_state(struct rnd_state *); void prandom_bytes_state(struct rnd_state *state, void *buf, int nbytes); +/** + * prandom_u32_max - returns a pseudo-random number in interval [0, ep_ro) + * @ep_ro: right open interval endpoint + * + * Returns a pseudo-random number that is in interval [0, ep_ro). Note + * that the result depends on PRNG being well distributed in [0, ~0U] + * u32 space. Here we use maximally equidistributed combined Tausworthe + * generator, that is, prandom_u32(). This is useful when requesting a + * random index of an array containing ep_ro elements, for example. + * + * Returns: pseudo-random number in interval [0, ep_ro) + */ +static inline u32 prandom_u32_max(u32 ep_ro) +{ + return (u32)(((u64) prandom_u32() * ep_ro) >> 32); +} + /* * Handle minimum values for seeds */ diff -Nurp a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c --- a/net/ipv4/tcp_input.c 2016-08-31 14:01:18.571384580 +0200 +++ b/net/ipv4/tcp_input.c 2016-09-02 11:45:26.295740789 +0200 @@ -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; @@ -3243,12 +3243,18 @@ static void tcp_send_challenge_ack(struc 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); }