From 8c27bd75f04fb9cb70c69c3cfe24f4e6d8e15906 Mon Sep 17 00:00:00 2001 From: Florian Westphal Date: Fri, 20 Sep 2013 22:32:55 +0200 Subject: [PATCH] tcp: syncookies: reduce cookie lifetime to 128 seconds We currently accept cookies that were created less than 4 minutes ago (ie, cookies with counter delta 0-3). Combined with the 8 mss table values, this yields 32 possible values (out of 2**32) that will be valid. Reducing the lifetime to < 2 minutes halves the guessing chance while still providing a large enough period. While at it, get rid of jiffies value -- they overflow too quickly on 32 bit platforms. getnstimeofday is used to create a counter that increments every 64s. perf shows getnstimeofday cost is negible compared to sha_transform; normal tcp initial sequence number generation uses getnstimeofday, too. Reported-by: Jakob Lell Signed-off-by: Florian Westphal Signed-off-by: David S. Miller --- include/net/tcp.h | 18 ++++++++++++++++++ net/ipv4/syncookies.c | 31 ++++++++++--------------------- net/ipv6/syncookies.c | 24 +++++++----------------- 3 files changed, 35 insertions(+), 38 deletions(-) diff --git a/include/net/tcp.h b/include/net/tcp.h index 0e47551e9bdb..de870ee5582d 100644 --- a/include/net/tcp.h +++ b/include/net/tcp.h @@ -481,6 +481,24 @@ int __cookie_v4_check(const struct iphdr *iph, const struct tcphdr *th, struct sock *cookie_v4_check(struct sock *sk, struct sk_buff *skb, struct ip_options *opt); #ifdef CONFIG_SYN_COOKIES +#include + +/* Syncookies use a monotonic timer which increments every 64 seconds. + * This counter is used both as a hash input and partially encoded into + * the cookie value. A cookie is only validated further if the delta + * between the current counter value and the encoded one is less than this, + * i.e. a sent cookie is valid only at most for 128 seconds (or less if + * the counter advances immediately after a cookie is generated). + */ +#define MAX_SYNCOOKIE_AGE 2 + +static inline u32 tcp_cookie_time(void) +{ + struct timespec now; + getnstimeofday(&now); + return now.tv_sec >> 6; /* 64 seconds granularity */ +} + u32 __cookie_v4_init_sequence(const struct iphdr *iph, const struct tcphdr *th, u16 *mssp); __u32 cookie_v4_init_sequence(struct sock *sk, struct sk_buff *skb, __u16 *mss); diff --git a/net/ipv4/syncookies.c b/net/ipv4/syncookies.c index 14a15c49129d..b6ea2979a2b7 100644 --- a/net/ipv4/syncookies.c +++ b/net/ipv4/syncookies.c @@ -89,8 +89,7 @@ __u32 cookie_init_timestamp(struct request_sock *req) static __u32 secure_tcp_syn_cookie(__be32 saddr, __be32 daddr, __be16 sport, - __be16 dport, __u32 sseq, __u32 count, - __u32 data) + __be16 dport, __u32 sseq, __u32 data) { /* * Compute the secure sequence number. @@ -102,7 +101,7 @@ static __u32 secure_tcp_syn_cookie(__be32 saddr, __be32 daddr, __be16 sport, * As an extra hack, we add a small "data" value that encodes the * MSS into the second hash value. */ - + u32 count = tcp_cookie_time(); return (cookie_hash(saddr, daddr, sport, dport, 0, 0) + sseq + (count << COOKIEBITS) + ((cookie_hash(saddr, daddr, sport, dport, count, 1) + data) @@ -114,22 +113,21 @@ static __u32 secure_tcp_syn_cookie(__be32 saddr, __be32 daddr, __be16 sport, * If the syncookie is bad, the data returned will be out of * range. This must be checked by the caller. * - * The count value used to generate the cookie must be within - * "maxdiff" if the current (passed-in) "count". The return value - * is (__u32)-1 if this test fails. + * The count value used to generate the cookie must be less than + * MAX_SYNCOOKIE_AGE minutes in the past. + * The return value (__u32)-1 if this test fails. */ static __u32 check_tcp_syn_cookie(__u32 cookie, __be32 saddr, __be32 daddr, - __be16 sport, __be16 dport, __u32 sseq, - __u32 count, __u32 maxdiff) + __be16 sport, __be16 dport, __u32 sseq) { - __u32 diff; + u32 diff, count = tcp_cookie_time(); /* Strip away the layers from the cookie */ cookie -= cookie_hash(saddr, daddr, sport, dport, 0, 0) + sseq; /* Cookie is now reduced to (count * 2^24) ^ (hash % 2^24) */ diff = (count - (cookie >> COOKIEBITS)) & ((__u32) - 1 >> COOKIEBITS); - if (diff >= maxdiff) + if (diff >= MAX_SYNCOOKIE_AGE) return (__u32)-1; return (cookie - @@ -173,7 +171,7 @@ u32 __cookie_v4_init_sequence(const struct iphdr *iph, const struct tcphdr *th, return secure_tcp_syn_cookie(iph->saddr, iph->daddr, th->source, th->dest, ntohl(th->seq), - jiffies / (HZ * 60), mssind); + mssind); } EXPORT_SYMBOL_GPL(__cookie_v4_init_sequence); @@ -188,13 +186,6 @@ __u32 cookie_v4_init_sequence(struct sock *sk, struct sk_buff *skb, __u16 *mssp) return __cookie_v4_init_sequence(iph, th, mssp); } -/* - * This (misnamed) value is the age of syncookie which is permitted. - * Its ideal value should be dependent on TCP_TIMEOUT_INIT and - * sysctl_tcp_retries1. It's a rather complicated formula (exponential - * backoff) to compute at runtime so it's currently hardcoded here. - */ -#define COUNTER_TRIES 4 /* * Check if a ack sequence number is a valid syncookie. * Return the decoded mss if it is, or 0 if not. @@ -204,9 +195,7 @@ int __cookie_v4_check(const struct iphdr *iph, const struct tcphdr *th, { __u32 seq = ntohl(th->seq) - 1; __u32 mssind = check_tcp_syn_cookie(cookie, iph->saddr, iph->daddr, - th->source, th->dest, seq, - jiffies / (HZ * 60), - COUNTER_TRIES); + th->source, th->dest, seq); return mssind < ARRAY_SIZE(msstab) ? msstab[mssind] : 0; } diff --git a/net/ipv6/syncookies.c b/net/ipv6/syncookies.c index bf63ac8a49b9..13ca0a0ea680 100644 --- a/net/ipv6/syncookies.c +++ b/net/ipv6/syncookies.c @@ -36,14 +36,6 @@ static __u16 const msstab[] = { 9000 - 60, }; -/* - * This (misnamed) value is the age of syncookie which is permitted. - * Its ideal value should be dependent on TCP_TIMEOUT_INIT and - * sysctl_tcp_retries1. It's a rather complicated formula (exponential - * backoff) to compute at runtime so it's currently hardcoded here. - */ -#define COUNTER_TRIES 4 - static inline struct sock *get_cookie_sock(struct sock *sk, struct sk_buff *skb, struct request_sock *req, struct dst_entry *dst) @@ -86,8 +78,9 @@ static u32 cookie_hash(const struct in6_addr *saddr, const struct in6_addr *dadd static __u32 secure_tcp_syn_cookie(const struct in6_addr *saddr, const struct in6_addr *daddr, __be16 sport, __be16 dport, __u32 sseq, - __u32 count, __u32 data) + __u32 data) { + u32 count = tcp_cookie_time(); return (cookie_hash(saddr, daddr, sport, dport, 0, 0) + sseq + (count << COOKIEBITS) + ((cookie_hash(saddr, daddr, sport, dport, count, 1) + data) @@ -96,15 +89,14 @@ static __u32 secure_tcp_syn_cookie(const struct in6_addr *saddr, static __u32 check_tcp_syn_cookie(__u32 cookie, const struct in6_addr *saddr, const struct in6_addr *daddr, __be16 sport, - __be16 dport, __u32 sseq, __u32 count, - __u32 maxdiff) + __be16 dport, __u32 sseq) { - __u32 diff; + __u32 diff, count = tcp_cookie_time(); cookie -= cookie_hash(saddr, daddr, sport, dport, 0, 0) + sseq; diff = (count - (cookie >> COOKIEBITS)) & ((__u32) -1 >> COOKIEBITS); - if (diff >= maxdiff) + if (diff >= MAX_SYNCOOKIE_AGE) return (__u32)-1; return (cookie - @@ -125,8 +117,7 @@ u32 __cookie_v6_init_sequence(const struct ipv6hdr *iph, *mssp = msstab[mssind]; return secure_tcp_syn_cookie(&iph->saddr, &iph->daddr, th->source, - th->dest, ntohl(th->seq), - jiffies / (HZ * 60), mssind); + th->dest, ntohl(th->seq), mssind); } EXPORT_SYMBOL_GPL(__cookie_v6_init_sequence); @@ -146,8 +137,7 @@ int __cookie_v6_check(const struct ipv6hdr *iph, const struct tcphdr *th, { __u32 seq = ntohl(th->seq) - 1; __u32 mssind = check_tcp_syn_cookie(cookie, &iph->saddr, &iph->daddr, - th->source, th->dest, seq, - jiffies / (HZ * 60), COUNTER_TRIES); + th->source, th->dest, seq); return mssind < ARRAY_SIZE(msstab) ? msstab[mssind] : 0; } -- 2.34.1