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 <jakob@jakoblell.com>
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 7 insertions, 17 deletions

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;
 }