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Commit 8c27bd75 authored by Florian Westphal's avatar Florian Westphal Committed by David S. Miller
Browse files

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: default avatarJakob Lell <jakob@jakoblell.com>
Signed-off-by: default avatarFlorian Westphal <fw@strlen.de>
Signed-off-by: default avatarDavid S. Miller <davem@davemloft.net>
parent 61f860c3

include/net/tcp.h

+18 −0
Original line number Diff line number Diff line
@@ -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 <linux/ktime.h>



/* 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);

net/ipv4/syncookies.c

+10 −21
Original line number Diff line number Diff line
@@ -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;

}

net/ipv6/syncookies.c

+7 −17
Original line number Diff line number Diff line
@@ -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;

}