Subject: Re: [PATCH] Add IPv6 support to TCP SYN cookies

From: Glenn Griffin <[email-masked]>
Date: Wed, 6 Feb 2008 15:03:34 -0800

Okay.  Round3.  Took into account that it was horribly broken when ipv6
was compiled as a module.  The fixes export a few more symbols, and now
the syncookie_secret is shared between the v4 and v6 code.  That should
be fine as it will be initialized when the v4 code starts, and it's not
currently possible to have v6 cookie support without v4.  At this point
I have not taken Evgeniy's feedback on the hash buffer being to large to
keep on the stack.  I was hoping to hear some other opinions on that.
Feedback is appreciated.  Thanks.

Signed-off-by: Glenn Griffin <[email-masked]>
---
 include/net/tcp.h        |    8 ++
 net/ipv4/syncookies.c    |    7 +-
 net/ipv4/tcp_input.c     |    1 +
 net/ipv4/tcp_minisocks.c |    2 +
 net/ipv4/tcp_output.c    |    1 +
 net/ipv6/Makefile        |    1 +
 net/ipv6/syncookies.c    |  265 ++++++++++++++++++++++++++++++++++++++++++++++
 net/ipv6/tcp_ipv6.c      |   77 ++++++++++----
 8 files changed, 336 insertions(+), 26 deletions(-)
 create mode 100644 net/ipv6/syncookies.c

diff --git a/include/net/tcp.h b/include/net/tcp.h
index cb5b033..58a2dda 100644
--- a/include/net/tcp.h
+++ b/include/net/tcp.h
@@ -29,6 +29,7 @@
 #include <linux/skbuff.h>
 #include <linux/dmaengine.h>
 #include <linux/crypto.h>
+#include <linux/cryptohash.h>
 
 #include <net/inet_connection_sock.h>
 #include <net/inet_timewait_sock.h>
@@ -431,11 +432,17 @@ extern int			tcp_disconnect(struct sock *sk, int flags);
 extern void			tcp_unhash(struct sock *sk);
 
 /* From syncookies.c */
+extern __u32 syncookie_secret[2][16-3+SHA_DIGEST_WORDS];
 extern struct sock *cookie_v4_check(struct sock *sk, struct sk_buff *skb, 
 				    struct ip_options *opt);
 extern __u32 cookie_v4_init_sequence(struct sock *sk, struct sk_buff *skb, 
 				     __u16 *mss);
 
+/* From net/ipv6/syncookies.c */
+extern struct sock *cookie_v6_check(struct sock *sk, struct sk_buff *skb);
+extern __u32 cookie_v6_init_sequence(struct sock *sk, struct sk_buff *skb,
+				     __u16 *mss);
+
 /* tcp_output.c */
 
 extern void __tcp_push_pending_frames(struct sock *sk, unsigned int cur_mss,
@@ -1337,6 +1344,7 @@ extern int tcp_proc_register(struct tcp_seq_afinfo *afinfo);
 extern void tcp_proc_unregister(struct tcp_seq_afinfo *afinfo);
 
 extern struct request_sock_ops tcp_request_sock_ops;
+extern struct request_sock_ops tcp6_request_sock_ops;
 
 extern int tcp_v4_destroy_sock(struct sock *sk);
 
diff --git a/net/ipv4/syncookies.c b/net/ipv4/syncookies.c
index 2da1be0..e4453fc 100644
--- a/net/ipv4/syncookies.c
+++ b/net/ipv4/syncookies.c
@@ -10,8 +10,6 @@
  *      2 of the License, or (at your option) any later version.
  *
  *  $Id: syncookies.c,v 1.18 2002/02/01 22:01:04 davem Exp $
- *
- *  Missing: IPv6 support.
  */
 
 #include <linux/tcp.h>
@@ -23,14 +21,15 @@
 
 extern int sysctl_tcp_syncookies;
 
-static __u32 syncookie_secret[2][16-3+SHA_DIGEST_WORDS];
+__u32 syncookie_secret[2][16-3+SHA_DIGEST_WORDS];
+EXPORT_SYMBOL(syncookie_secret);
 
 static __init int init_syncookies(void)
 {
 	get_random_bytes(syncookie_secret, sizeof(syncookie_secret));
 	return 0;
 }
-module_init(init_syncookies);
+__initcall(init_syncookies);
 
 #define COOKIEBITS 24	/* Upper bits store count */
 #define COOKIEMASK (((__u32)1 << COOKIEBITS) - 1)
diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
index b39f0d8..5180c7d 100644
--- a/net/ipv4/tcp_input.c
+++ b/net/ipv4/tcp_input.c
@@ -5207,6 +5207,7 @@ discard:
 
 EXPORT_SYMBOL(sysctl_tcp_ecn);
 EXPORT_SYMBOL(sysctl_tcp_reordering);
+EXPORT_SYMBOL(sysctl_tcp_adv_win_scale);
 EXPORT_SYMBOL(tcp_parse_options);
 EXPORT_SYMBOL(tcp_rcv_established);
 EXPORT_SYMBOL(tcp_rcv_state_process);
diff --git a/net/ipv4/tcp_minisocks.c b/net/ipv4/tcp_minisocks.c
index b61b768..0f494cd 100644
--- a/net/ipv4/tcp_minisocks.c
+++ b/net/ipv4/tcp_minisocks.c
@@ -35,6 +35,8 @@
 #endif
 
 int sysctl_tcp_syncookies __read_mostly = SYNC_INIT;
+EXPORT_SYMBOL(sysctl_tcp_syncookies);
+
 int sysctl_tcp_abort_on_overflow __read_mostly;
 
 struct inet_timewait_death_row tcp_death_row = {
diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c
index f4c1eef..613c64b 100644
--- a/net/ipv4/tcp_output.c
+++ b/net/ipv4/tcp_output.c
@@ -2591,6 +2591,7 @@ void tcp_send_probe0(struct sock *sk)
 	}
 }
 
+EXPORT_SYMBOL(tcp_select_initial_window);
 EXPORT_SYMBOL(tcp_connect);
 EXPORT_SYMBOL(tcp_make_synack);
 EXPORT_SYMBOL(tcp_simple_retransmit);
diff --git a/net/ipv6/Makefile b/net/ipv6/Makefile
index 87c23a7..d1a1056 100644
--- a/net/ipv6/Makefile
+++ b/net/ipv6/Makefile
@@ -15,6 +15,7 @@ ipv6-$(CONFIG_XFRM) += xfrm6_policy.o xfrm6_state.o xfrm6_input.o \
 ipv6-$(CONFIG_NETFILTER) += netfilter.o
 ipv6-$(CONFIG_IPV6_MULTIPLE_TABLES) += fib6_rules.o
 ipv6-$(CONFIG_PROC_FS) += proc.o
+ipv6-$(CONFIG_SYN_COOKIES) += syncookies.o
 
 ipv6-objs += $(ipv6-y)
 
diff --git a/net/ipv6/syncookies.c b/net/ipv6/syncookies.c
new file mode 100644
index 0000000..cef5d39
--- /dev/null
+++ b/net/ipv6/syncookies.c
@@ -0,0 +1,265 @@
+/*
+ *  IPv6 Syncookies implementation for the Linux kernel
+ *
+ *  Authors:
+ *  Glenn Griffin	<[email-masked]>
+ *
+ *  Based on IPv4 implementation by Andi Kleen
+ *  linux/net/ipv4/syncookies.c
+ *
+ *	This program is free software; you can redistribute it and/or
+ *      modify it under the terms of the GNU General Public License
+ *      as published by the Free Software Foundation; either version
+ *      2 of the License, or (at your option) any later version.
+ *
+ */
+
+#include <linux/tcp.h>
+#include <linux/random.h>
+#include <linux/cryptohash.h>
+#include <linux/kernel.h>
+#include <net/ipv6.h>
+#include <net/tcp.h>
+
+extern int sysctl_tcp_syncookies;
+extern __u32 syncookie_secret[2][16-3+SHA_DIGEST_WORDS];
+
+#define COOKIEBITS 24	/* Upper bits store count */
+#define COOKIEMASK (((__u32)1 << COOKIEBITS) - 1)
+
+/*
+ * This table has to be sorted and terminated with (__u16)-1.
+ * XXX generate a better table.
+ * Unresolved Issues: HIPPI with a 64k MSS is not well supported.
+ * 
+ * Taken directly from ipv4 implementation. 
+ * Should this list be modified for ipv6 use or is it close enough?
+ * rfc 2460 8.3 suggests mss values 20 bytes less than ipv4 counterpart
+ */
+static __u16 const msstab[] = {
+	64 - 1,
+	256 - 1,
+	512 - 1,
+	536 - 1,
+	1024 - 1,
+	1440 - 1,
+	1460 - 1,
+	4312 - 1,
+	(__u16)-1
+};
+/* The number doesn't include the -1 terminator */
+#define NUM_MSS (ARRAY_SIZE(msstab) - 1)
+
+/*
+ * 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)
+{
+	struct inet_connection_sock *icsk = inet_csk(sk);
+	struct sock *child;
+
+	child = icsk->icsk_af_ops->syn_recv_sock(sk, skb, req, dst);
+	if (child)
+		inet_csk_reqsk_queue_add(sk, req, child);
+	else
+		reqsk_free(req);
+
+	return child;
+}
+
+static u32 cookie_hash(struct in6_addr *saddr, struct in6_addr *daddr,
+		       __be16 sport, __be16 dport, u32 count, int c)
+{
+	__u32 tmp[16 + 5 + SHA_WORKSPACE_WORDS];
+
+	/*
+	 * we have 320 bits of information to hash, copy in the remaining
+	 * 192 bits required for sha_transform, from the syncookie_secret
+	 * and overwrite the digest with the secret
+	 */
+	memcpy(tmp + 10, syncookie_secret[c], 44);
+	memcpy(tmp, saddr, 16);
+	memcpy(tmp + 4, daddr, 16);
+	tmp[8] = ((__force u32)sport << 16) + (__force u32)dport;
+	tmp[9] = count;
+	sha_transform(tmp + 16, (__u8 *)tmp, tmp + 16 + 5);
+
+	return tmp[17];
+}
+
+static __u32 secure_tcp_syn_cookie(struct in6_addr *saddr, struct in6_addr *daddr,
+				   __be16 sport, __be16 dport, __u32 sseq,
+				   __u32 count, __u32 data)
+{
+	return (cookie_hash(saddr, daddr, sport, dport, 0, 0) +
+		sseq + (count << COOKIEBITS) +
+		((cookie_hash(saddr, daddr, sport, dport, count, 1) + data)
+		& COOKIEMASK));
+}
+
+static __u32 check_tcp_syn_cookie(__u32 cookie, struct in6_addr *saddr,
+				  struct in6_addr *daddr, __be16 sport,
+				  __be16 dport, __u32 sseq, __u32 count,
+				  __u32 maxdiff)
+{
+	__u32 diff;
+
+	cookie -= cookie_hash(saddr, daddr, sport, dport, 0, 0) + sseq;
+
+	diff = (count - (cookie >> COOKIEBITS)) & ((__u32) -1 >> COOKIEBITS);
+	if (diff >= maxdiff)
+		return (__u32)-1;
+
+	return (cookie -
+		cookie_hash(saddr, daddr, sport, dport, count - diff, 1))
+		& COOKIEMASK;
+}
+
+__u32 cookie_v6_init_sequence(struct sock *sk, struct sk_buff *skb, __u16 *mssp)
+{
+	struct ipv6hdr *iph = ipv6_hdr(skb);
+	const struct tcphdr *th = tcp_hdr(skb);
+	int mssind;
+	const __u16 mss = *mssp;
+
+	tcp_sk(sk)->last_synq_overflow = jiffies;
+
+	for (mssind = 0; mss > msstab[mssind + 1]; mssind++)
+		;
+	*mssp = msstab[mssind] + 1;
+
+	NET_INC_STATS_BH(LINUX_MIB_SYNCOOKIESSENT);
+
+	return secure_tcp_syn_cookie(&iph->saddr, &iph->daddr, th->source,
+				     th->dest, ntohl(th->seq),
+				     jiffies / (HZ * 60), mssind);
+}
+
+static inline int cookie_check(struct sk_buff *skb, __u32 cookie)
+{
+	struct ipv6hdr *iph = ipv6_hdr(skb);
+	const struct tcphdr *th = tcp_hdr(skb);
+	__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);
+
+	return mssind < NUM_MSS ? msstab[mssind] + 1 : 0;
+}
+
+struct sock *cookie_v6_check(struct sock *sk, struct sk_buff *skb)
+{
+	struct inet_request_sock *ireq;
+	struct inet6_request_sock *ireq6;
+	struct tcp_request_sock *treq;
+	struct ipv6_pinfo *np = inet6_sk(sk);
+	struct tcp_sock *tp = tcp_sk(sk);
+	const struct tcphdr *th = tcp_hdr(skb);
+	__u32 cookie = ntohl(th->ack_seq) - 1;
+	struct sock *ret = sk;
+	struct request_sock *req;
+	int mss;
+	struct dst_entry *dst;
+	__u8 rcv_wscale;
+
+	if (!sysctl_tcp_syncookies || !th->ack)
+		goto out;
+
+	if (time_after(jiffies, tp->last_synq_overflow + TCP_TIMEOUT_INIT) ||
+		(mss = cookie_check(skb, cookie)) == 0) {
+		NET_INC_STATS_BH(LINUX_MIB_SYNCOOKIESFAILED);
+		goto out;
+	}
+
+	NET_INC_STATS_BH(LINUX_MIB_SYNCOOKIESRECV);
+
+	ret = NULL;
+	req = inet6_reqsk_alloc(&tcp6_request_sock_ops);
+	if (!req)
+		goto out;
+
+	ireq = inet_rsk(req);
+	ireq6 = inet6_rsk(req);
+	treq = tcp_rsk(req);
+	ireq6->pktopts = NULL;
+
+	if (security_inet_conn_request(sk, skb, req)) {
+		reqsk_free(req);
+		goto out;
+	}
+
+	req->mss = mss;
+	ireq->rmt_port = th->source;
+	ipv6_addr_copy(&ireq6->rmt_addr, &ipv6_hdr(skb)->saddr);
+	ipv6_addr_copy(&ireq6->loc_addr, &ipv6_hdr(skb)->daddr);
+	if (ipv6_opt_accepted(sk, skb) ||
+	    np->rxopt.bits.rxinfo || np->rxopt.bits.rxoinfo ||
+	    np->rxopt.bits.rxhlim || np->rxopt.bits.rxohlim) {
+		atomic_inc(&skb->users);
+		ireq6->pktopts = skb;
+	}
+
+	ireq6->iif = sk->sk_bound_dev_if;
+	/* So that link locals have meaning */
+	if (!sk->sk_bound_dev_if &&
+	    ipv6_addr_type(&ireq6->rmt_addr) & IPV6_ADDR_LINKLOCAL)
+		ireq6->iif = inet6_iif(skb);
+
+	req->expires = 0UL;
+	req->retrans = 0;
+	ireq->snd_wscale = ireq->rcv_wscale = ireq->tstamp_ok = 0;
+	ireq->wscale_ok = ireq->sack_ok = 0;
+	treq->rcv_isn = ntohl(th->seq) - 1;
+	treq->snt_isn = cookie;
+
+	/*
+	 * We need to lookup the dst_entry to get the correct window size.
+	 * This is taken from tcp_v6_syn_recv_sock.  Somebody please enlighten
+	 * me if there is a preferred way.
+	 */
+	{
+		struct in6_addr *final_p = NULL, final;
+		struct flowi fl;
+		memset(&fl, 0, sizeof(fl));
+		fl.proto = IPPROTO_TCP;
+		ipv6_addr_copy(&fl.fl6_dst, &ireq6->rmt_addr);
+		if (np->opt && np->opt->srcrt) {
+			struct rt0_hdr *rt0 = (struct rt0_hdr *) np->opt->srcrt;
+			ipv6_addr_copy(&final, &fl.fl6_dst);
+			ipv6_addr_copy(&fl.fl6_dst, rt0->addr);
+			final_p = &final;
+		}
+		ipv6_addr_copy(&fl.fl6_src, &ireq6->loc_addr);
+		fl.oif = sk->sk_bound_dev_if;
+		fl.fl_ip_dport = inet_rsk(req)->rmt_port;
+		fl.fl_ip_sport = inet_sk(sk)->sport;
+		security_req_classify_flow(req, &fl);
+		if (ip6_dst_lookup(sk, &dst, &fl)) {
+			reqsk_free(req);
+			goto out;
+		}
+		if (final_p)
+			ipv6_addr_copy(&fl.fl6_dst, final_p);
+		if ((xfrm_lookup(&dst, &fl, sk, 0)) < 0)
+			goto out;
+	}
+
+	req->window_clamp = dst_metric(dst, RTAX_WINDOW);
+	tcp_select_initial_window(tcp_full_space(sk), req->mss,
+				  &req->rcv_wnd, &req->window_clamp,
+				  0, &rcv_wscale);
+
+	ireq->rcv_wscale = rcv_wscale;
+
+	ret = get_cookie_sock(sk, skb, req, dst);
+
+out:	return ret;
+}
+
diff --git a/net/ipv6/tcp_ipv6.c b/net/ipv6/tcp_ipv6.c
index 93980c3..ad39bd1 100644
--- a/net/ipv6/tcp_ipv6.c
+++ b/net/ipv6/tcp_ipv6.c
@@ -520,6 +520,20 @@ done:
 	return err;
 }
 
+static inline void syn_flood_warning(struct sk_buff *skb)
+{
+#ifdef CONFIG_SYN_COOKIES
+	if (sysctl_tcp_syncookies)
+		printk(KERN_INFO
+		       "TCPv6: Possible SYN flooding on port %d. "
+		       "Sending cookies.\n", ntohs(tcp_hdr(skb)->dest));
+	else
+#endif
+		printk(KERN_INFO
+		       "TCPv6: Possible SYN flooding on port %d. "
+		       "Dropping request.\n", ntohs(tcp_hdr(skb)->dest));
+}
+
 static void tcp_v6_reqsk_destructor(struct request_sock *req)
 {
 	if (inet6_rsk(req)->pktopts)
@@ -923,7 +937,7 @@ done_opts:
 }
 #endif
 
-static struct request_sock_ops tcp6_request_sock_ops __read_mostly = {
+struct request_sock_ops tcp6_request_sock_ops __read_mostly = {
 	.family		=	AF_INET6,
 	.obj_size	=	sizeof(struct tcp6_request_sock),
 	.rtx_syn_ack	=	tcp_v6_send_synack,
@@ -1221,9 +1235,9 @@ static struct sock *tcp_v6_hnd_req(struct sock *sk,struct sk_buff *skb)
 		return NULL;
 	}
 
-#if 0 /*def CONFIG_SYN_COOKIES*/
+#ifdef CONFIG_SYN_COOKIES
 	if (!th->rst && !th->syn && th->ack)
-		sk = cookie_v6_check(sk, skb, &(IPCB(skb)->opt));
+		sk = cookie_v6_check(sk, skb);
 #endif
 	return sk;
 }
@@ -1239,6 +1253,11 @@ static int tcp_v6_conn_request(struct sock *sk, struct sk_buff *skb)
 	struct tcp_sock *tp = tcp_sk(sk);
 	struct request_sock *req = NULL;
 	__u32 isn = TCP_SKB_CB(skb)->when;
+#ifdef CONFIG_SYN_COOKIES
+	int want_cookie = 0;
+#else
+#define want_cookie 0
+#endif
 
 	if (skb->protocol == htons(ETH_P_IP))
 		return tcp_v4_conn_request(sk, skb);
@@ -1246,12 +1265,14 @@ static int tcp_v6_conn_request(struct sock *sk, struct sk_buff *skb)
 	if (!ipv6_unicast_destination(skb))
 		goto drop;
 
-	/*
-	 *	There are no SYN attacks on IPv6, yet...
-	 */
 	if (inet_csk_reqsk_queue_is_full(sk) && !isn) {
 		if (net_ratelimit())
-			printk(KERN_INFO "TCPv6: dropping request, synflood is possible\n");
+			syn_flood_warning(skb);
+#ifdef CONFIG_SYN_COOKIES
+		if (sysctl_tcp_syncookies)
+			want_cookie = 1;
+		else
+#endif
 		goto drop;
 	}
 
@@ -1272,29 +1293,39 @@ static int tcp_v6_conn_request(struct sock *sk, struct sk_buff *skb)
 
 	tcp_parse_options(skb, &tmp_opt, 0);
 
+	if (want_cookie) {
+		tcp_clear_options(&tmp_opt);
+		tmp_opt.saw_tstamp = 0;
+	}
+
 	tmp_opt.tstamp_ok = tmp_opt.saw_tstamp;
 	tcp_openreq_init(req, &tmp_opt, skb);
 
 	treq = inet6_rsk(req);
 	ipv6_addr_copy(&treq->rmt_addr, &ipv6_hdr(skb)->saddr);
 	ipv6_addr_copy(&treq->loc_addr, &ipv6_hdr(skb)->daddr);
-	TCP_ECN_create_request(req, tcp_hdr(skb));
 	treq->pktopts = NULL;
-	if (ipv6_opt_accepted(sk, skb) ||
-	    np->rxopt.bits.rxinfo || np->rxopt.bits.rxoinfo ||
-	    np->rxopt.bits.rxhlim || np->rxopt.bits.rxohlim) {
-		atomic_inc(&skb->users);
-		treq->pktopts = skb;
-	}
-	treq->iif = sk->sk_bound_dev_if;
+	if (!want_cookie)
+		TCP_ECN_create_request(req, tcp_hdr(skb));
+
+	if (want_cookie) {
+		isn = cookie_v6_init_sequence(sk, skb, &req->mss);
+	} else if (!isn) {
+		if (ipv6_opt_accepted(sk, skb) ||
+		    np->rxopt.bits.rxinfo || np->rxopt.bits.rxoinfo ||
+		    np->rxopt.bits.rxhlim || np->rxopt.bits.rxohlim) {
+			atomic_inc(&skb->users);
+			treq->pktopts = skb;
+		}
+		treq->iif = sk->sk_bound_dev_if;
 
-	/* So that link locals have meaning */
-	if (!sk->sk_bound_dev_if &&
-	    ipv6_addr_type(&treq->rmt_addr) & IPV6_ADDR_LINKLOCAL)
-		treq->iif = inet6_iif(skb);
+		/* So that link locals have meaning */
+		if (!sk->sk_bound_dev_if &&
+		    ipv6_addr_type(&treq->rmt_addr) & IPV6_ADDR_LINKLOCAL)
+			treq->iif = inet6_iif(skb);
 
-	if (isn == 0)
 		isn = tcp_v6_init_sequence(skb);
+	}
 
 	tcp_rsk(req)->snt_isn = isn;
 
@@ -1303,8 +1334,10 @@ static int tcp_v6_conn_request(struct sock *sk, struct sk_buff *skb)
 	if (tcp_v6_send_synack(sk, req, NULL))
 		goto drop;
 
-	inet6_csk_reqsk_queue_hash_add(sk, req, TCP_TIMEOUT_INIT);
-	return 0;
+	if (!want_cookie) {
+		inet6_csk_reqsk_queue_hash_add(sk, req, TCP_TIMEOUT_INIT);
+		return 0;
+	}
 
 drop:
 	if (req)
-- 
1.5.3.4