[Syncookies]: Add support for TCP options via timestamps.

Allow the use of SACK and window scaling when syncookies are used
and the client supports tcp timestamps. Options are encoded into
the timestamp sent in the syn-ack and restored from the timestamp
echo when the ack is received.

Based on earlier work by Glenn Griffin.
This patch avoids increasing the size of structs by encoding TCP
options into the least significant bits of the timestamp and
by not using any 'timestamp offset'.

The downside is that the timestamp sent in the packet after the synack
will increase by several seconds.

changes since v1:
 don't duplicate timestamp echo decoding function, put it into ipv4/syncookie.c
 and have ipv6/syncookies.c use it.
 Feedback from Glenn Griffin: fix line indented with spaces, kill redundant if ()

Reviewed-by: Hagen Paul Pfeifer <hagen@jauu.net>
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: David S. Miller <davem@davemloft.net>

2 files changed, 18 insertions, 7 deletions

diff --git a/net/ipv6/syncookies.c b/net/ipv6/syncookies.c
index 3a622e7abc02..938ce4ecde55 100644
--- a/net/ipv6/syncookies.c
+++ b/net/ipv6/syncookies.c
@@ -170,6 +170,7 @@ struct sock *cookie_v6_check(struct sock *sk, struct sk_buff *skb)
         int mss;
         struct dst_entry *dst;
         __u8 rcv_wscale;
+        struct tcp_options_received tcp_opt;
 
         if (!sysctl_tcp_syncookies || !th->ack)
                 goto out;
@@ -182,6 +183,13 @@ struct sock *cookie_v6_check(struct sock *sk, struct sk_buff *skb)
 
         NET_INC_STATS_BH(LINUX_MIB_SYNCOOKIESRECV);
 
+        /* check for timestamp cookie support */
+        memset(&tcp_opt, 0, sizeof(tcp_opt));
+        tcp_parse_options(skb, &tcp_opt, 0);
+
+        if (tcp_opt.saw_tstamp)
+                cookie_check_timestamp(&tcp_opt);
+
         ret = NULL;
         req = inet6_reqsk_alloc(&tcp6_request_sock_ops);
         if (!req)
@@ -216,8 +224,12 @@ struct sock *cookie_v6_check(struct sock *sk, struct sk_buff *skb)
 
         req->expires = 0UL;
         req->retrans = 0;
-        ireq->snd_wscale = ireq->rcv_wscale = ireq->tstamp_ok = 0;
-        ireq->wscale_ok = ireq->sack_ok = 0;
+        ireq->snd_wscale        = tcp_opt.snd_wscale;
+        ireq->rcv_wscale        = tcp_opt.rcv_wscale;
+        ireq->sack_ok           = tcp_opt.sack_ok;
+        ireq->wscale_ok         = tcp_opt.wscale_ok;
+        ireq->tstamp_ok         = tcp_opt.saw_tstamp;
+        req->ts_recent          = tcp_opt.saw_tstamp ? tcp_opt.rcv_tsval : 0;
         treq->rcv_isn = ntohl(th->seq) - 1;
         treq->snt_isn = cookie;
 
@@ -253,10 +265,10 @@ struct sock *cookie_v6_check(struct sock *sk, struct sk_buff *skb)
                         goto out;
         }
 
-        req->window_clamp = dst_metric(dst, RTAX_WINDOW);
+        req->window_clamp = tp->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->wscale_ok, &rcv_wscale);
 
         ireq->rcv_wscale = rcv_wscale;
 
diff --git a/net/ipv6/tcp_ipv6.c b/net/ipv6/tcp_ipv6.c
index 378cc4002a76..8ebf6de29562 100644
--- a/net/ipv6/tcp_ipv6.c
+++ b/net/ipv6/tcp_ipv6.c
@@ -1290,10 +1290,8 @@ static int tcp_v6_conn_request(struct sock *sk, struct sk_buff *skb)
 
         tcp_parse_options(skb, &tmp_opt, 0);
 
-        if (want_cookie) {
+        if (want_cookie && !tmp_opt.saw_tstamp)
                 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);
@@ -1307,6 +1305,7 @@ static int tcp_v6_conn_request(struct sock *sk, struct sk_buff *skb)
 
         if (want_cookie) {
                 isn = cookie_v6_init_sequence(sk, skb, &req->mss);
+                req->cookie_ts = tmp_opt.tstamp_ok;
         } else if (!isn) {
                 if (ipv6_opt_accepted(sk, skb) ||
                     np->rxopt.bits.rxinfo || np->rxopt.bits.rxoinfo ||