syncookies: check decoded options against sysctl settings

Discard the ACK if we find options that do not match current sysctl
settings.

Previously it was possible to create a connection with sack, wscale,
etc. enabled even if the feature was disabled via sysctl.

Also remove an unneeded call to tcp_sack_reset() in
cookie_check_timestamp: Both call sites (cookie_v4_check,
cookie_v6_check) zero "struct tcp_options_received", hand it to
tcp_parse_options() (which does not change tcp_opt->num_sacks/dsack)
and then call cookie_check_timestamp().

Even if num_sacks/dsacks were changed, the structure is allocated on
the stack and after cookie_check_timestamp returns only a few selected
members are copied to the inet_request_sock.
Signed-off-by: NFlorian Westphal <fw@strlen.de>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

include/net/tcp.h

@@ -464,7 +464,7 @@ extern __u32 cookie_v4_init_sequence(struct sock *sk, struct sk_buff *skb,
 				     __u16 *mss);
 
 extern __u32 cookie_init_timestamp(struct request_sock *req);
-extern void cookie_check_timestamp(struct tcp_options_received *tcp_opt);
+extern bool cookie_check_timestamp(struct tcp_options_received *tcp_opt);
 
 /* From net/ipv6/syncookies.c */
 extern struct sock *cookie_v6_check(struct sock *sk, struct sk_buff *skb);

net/ipv4/syncookies.c

@@ -230,23 +230,36 @@ static inline struct sock *get_cookie_sock(struct sock *sk, struct sk_buff *skb,
  * The lowest 4 bits are for snd_wscale
  * The next 4 lsb are for rcv_wscale
  * The next lsb is for sack_ok
+ *
+ * return false if we decode an option that should not be.
  */
-void cookie_check_timestamp(struct tcp_options_received *tcp_opt)
+bool cookie_check_timestamp(struct tcp_options_received *tcp_opt)
 {
 	/* echoed timestamp, 9 lowest bits contain options */
 	u32 options = tcp_opt->rcv_tsecr & TSMASK;
 
+	if (!tcp_opt->saw_tstamp)  {
+		tcp_clear_options(tcp_opt);
+		return true;
+	}
+
+	if (!sysctl_tcp_timestamps)
+		return false;
+
 	tcp_opt->snd_wscale = options & 0xf;
 	options >>= 4;
 	tcp_opt->rcv_wscale = options & 0xf;
 
 	tcp_opt->sack_ok = (options >> 4) & 0x1;
 
-	if (tcp_opt->sack_ok)
-		tcp_sack_reset(tcp_opt);
+	if (tcp_opt->sack_ok && !sysctl_tcp_sack)
+		return false;
 
-	if (tcp_opt->snd_wscale || tcp_opt->rcv_wscale)
+	if (tcp_opt->snd_wscale || tcp_opt->rcv_wscale) {
 		tcp_opt->wscale_ok = 1;
+		return sysctl_tcp_window_scaling != 0;
+	}
+	return true;
 }
 EXPORT_SYMBOL(cookie_check_timestamp);
 
@@ -281,8 +294,8 @@ struct sock *cookie_v4_check(struct sock *sk, struct sk_buff *skb,
 	memset(&tcp_opt, 0, sizeof(tcp_opt));
 	tcp_parse_options(skb, &tcp_opt, &hash_location, 0);
 
-	if (tcp_opt.saw_tstamp)
-		cookie_check_timestamp(&tcp_opt);
+	if (!cookie_check_timestamp(&tcp_opt))
+		goto out;
 
 	ret = NULL;
 	req = inet_reqsk_alloc(&tcp_request_sock_ops); /* for safety */

net/ipv6/syncookies.c

@@ -180,8 +180,8 @@ struct sock *cookie_v6_check(struct sock *sk, struct sk_buff *skb)
 	memset(&tcp_opt, 0, sizeof(tcp_opt));
 	tcp_parse_options(skb, &tcp_opt, &hash_location, 0);
 
-	if (tcp_opt.saw_tstamp)
-		cookie_check_timestamp(&tcp_opt);
+	if (!cookie_check_timestamp(&tcp_opt))
+		goto out;
 
 	ret = NULL;
 	req = inet6_reqsk_alloc(&tcp6_request_sock_ops);