o IPFW incorrectly handled filtering in the presence of previously

  reserved and now allocated TCP flags in incoming packets.  This patch
  stops overloading those bits in the IP firewall rules, and moves
  colliding flags to a seperate field, ipflg.  The IPFW userland
  management tool, ipfw(8), is updated to reflect this change.  New TCP
  flags related to ECN are now included in tcp.h for reference, although
  we don't currently implement TCP+ECN.

o To use this fix without completely rebuilding, it is sufficient to copy
  ip_fw.h and tcp.h into your appropriate include directory, then rebuild
  the ipfw kernel module, and ipfw tool, and install both.  Note that a
  mismatch between module and userland tool will result in incorrect
  installation of firewall rules that may have unexpected effects.  This
  is an MFC candidate, following shakedown.  This bug does not appear
  to affect ipfilter.

Reviewed by:	security-officer, billf
Reported by:	Aragon Gouveia <aragon@phat.za.net>

1 files changed, 13 insertions, 5 deletions

diff --git a/sys/netinet/ip_fw.c b/sys/netinet/ip_fw.c
index 7e69223..3f3d325 100644
--- a/sys/netinet/ip_fw.c
+++ b/sys/netinet/ip_fw.c
@@ -244,10 +244,16 @@ static int
 tcpflg_match(struct tcphdr *tcp, struct ip_fw *f)
 {
 	u_char		flg_set, flg_clr;
-	
-	if ((f->fw_tcpf & IP_FW_TCPF_ESTAB) &&
-	    (tcp->th_flags & (IP_FW_TCPF_RST | IP_FW_TCPF_ACK)))
-		return 1;
+
+	/*
+	 * If an established connection is required, reject packets that
+	 * have only SYN of RST|ACK|SYN set.  Otherwise, fall through to
+	 * other flag requirements.
+	 */
+	if ((f->fw_ipflg & IP_FW_IF_TCPEST) &&
+	    ((tcp->th_flags & (IP_FW_TCPF_RST | IP_FW_TCPF_ACK |
+	    IP_FW_TCPF_SYN)) == IP_FW_TCPF_SYN))
+		return 0;
 
 	flg_set = tcp->th_flags & f->fw_tcpf;
 	flg_clr = tcp->th_flags & f->fw_tcpnf;
@@ -1243,7 +1249,9 @@ again:
 
 			if (f->fw_ipflg & IP_FW_IF_TCPOPT && !tcpopts_match(tcp, f))
 				continue;
-			if (f->fw_ipflg & IP_FW_IF_TCPFLG && !tcpflg_match(tcp, f))
+			if (((f->fw_ipflg & IP_FW_IF_TCPFLG) ||
+			    (f->fw_ipflg & IP_FW_IF_TCPEST)) &&
+			    !tcpflg_match(tcp, f))
 				continue;
 			if (f->fw_ipflg & IP_FW_IF_TCPSEQ && tcp->th_seq != f->fw_tcpseq)
 				continue;