Move Path MTU discovery ICMP processing from icmp_input() to

tcp_ctlinput() and subject it to active tcpcb and sequence
number checking.  Previously any ICMP unreachable/needfrag
message would cause an update to the TCP hostcache.  Now only
ICMP PMTU messages belonging to an active TCP session with
the correct src/dst/port and sequence number will update the
hostcache and complete the path MTU discovery process.

Note that we don't entirely implement the recommended counter
measures of Section 7.2 of the paper.  However we close down
the possible degradation vector from trivially easy to really
complex and resource intensive.  In addition we have limited
the smallest acceptable MTU with net.inet.tcp.minmss sysctl
for some time already, further reducing the effect of any
degradation due to an attack.

Security:	draft-gont-tcpm-icmp-attacks-03.txt Section 7.2
MFC after:	3 days

1 files changed, 1 insertions, 45 deletions

diff --git a/sys/netinet/ip_icmp.c b/sys/netinet/ip_icmp.c
index 4547886..c57b684 100644
--- a/sys/netinet/ip_icmp.c
+++ b/sys/netinet/ip_icmp.c
@@ -125,7 +125,6 @@ int	icmpprintfs = 0;
 
 static void	icmp_reflect(struct mbuf *);
 static void	icmp_send(struct mbuf *, struct mbuf *);
-static int	ip_next_mtu(int, int);
 
 extern	struct protosw inetsw[];
 
@@ -407,49 +406,6 @@ icmp_input(m, off)
 			printf("deliver to protocol %d\n", icp->icmp_ip.ip_p);
 #endif
 		icmpsrc.sin_addr = icp->icmp_ip.ip_dst;
-
-		/*
-		 * MTU discovery:
-		 * If we got a needfrag and there is a host route to the
-		 * original destination, and the MTU is not locked, then
-		 * set the MTU in the route to the suggested new value
-		 * (if given) and then notify as usual.  The ULPs will
-		 * notice that the MTU has changed and adapt accordingly.
-		 * If no new MTU was suggested, then we guess a new one
-		 * less than the current value.  If the new MTU is
-		 * unreasonably small (defined by sysctl tcp_minmss), then
-		 * we don't update the MTU value.
-		 *
-		 * XXX: All this should be done in tcp_mtudisc() because
-		 * the way we do it now, everyone can send us bogus ICMP
-		 * MSGSIZE packets for any destination. By doing this far
-		 * higher in the chain we have a matching tcp connection.
-		 * Thus spoofing is much harder. However there is no easy
-		 * non-hackish way to pass the new MTU up to tcp_mtudisc().
-		 * Also see next XXX regarding IPv4 AH TCP.
-		 */
-		if (code == PRC_MSGSIZE) {
-			int mtu;
-			struct in_conninfo inc;
-
-			bzero(&inc, sizeof(inc));
-			inc.inc_flags = 0; /* IPv4 */
-			inc.inc_faddr = icmpsrc.sin_addr;
-
-			mtu = ntohs(icp->icmp_nextmtu);
-			if (!mtu)
-				mtu = ip_next_mtu(mtu, 1);
-
-			if (mtu >= max(296, (tcp_minmss +
-					sizeof(struct tcpiphdr))))
-				tcp_hc_updatemtu(&inc, mtu);
-
-#ifdef DEBUG_MTUDISC
-			printf("MTU for %s reduced to %d\n",
-				inet_ntoa(icmpsrc.sin_addr), mtu);
-#endif
-		}
-
 		/*
 		 * XXX if the packet contains [IPv4 AH TCP], we can't make a
 		 * notification to TCP layer.
@@ -831,7 +787,7 @@ iptime()
  * given current value MTU.  If DIR is less than zero, a larger plateau
  * is returned; otherwise, a smaller value is returned.
  */
-static int
+int
 ip_next_mtu(mtu, dir)
 	int mtu;
 	int dir;