Limiters and sanity checks for TCP MSS (maximum segement size)

resource exhaustion attacks.

For network link optimization TCP can adjust its MSS and thus
packet size according to the observed path MTU.  This is done
dynamically based on feedback from the remote host and network
components along the packet path.  This information can be
abused to pretend an extremely low path MTU.

The resource exhaustion works in two ways:

 o during tcp connection setup the advertized local MSS is
   exchanged between the endpoints.  The remote endpoint can
   set this arbitrarily low (except for a minimum MTU of 64
   octets enforced in the BSD code).  When the local host is
   sending data it is forced to send many small IP packets
   instead of a large one.

   For example instead of the normal TCP payload size of 1448
   it forces TCP payload size of 12 (MTU 64) and thus we have
   a 120 times increase in workload and packets. On fast links
   this quickly saturates the local CPU and may also hit pps
   processing limites of network components along the path.

   This type of attack is particularly effective for servers
   where the attacker can download large files (WWW and FTP).

   We mitigate it by enforcing a minimum MTU settable by sysctl
   net.inet.tcp.minmss defaulting to 256 octets.

 o the local host is reveiving data on a TCP connection from
   the remote host.  The local host has no control over the
   packet size the remote host is sending.  The remote host
   may chose to do what is described in the first attack and
   send the data in packets with an TCP payload of at least
   one byte.  For each packet the tcp_input() function will
   be entered, the packet is processed and a sowakeup() is
   signalled to the connected process.

   For example an attack with 2 Mbit/s gives 4716 packets per
   second and the same amount of sowakeup()s to the process
   (and context switches).

   This type of attack is particularly effective for servers
   where the attacker can upload large amounts of data.
   Normally this is the case with WWW server where large POSTs
   can be made.

   We mitigate this by calculating the average MSS payload per
   second.  If it goes below 'net.inet.tcp.minmss' and the pps
   rate is above 'net.inet.tcp.minmssoverload' defaulting to
   1000 this particular TCP connection is resetted and dropped.

MITRE CVE:	CAN-2004-0002
Reviewed by:	sam (mentor)
MFC after:	1 day

1 files changed, 19 insertions, 1 deletions

diff --git a/sys/netinet/tcp.h b/sys/netinet/tcp.h
index e189600..0143bca 100644
--- a/sys/netinet/tcp.h
+++ b/sys/netinet/tcp.h
@@ -105,11 +105,29 @@ struct tcphdr {
 
 /*
  * Default maximum segment size for TCP.
- * With an IP MSS of 576, this is 536,
+ * With an IP MTU of 576, this is 536,
  * but 512 is probably more convenient.
  * This should be defined as MIN(512, IP_MSS - sizeof (struct tcpiphdr)).
  */
 #define	TCP_MSS	512
+/*
+ * TCP_MINMSS is defined to be 256 which is fine for the smallest
+ * link MTU (296 bytes, SLIP interface) in the Internet.
+ * However it is very unlikely to come across such low MTU interfaces
+ * these days (anno dato 2003).
+ * Probably it can be set to 512 without ill effects. But we play safe.
+ * See tcp_subr.c tcp_minmss SYSCTL declaration for more comments.
+ * Setting this to "0" disables the minmss check.
+ */
+#define	TCP_MINMSS 256
+/*
+ * TCP_MINMSSOVERLOAD is defined to be 1000 which should cover any type
+ * of interactive TCP session.
+ * See tcp_subr.c tcp_minmssoverload SYSCTL declaration and tcp_input.c
+ * for more comments.
+ * Setting this to "0" disables the minmssoverload check.
+ */
+#define	TCP_MINMSSOVERLOAD 1000
 
 /*
  * Default maximum segment size for TCP6.