Re: ECN target bug report

[Andrea Rossato <mailing_list@istitutocolli.org>]

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attached you will find 2 patches. the first one is just a hack that 
provides a solution, the second is an attempt to prove the existence of 
the problem.
this is far from being the solution, it's just a workaround: checksum of 
the packets will be recalculated from scratch. this means that unclean 
packets with ec e cwr bits set will be cleaned after passing through 
this rule. that's the reason for partial checksum recalculation: if a 
packet was unclean it must remain as such.

The problem, as far as I can see it, could be located in csum_partial 
(arch/i386/checksum.S): i'm not a kernel hacker (i'm a lawyer, a legal 
scholar actually), but i do not see any mistake in the way partial 
checksum is carried out in tcp_etc_set. anyway checksum after partial or 
total recalculation differ. That's a fact. Evidence of the fact  can be 
gained with the second patch: in this case the kernel will log the 
checksum after partial recalculation and after total recalculation (that 
means that two calculations will take place). The two values differ!

now, is this the right place for such a bug submission? is this code 
manteined? should i refer to the kernel mailing list?

If someone could give me directions I would very much appreciate. I 
would like to see this problem solved.
Thanks for your attention.

1. check the bug:
echo 1 /proc/sys/net/ipv4/tcp_ecn
iptables -A OUTPUT -t mangle -o ppp0 -p tcp -d my.host.org --dport 80 -j 
ECN --ecn-tcp-remove
iptables -A OUTPUT -o ppp0 -p tcp -d my.host.org --dport 80 -m unclean 
-j DROP
packets will be dropped


2. apply one of the patches and try again:
packets will get though and the connection will be established.

andrea

[-- Attachment #2: ecn_csum.patch --]
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--- linux-2.4.20/net/ipv4/netfilter/ipt_ECN.c.orig	2002-12-07 23:22:37.000000000 +0100
+++ linux-2.4.20/net/ipv4/netfilter/ipt_ECN.c	2002-12-09 10:13:56.000000000 +0100
@@ -11,6 +11,7 @@
 #include <linux/skbuff.h>
 #include <linux/ip.h>
 #include <net/checksum.h>
+#include <net/tcp.h>
 
 #include <linux/netfilter_ipv4/ip_tables.h>
 #include <linux/netfilter_ipv4/ipt_ECN.h>
@@ -62,6 +63,7 @@
 	struct tcphdr *tcph = (void *) iph + iph->ihl * 4;
 	u_int16_t *tcpflags = (u_int16_t *)tcph + 6;
 	u_int16_t diffs[2];
+	u_int32_t tcplen;
 
 	/* raw socket (tcpdump) may have clone of incoming
 	 * skb: don't disturb it --RR */
@@ -87,13 +89,14 @@
 	}
 	
 	if (diffs[0] != *tcpflags) {
-		diffs[0] = htons(diffs[0]) ^ 0xFFFF;
-		diffs[1] = htons(*tcpflags);
-		tcph->check = csum_fold(csum_partial((char *)diffs,
-		                                    sizeof(diffs),
-		                                    tcph->check^0xFFFF));
-		(*pskb)->nfcache |= NFC_ALTERED;
 
+		tcplen = (*pskb)->len - iph->ihl*4;
+		tcph->check = 0;
+		tcph->check = tcp_v4_check(tcph, tcplen, iph->saddr, iph->daddr,
+				   csum_partial((char *)tcph, tcph->doff*4,
+					   (*pskb)->csum));
+		(*pskb)->nfcache |= NFC_ALTERED;
+	
 		return 1;
 	}
 

[-- Attachment #3: ecn_csum_with_debug_info.patch --]
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--- linux-2.4.20/net/ipv4/netfilter/ipt_ECN.c.orig	2002-12-09 10:44:03.000000000 +0100
+++ linux-2.4.20/net/ipv4/netfilter/ipt_ECN.c	2002-12-09 10:48:46.000000000 +0100
@@ -11,6 +11,7 @@
 #include <linux/skbuff.h>
 #include <linux/ip.h>
 #include <net/checksum.h>
+#include <net/tcp.h>
 
 #include <linux/netfilter_ipv4/ip_tables.h>
 #include <linux/netfilter_ipv4/ipt_ECN.h>
@@ -62,6 +63,7 @@
 	struct tcphdr *tcph = (void *) iph + iph->ihl * 4;
 	u_int16_t *tcpflags = (u_int16_t *)tcph + 6;
 	u_int16_t diffs[2];
+	u_int32_t tcplen;
 
 	/* raw socket (tcpdump) may have clone of incoming
 	 * skb: don't disturb it --RR */
@@ -92,6 +94,17 @@
 		tcph->check = csum_fold(csum_partial((char *)diffs,
 		                                    sizeof(diffs),
 		                                    tcph->check^0xFFFF));
+
+		printk(KERN_WARNING "ECN: checksum after partial recalculation \"%x\"\n", tcph->check );
+		
+		tcplen = (*pskb)->len - iph->ihl*4;
+		tcph->check = 0;
+		tcph->check = tcp_v4_check(tcph, tcplen, iph->saddr, iph->daddr,
+				   csum_partial((char *)tcph, tcph->doff*4,
+					   (*pskb)->csum));
+
+		printk(KERN_WARNING "ECN: checksum after total recalculation \"%x\"\n", tcph->check );
+
 		(*pskb)->nfcache |= NFC_ALTERED;
 
 		return 1;