Merge branch 'ipv6-reply-icmp-error-if-fragment-doesn-t-contain-all-headers'

Hangbin Liu says:

====================
IPv6: reply ICMP error if fragment doesn't contain all headers

When our Engineer run latest IPv6 Core Conformance test, test v6LC.1.3.6:
First Fragment Doesn’t Contain All Headers[1] failed. The test purpose is to
verify that the node (Linux for example) should properly process IPv6 packets
that don’t include all the headers through the Upper-Layer header.

Based on RFC 8200, Section 4.5 Fragment Header

  -  If the first fragment does not include all headers through an
     Upper-Layer header, then that fragment should be discarded and
     an ICMP Parameter Problem, Code 3, message should be sent to
     the source of the fragment, with the Pointer field set to zero.

The first patch add a definition for ICMPv6 Parameter Problem, code 3.
The second patch add a check for the 1st fragment packet to make sure
Upper-Layer header exist.

[1] Page 68, v6LC.1.3.6: First Fragment Doesn’t Contain All Headers part A, B,
C and D at https://ipv6ready.org/docs/Core_Conformance_5_0_0.pdf
[2] My reproducer:

import sys, os
from scapy.all import *

def send_frag_dst_opt(src_ip6, dst_ip6):
    ip6 = IPv6(src = src_ip6, dst = dst_ip6, nh = 44)

    frag_1 = IPv6ExtHdrFragment(nh = 60, m = 1)
    dst_opt = IPv6ExtHdrDestOpt(nh = 58)

    frag_2 = IPv6ExtHdrFragment(nh = 58, offset = 4, m = 1)
    icmp_echo = ICMPv6EchoRequest(seq = 1)

    pkt_1 = ip6/frag_1/dst_opt
    pkt_2 = ip6/frag_2/icmp_echo

    send(pkt_1)
    send(pkt_2)

def send_frag_route_opt(src_ip6, dst_ip6):
    ip6 = IPv6(src = src_ip6, dst = dst_ip6, nh = 44)

    frag_1 = IPv6ExtHdrFragment(nh = 43, m = 1)
    route_opt = IPv6ExtHdrRouting(nh = 58)

    frag_2 = IPv6ExtHdrFragment(nh = 58, offset = 4, m = 1)
    icmp_echo = ICMPv6EchoRequest(seq = 2)

    pkt_1 = ip6/frag_1/route_opt
    pkt_2 = ip6/frag_2/icmp_echo

    send(pkt_1)
    send(pkt_2)

if __name__ == '__main__':
    src = sys.argv[1]
    dst = sys.argv[2]
    conf.iface = sys.argv[3]
    send_frag_dst_opt(src, dst)
    send_frag_route_opt(src, dst)
====================

Link: https://lore.kernel.org/r/20201027123313.3717941-1-liuhangbin@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

include/uapi/linux/icmpv6.h

@@ -138,6 +138,7 @@ struct icmp6hdr {
 #define ICMPV6_HDR_FIELD		0
 #define ICMPV6_UNK_NEXTHDR		1
 #define ICMPV6_UNK_OPTION		2
+#define ICMPV6_HDR_INCOMP		3
 
 /*
  *	constants for (set|get)sockopt

net/ipv6/icmp.c

@@ -158,7 +158,13 @@ static bool is_ineligible(const struct sk_buff *skb)
 		tp = skb_header_pointer(skb,
 			ptr+offsetof(struct icmp6hdr, icmp6_type),
 			sizeof(_type), &_type);
-		if (!tp || !(*tp & ICMPV6_INFOMSG_MASK))
+
+		/* Based on RFC 8200, Section 4.5 Fragment Header, return
+		 * false if this is a fragment packet with no icmp header info.
+		 */
+		if (!tp && frag_off != 0)
+			return false;
+		else if (!tp || !(*tp & ICMPV6_INFOMSG_MASK))
 			return true;
 	}
 	return false;

net/ipv6/reassembly.c

@@ -42,6 +42,8 @@
 #include <linux/skbuff.h>
 #include <linux/slab.h>
 #include <linux/export.h>
+#include <linux/tcp.h>
+#include <linux/udp.h>
 
 #include <net/sock.h>
 #include <net/snmp.h>
@@ -322,7 +324,9 @@ static int ipv6_frag_rcv(struct sk_buff *skb)
 	struct frag_queue *fq;
 	const struct ipv6hdr *hdr = ipv6_hdr(skb);
 	struct net *net = dev_net(skb_dst(skb)->dev);
-	int iif;
+	__be16 frag_off;
+	int iif, offset;
+	u8 nexthdr;
 
 	if (IP6CB(skb)->flags & IP6SKB_FRAGMENTED)
 		goto fail_hdr;
@@ -351,6 +355,33 @@ static int ipv6_frag_rcv(struct sk_buff *skb)
 		return 1;
 	}
 
+	/* RFC 8200, Section 4.5 Fragment Header:
+	 * If the first fragment does not include all headers through an
+	 * Upper-Layer header, then that fragment should be discarded and
+	 * an ICMP Parameter Problem, Code 3, message should be sent to
+	 * the source of the fragment, with the Pointer field set to zero.
+	 */
+	nexthdr = hdr->nexthdr;
+	offset = ipv6_skip_exthdr(skb, skb_transport_offset(skb), &nexthdr, &frag_off);
+	if (offset >= 0) {
+		/* Check some common protocols' header */
+		if (nexthdr == IPPROTO_TCP)
+			offset += sizeof(struct tcphdr);
+		else if (nexthdr == IPPROTO_UDP)
+			offset += sizeof(struct udphdr);
+		else if (nexthdr == IPPROTO_ICMPV6)
+			offset += sizeof(struct icmp6hdr);
+		else
+			offset += 1;
+
+		if (!(frag_off & htons(IP6_OFFSET)) && offset > skb->len) {
+			__IP6_INC_STATS(net, __in6_dev_get_safely(skb->dev),
+					IPSTATS_MIB_INHDRERRORS);
+			icmpv6_param_prob(skb, ICMPV6_HDR_INCOMP, 0);
+			return -1;
+		}
+	}
+
 	iif = skb->dev ? skb->dev->ifindex : 0;
 	fq = fq_find(net, fhdr->identification, hdr, iif);
 	if (fq) {