netdev - [PATCHv3 net 2/2] IPv6: reply ICMP error if the first fragment doesn't include all headers

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Date:   Fri, 23 Oct 2020 14:43:47 +0800
From:   Hangbin Liu <liuhangbin@...il.com>
To:     netdev@...r.kernel.org
Cc:     Alexey Kuznetsov <kuznet@....inr.ac.ru>,
        Hideaki YOSHIFUJI <yoshfuji@...ux-ipv6.org>,
        Jakub Kicinski <kuba@...nel.org>,
        Eric Dumazet <eric.dumazet@...il.com>,
        "David S . Miller" <davem@...emloft.net>,
        Willem de Bruijn <willemdebruijn.kernel@...il.com>,
        Hangbin Liu <liuhangbin@...il.com>
Subject: [PATCHv3 net 2/2] IPv6: reply ICMP error if the first fragment doesn't include all headers

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.

As the packet may be any kind of L4 protocol, I only checked some common
protocols' header length and handle others by (offset + 1) > skb->len.
Checking each packet header in IPv6 fast path will have performance impact,
so I put the checking in ipv6_frag_rcv().

When send ICMP error message, if the 1st truncated fragment is ICMP message,
icmp6_send() will break as is_ineligible() return true. So I added a check
in is_ineligible() to let fragment packet with nexthdr ICMP but no ICMP header
return false.

v3:
a) use frag_off to check if this is a fragment packet
b) check some common protocols' header length

v2:
a) Move header check to ipv6_frag_rcv(). Also check the ipv6_skip_exthdr()
   return value
b) Fix ipv6_find_hdr() parameter type miss match in is_ineligible()

Signed-off-by: Hangbin Liu <liuhangbin@...il.com>
---
 net/ipv6/icmp.c       | 10 +++++++++-
 net/ipv6/reassembly.c | 33 ++++++++++++++++++++++++++++++++-
 2 files changed, 41 insertions(+), 2 deletions(-)

diff --git a/net/ipv6/icmp.c b/net/ipv6/icmp.c
index ec448b71bf9a..0bda77d7e6b8 100644
--- a/net/ipv6/icmp.c
+++ b/net/ipv6/icmp.c
@@ -145,6 +145,7 @@ static bool is_ineligible(const struct sk_buff *skb)
 	int ptr = (u8 *)(ipv6_hdr(skb) + 1) - skb->data;
 	int len = skb->len - ptr;
 	__u8 nexthdr = ipv6_hdr(skb)->nexthdr;
+	unsigned int offs = 0;
 	__be16 frag_off;
 
 	if (len < 0)
@@ -153,12 +154,19 @@ static bool is_ineligible(const struct sk_buff *skb)
 	ptr = ipv6_skip_exthdr(skb, ptr, &nexthdr, &frag_off);
 	if (ptr < 0)
 		return false;
+
 	if (nexthdr == IPPROTO_ICMPV6) {
 		u8 _type, *tp;
 		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;
diff --git a/net/ipv6/reassembly.c b/net/ipv6/reassembly.c
index 1f5d4d196dcc..bf042bcb5a47 100644
--- a/net/ipv6/reassembly.c
+++ b/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)
+		goto fail_hdr;
+
+	/* 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_MF) && 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) {
-- 
2.25.4