icmpv6_filter: allow ICMPv6 messages with bodies < 4 bytes

By using sizeof(_hdr), net/ipv6/raw.c:icmpv6_filter implicitly assumes
that any valid ICMPv6 message is at least eight bytes long, i.e., that
the message body is at least four bytes.

The DIS message of RPL (RFC 6550 section 6.2, from the 6LoWPAN world),
has a minimum length of only six bytes, and is thus blocked by
icmpv6_filter.

RFC 4443 seems to allow even a zero-sized body, making the minimum
allowable message size four bytes.
Signed-off-by: NWerner Almesberger <werner@almesberger.net>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/ipv6/raw.c

@@ -63,6 +63,8 @@
 #include <linux/seq_file.h>
 #include <linux/export.h>
 
+#define	ICMPV6_HDRLEN	4	/* ICMPv6 header, RFC 4443 Section 2.1 */
+
 static struct raw_hashinfo raw_v6_hashinfo = {
 	.lock = __RW_LOCK_UNLOCKED(raw_v6_hashinfo.lock),
 };
@@ -111,8 +113,11 @@ static int icmpv6_filter(const struct sock *sk, const struct sk_buff *skb)
 	struct icmp6hdr _hdr;
 	const struct icmp6hdr *hdr;
 
+	/* We require only the four bytes of the ICMPv6 header, not any
+	 * additional bytes of message body in "struct icmp6hdr".
+	 */
 	hdr = skb_header_pointer(skb, skb_transport_offset(skb),
-				 sizeof(_hdr), &_hdr);
+				 ICMPV6_HDRLEN, &_hdr);
 	if (hdr) {
 		const __u32 *data = &raw6_sk(sk)->filter.data[0];
 		unsigned int type = hdr->icmp6_type;