ipv6: ip6_fragment() should check CHECKSUM_PARTIAL

Quoting Tore Anderson from :

If the allfrag feature has been set on a host route (due to an ICMPv6
Packet Too Big received indicating a MTU of less than 1280),
TCP SYN/ACK packets to that destination appears to get an incorrect
TCP checksum. This in turn means they are thrown away as invalid.

In the case of an IPv4 client behind a link with a MTU of less than
1260, accessing an IPv6 server through a stateless translator,
this means that the client can only download a single large file
from the server, because once it is in the server's routing cache
with the allfrag feature set, new TCP connections can no longer
be established.

</endquote>

It appears ip6_fragment() doesn't handle CHECKSUM_PARTIAL properly.

As network drivers are not prepared to fetch correct transport header, a
safe fix is to call skb_checksum_help() before fragmenting packet.
Reported-by: Tore Anderson <tore@fud.no>
Signed-off-by: Eric Dumazet <edumazet@google.com>
Tested-by: Tore Anderson <tore@fud.no>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv6/ip6_output.c

@@ -788,6 +788,10 @@ int ip6_fragment(struct sk_buff *skb, int (*output)(struct sk_buff *))
 	}
 
 slow_path:
+	if ((skb->ip_summed == CHECKSUM_PARTIAL) &&
+	    skb_checksum_help(skb))
+		goto fail;
+
 	left = skb->len - hlen;		/* Space per frame */
 	ptr = hlen;			/* Where to start from */