netfilter: IPv6: initialize TOS field in REJECT target module

The IPv6 header is not zeroed out in alloc_skb so we must initialize
it properly unless we want to see IPv6 packets with random TOS fields
floating around. The current implementation resets the flow label
but this could be changed if deemed necessary.

We stumbled upon this issue when trying to apply a mangle rule to
the RST packet generated by the REJECT target module.

Signed-off-by: Fernando Luis Vazquez Cao <[e-mail]>
Signed-off-by: Pablo Neira Ayuso <[e-mail]>

net/ipv6/netfilter/ip6t_REJECT.c

···
 	int tcphoff, needs_ack;
 	const struct ipv6hdr *oip6h = ipv6_hdr(oldskb);
 	struct ipv6hdr *ip6h;
+#define DEFAULT_TOS_VALUE	0x0U
+	const __u8 tclass = DEFAULT_TOS_VALUE;
 	struct dst_entry *dst = NULL;
 	u8 proto;
 	struct flowi6 fl6;
···
 	skb_put(nskb, sizeof(struct ipv6hdr));
 	skb_reset_network_header(nskb);
 	ip6h = ipv6_hdr(nskb);
-	ip6h->version = 6;
+	*(__be32 *)ip6h =  htonl(0x60000000 | (tclass << 20));
 	ip6h->hop_limit = ip6_dst_hoplimit(dst);
 	ip6h->nexthdr = IPPROTO_TCP;
 	ipv6_addr_copy(&ip6h->saddr, &oip6h->daddr);