ipvs: use parsed transport offset in TCP state lookup

[ Upstream commit 2500fa3 ]

TCP state handling reparses the skb to find the TCP header. For IPv6 it
uses sizeof(struct ipv6hdr), while the surrounding IPVS code already
parsed the packet with ip_vs_fill_iph_skb() and has the real
transport-header offset in iph.len.

This makes TCP state handling look at the wrong bytes when an IPv6
packet carries extension headers. Use the parsed transport offset passed
down from ip_vs_set_state() when reading the TCP header.

For IPv4 and for IPv6 packets without extension headers, the passed
offset matches the previous value.

Fixes: 0bbdd42 ("IPVS: Extend protocol DNAT/SNAT and state handlers")
Link: https://lore.kernel.org/netdev/20260705125659.37744-1-zhaoyz24@mails.tsinghua.edu.cn/
Reported-by: Yizhou Zhao <zhaoyz24@mails.tsinghua.edu.cn>
Reported-by: Yuxiang Yang <yangyx22@mails.tsinghua.edu.cn>
Reported-by: Ao Wang <wangao@seu.edu.cn>
Reported-by: Xuewei Feng <fengxw06@126.com>
Reported-by: Qi Li <qli01@tsinghua.edu.cn>
Reported-by: Ke Xu <xuke@tsinghua.edu.cn>
Assisted-by: Claude Code:GLM-5.2
Signed-off-by: Yizhou Zhao <zhaoyz24@mails.tsinghua.edu.cn>
Acked-by: Julian Anastasov <ja@ssi.bg>
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Sasha Levin <sashal@kernel.org>

Author: ArtisticFantasy

net/netfilter/ipvs/ip_vs_proto_tcp.c

@@ -585,13 +585,7 @@ tcp_state_transition(struct ip_vs_conn *cp, int direction,
 {
 	struct tcphdr _tcph, *th;
 
-#ifdef CONFIG_IP_VS_IPV6
-	int ihl = cp->af == AF_INET ? ip_hdrlen(skb) : sizeof(struct ipv6hdr);
-#else
-	int ihl = ip_hdrlen(skb);
-#endif
-
-	th = skb_header_pointer(skb, ihl, sizeof(_tcph), &_tcph);
+	th = skb_header_pointer(skb, iph_len, sizeof(_tcph), &_tcph);
 	if (th == NULL)
 		return;