ipv6: fix use-after-free in ip6_finish_output2()

ip6_finish_output2() caches a pointer to the IPv6 destination
address (daddr) before invoking lwtunnel_xmit().  The LWT-BPF
transmit path or other encapsulation operations within
lwtunnel_xmit() can reallocate the skb head, freeing the memory
that daddr points to.  When lwtunnel_xmit() returns
LWTUNNEL_XMIT_CONTINUE, the function continues to use the stale
daddr pointer to compute the nexthop and to look up or create the
neighbour entry.  This results in a use-after-free read, which can
leak sensitive kernel data, pollute the neighbour table with
arbitrary values, misdirect traffic, or crash the system.

Fix this by re-fetching the IPv6 header and the destination
address pointer after lwtunnel_xmit() returns
LWTUNNEL_XMIT_CONTINUE, ensuring that the subsequent nexthop
computation and neighbour lookup operate on valid memory.

Fixes: e415ed3a4b ("ipv6: use skb_expand_head in ip6_finish_output2")
Cc: stable@vger.kernel.org
Reported-by: Vega <vega@nebusec.ai>
Signed-off-by: Luxiao Xu <rakukuip@gmail.com>
Signed-off-by: Ren Wei <weir@nebusec.ai>
Reviewed-by: Vadim Fedorenko <vadim.fedorenko@linux.dev>
Reviewed-by: Ido Schimmel <idosch@nvidia.com>
Link: https://patch.msgid.link/4aa3f53bc44e79572c6dd2340ec7b68ef1a3d87d.1786516730.git.rakukuip@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

net/ipv6/ip6_output.c

@@ -116,6 +116,8 @@ static int ip6_finish_output2(struct net *net, struct sock *sk, struct sk_buff *
 
 		if (res != LWTUNNEL_XMIT_CONTINUE)
 			return res;
+		hdr = ipv6_hdr(skb);
+		daddr = &hdr->daddr;
 	}
 
 	IP6_UPD_PO_STATS(net, idev, IPSTATS_MIB_OUT, skb->len);