CVE-2026-43037: ip6_tunnel: clear skb2->cb[] in ip4ip6_err()

In the Linux kernel, the following vulnerability has been resolved:

ip6tunnel: clear skb2->cb[] in ip4ip6err()

Oskar Kjos reported the following problem.

ip4ip6err() calls icmpsend() on a cloned skb whose cb[] was written by the IPv6 receive path as struct inet6skbparm. icmpsend() passes IPCB(skb2) to ipoptionsecho(), which interprets that cb[] region as struct inetskbparm (IPv4). The layouts differ: inet6skbparm.nhoff at offset 14 overlaps inetskbparm.opt.rr, producing a non-zero rr value. ipoptionsecho() then reads optlen from attacker-controlled packet data at sptr[rr+1] and copies that many bytes into dopt->data, a fixed 40-byte stack buffer (IPOPTIONSDATAFIXEDSIZE).

To fix this we clear skb2->cb[], as suggested by Oskar Kjos.

Also add minimal IPv4 header validation (version == 4, ihl >= 5).