CVE-2026-43037

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

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

Oskar Kjos reported the following problem.

ip4ip6_err() calls icmp_send() on a cloned skb whose cb[] was written

by the IPv6 receive path as struct inet6_skb_parm. icmp_send() passes

IPCB(skb2) to __ip_options_echo(), which interprets that cb[] region

as struct inet_skb_parm (IPv4). The layouts differ: inet6_skb_parm.nhoff

at offset 14 overlaps inet_skb_parm.opt.rr, producing a non-zero rr

value. __ip_options_echo() 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 (IP_OPTIONS_DATA_FIXED_SIZE).

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

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