CVE-2026-74597: ip6_tunnel: clear skb2->cb[] in ip6ip6_err()

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

ip6tunnel: clear skb2->cb[] in ip6ip6err()

ip6ip6err() clones an outer IPv6 ICMP error skb, pulls it to the quoted inner IPv6 packet, and then passes the clone to icmpv6send(). The clone still carries the outer packet's inet6skbparm in skb->cb.

If the outer packet had a Home Address Option, IP6CB(skb2)->dsthao remains non-zero after skbpull(). icmpv6send() later calls mip6addrswap(), which uses that stale dsthao offset against the quoted inner packet. A malformed inner destination-options header can then make the HAO lookup and address swap run past the end of the quoted packet and corrupt skbsharedinfo.

Clear skb2->cb[] before pulling the quoted inner IPv6 packet so the reply path does not reuse metadata left by the outer IPv6 stack.