CVE-2026-43038

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

ipv6: icmp: clear skb2->cb[] in ip6_err_gen_icmpv6_unreach()

Sashiko AI-review observed:

In ip6_err_gen_icmpv6_unreach(), the skb is an outer IPv4 ICMP error packet where its cb contains an IPv4 inet_skb_parm. When skb is cloned into skb2 and passed to icmp6_send(), it uses IP6CB(skb2).

IP6CB interprets the IPv4 inet_skb_parm as an inet6_skb_parm. The cipso offset in inet_skb_parm.opt directly overlaps with dsthao in inet6_skb_parm at offset 18.

If an attacker sends a forged ICMPv4 error with a CIPSO IP option, dsthao would be a non-zero offset. Inside icmp6_send(), mip6_addr_swap() is called and uses ipv6_find_tlv(skb, opt->dsthao, IPV6_TLV_HAO).

This would scan the inner, attacker-controlled IPv6 packet starting at that offset, potentially returning a fake TLV without checking if the remaining packet length can hold the full 18-byte struct ipv6_destopt_hao.

Could mip6_addr_swap() then perform a 16-byte swap that extends past the end of the packet data into skb_shared_info?

Should the cb array also be cleared in ip6_err_gen_icmpv6_unreach() and ip6ip6_err() to prevent this?

This patch implements the first suggestion.

I am not sure if ip6ip6_err() needs to be changed. A separate patch would be better anyway.

Affected Software

References

• https://git.kernel.org/stable/c/0452b6526b2f54b2413b9cb4ff1ea2ac542c99c7
• https://git.kernel.org/stable/c/1ceeebd5bd6d855b17a5df625109bfe29129d7cf
• https://git.kernel.org/stable/c/3d5127d998de617b130aae96b138dba22ac6a8a7
• https://git.kernel.org/stable/c/86ab3e55673a7a49a841838776f1ab18d23a67b5
• https://git.kernel.org/stable/c/a2edbb6393972a02114b6003953a5cef3104fada
• https://git.kernel.org/stable/c/a4437faf135da293d16fcc4cc607316742bd0ebb
• https://git.kernel.org/stable/c/c438ba010171b70bad22fc18b1d5bdc3627476e8
• https://git.kernel.org/stable/c/e41953e7d118e2702bcb217879c173d9d1d3cd4e