Medium [CVE-2026-72429] fix type confusion of dst_entry

In the Linux kernel, the following vulnerability has been resolved: ipv6: ioam: fix type confusion of dst_entry IOAM uses a dummy dst_entry(null_dst) to mark that the destination should not be changed after the transformation. This dst is stored in the IOAM lwt state and may be passed to dst_cache_set_ip6().

However, the IPv6 dst cache path eventually calls rt6_get_cookie(), which treats the dst_entry as part of a struct rt6_info. Since the null_dst was embedded directly as a struct dst_entry in struct ioam6_lwt, this resulted in an invalid cast and rt6_get_cookie() reading fields from the wrong object.

In practice, the wrong cookie is not used while dst->obsolete is zero, but rt6_get_cookie() may also access per-cpu value when rt->sernum is zero. In this case, rt->sernum aliases ioam6_lwt::cache::reset_ts, which can become zero, making this a potential invalid pointer access.

Fix this by embedding a full struct rt6_info for the dummy IPv6 route and passing its dst member to the dst APIs. A flaw was found in the Linux kernel's IPv6 In-situ Operations, Administration, and Maintenance (IOAM) subsystem.

This vulnerability arises from a type confusion where a dummy network destination entry (dst_entry) is incorrectly handled as a routing information structure (rt6_info). This can result in an invalid pointer access, potentially causing system instability or a denial of service.