CVE-2026-72429: ipv6: ioam: fix type confusion of dst_entry

CVE-2026-72429: ipv6: ioam: fix type confusion of dst_entry

[Greg Kroah-Hartman]

From: Greg Kroah-Hartman <gregkh@kernel.org>

Description
===========

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.

The Linux kernel CVE team has assigned CVE-2026-72429 to this issue.


Affected and fixed versions
===========================

	Issue introduced in 6.16 with commit 47ce7c854563fe8450e9cb8dcd62c6470e28076b and fixed in 6.18.40 with commit ea24f911ead85ba3d570a31e37c52b6c949f6928
	Issue introduced in 6.16 with commit 47ce7c854563fe8450e9cb8dcd62c6470e28076b and fixed in 7.1.5 with commit 5a3b2ee1e96d0580a8ed8deda6dfa430604f9ab0
	Issue introduced in 6.16 with commit 47ce7c854563fe8450e9cb8dcd62c6470e28076b and fixed in 7.2-rc1 with commit 9ed19e11d2146076d117d51a940643990118449b

Please see https://www.kernel.org for a full list of currently supported
kernel versions by the kernel community.

Unaffected versions might change over time as fixes are backported to
older supported kernel versions.  The official CVE entry at
	https://cve.org/CVERecord/?id=CVE-2026-72429
will be updated if fixes are backported, please check that for the most
up to date information about this issue.


Affected files
==============

The file(s) affected by this issue are:
	net/ipv6/ioam6_iptunnel.c


Mitigation
==========

The Linux kernel CVE team recommends that you update to the latest
stable kernel version for this, and many other bugfixes.  Individual
changes are never tested alone, but rather are part of a larger kernel
release.  Cherry-picking individual commits is not recommended or
supported by the Linux kernel community at all.  If however, updating to
the latest release is impossible, the individual changes to resolve this
issue can be found at these commits:
	https://git.kernel.org/stable/c/ea24f911ead85ba3d570a31e37c52b6c949f6928
	https://git.kernel.org/stable/c/5a3b2ee1e96d0580a8ed8deda6dfa430604f9ab0
	https://git.kernel.org/stable/c/9ed19e11d2146076d117d51a940643990118449b