CVE-2026-99072: ipv4: stop route notification sizing when nexthop group shrinks

CVE-2026-99072: ipv4: stop route notification sizing when nexthop group shrinks

[Greg Kroah-Hartman]

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

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

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

ipv4: stop route notification sizing when nexthop group shrinks

Commit 680aea08e78c ("net: ipv4: Emit notification when fib
hardware flags are changed") added an RCU-only fib_nlmsg_size() call
for asynchronous hardware flag notifications.

fib_nlmsg_size() checks fib_info_num_path() before each iteration,
then fib_info_nhc() independently reloads nh->nh_grp. If
RTM_NEWNEXTHOP replaces a group with fewer paths between those loads,
fib_info_nhc() returns NULL and fib_nexthop_nlmsg_size() dereferences
it.

Stop sizing when the indexed path is absent. Nexthop groups are
dense, so the current snapshot has no later path.

The Linux kernel CVE team has assigned CVE-2026-99072 to this issue.


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

	Issue introduced in 5.12 with commit 680aea08e78c003292415518ad270bc20f9c80b1 and fixed in 6.6.160 with commit 72d9d17092bb37b0717f12042a0817d01a1f9e78
	Issue introduced in 5.12 with commit 680aea08e78c003292415518ad270bc20f9c80b1 and fixed in 6.12.113 with commit ad1c9f1b98c92609e9bb2ae8f05916defc3b0a54
	Issue introduced in 5.12 with commit 680aea08e78c003292415518ad270bc20f9c80b1 and fixed in 6.18.57 with commit c1f48d1c2d5e9efd904dbbc2f2b3d41543e55c49
	Issue introduced in 5.12 with commit 680aea08e78c003292415518ad270bc20f9c80b1 and fixed in 7.2.11 with commit ad2b3a68b8a7d5d27de88e05674e4adb1ce46d88
	Issue introduced in 5.12 with commit 680aea08e78c003292415518ad270bc20f9c80b1 and fixed in 7.3 with commit d3daa33c7af92e38f2274bbd2576f58bfbaf89e6

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-2026-99072
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/ipv4/fib_semantics.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/72d9d17092bb37b0717f12042a0817d01a1f9e78
	https://git.kernel.org/stable/c/ad1c9f1b98c92609e9bb2ae8f05916defc3b0a54
	https://git.kernel.org/stable/c/c1f48d1c2d5e9efd904dbbc2f2b3d41543e55c49
	https://git.kernel.org/stable/c/ad2b3a68b8a7d5d27de88e05674e4adb1ce46d88
	https://git.kernel.org/stable/c/d3daa33c7af92e38f2274bbd2576f58bfbaf89e6