CVE-2026-72421: ipv4: fib: Don't ignore error route in local/main tables.

CVE-2026-72421: ipv4: fib: Don't ignore error route in local/main tables.

[Greg Kroah-Hartman]

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

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

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

ipv4: fib: Don't ignore error route in local/main tables.

When CONFIG_IP_MULTIPLE_TABLES is enabled but no rule is added,
fib_lookup() performs route lookup directly on two tables.

Since the first lookup does not properly bail out, the result
of an error route in the merged local/main table could be
overwritten by another route in the default table:

  # unshare -n
  # ip link set lo up
  # ip route add 192.168.0.0/24 dev lo table 253
  # ip route add unreachable 192.168.0.0/24
  # ip route get 192.168.0.1
  192.168.0.1 dev lo table default uid 0
      cache <local>

Once a random rule is added, the error route is respected:

  # ip rule add table 0
  # ip rule del table 0
  # ip route get 192.168.0.1
  RTNETLINK answers: No route to host

Let's fix the inconsistent behaviour.

The Linux kernel CVE team has assigned CVE-2026-72421 to this issue.


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

	Issue introduced in 3.6 with commit f4530fa574df4d833506c53697ed1daa0d390bf4 and fixed in 5.10.261 with commit a29e95fbc51e8a1b932773fd0e259b2080881443
	Issue introduced in 3.6 with commit f4530fa574df4d833506c53697ed1daa0d390bf4 and fixed in 5.15.212 with commit 5ae18d87a45698e8244d0fcba64c658c35a7dd3d
	Issue introduced in 3.6 with commit f4530fa574df4d833506c53697ed1daa0d390bf4 and fixed in 6.1.178 with commit 9127589aabdee588278e8d0d0bd3709a760a92c8
	Issue introduced in 3.6 with commit f4530fa574df4d833506c53697ed1daa0d390bf4 and fixed in 6.6.145 with commit 49eaf1403201357762d745a35882fb734107d763
	Issue introduced in 3.6 with commit f4530fa574df4d833506c53697ed1daa0d390bf4 and fixed in 6.12.97 with commit fd25996f57a95d56bc568c89b4921edcf334b7d8
	Issue introduced in 3.6 with commit f4530fa574df4d833506c53697ed1daa0d390bf4 and fixed in 6.18.40 with commit 828fad4fd418bcdb9f5d66fec0d184c52a85ec31
	Issue introduced in 3.6 with commit f4530fa574df4d833506c53697ed1daa0d390bf4 and fixed in 7.1.5 with commit a668fa160247d7bbe921548cb845f607b8b9305f
	Issue introduced in 3.6 with commit f4530fa574df4d833506c53697ed1daa0d390bf4 and fixed in 7.2-rc1 with commit b72f0db64205d9ce462038ba995d5d31eff32dc1

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-2026-72421
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:
	include/net/ip_fib.h


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/a29e95fbc51e8a1b932773fd0e259b2080881443
	https://git.kernel.org/stable/c/5ae18d87a45698e8244d0fcba64c658c35a7dd3d
	https://git.kernel.org/stable/c/9127589aabdee588278e8d0d0bd3709a760a92c8
	https://git.kernel.org/stable/c/49eaf1403201357762d745a35882fb734107d763
	https://git.kernel.org/stable/c/fd25996f57a95d56bc568c89b4921edcf334b7d8
	https://git.kernel.org/stable/c/828fad4fd418bcdb9f5d66fec0d184c52a85ec31
	https://git.kernel.org/stable/c/a668fa160247d7bbe921548cb845f607b8b9305f
	https://git.kernel.org/stable/c/b72f0db64205d9ce462038ba995d5d31eff32dc1