CVE-2026-72421

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.