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

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.