CVE-2025-71097: ipv4: Fix reference count leak when using error routes with nexthop objects

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

ipv4: Fix reference count leak when using error routes with nexthop objects

When a nexthop object is deleted, it is marked as dead and then fibtableflush() is called to flush all the routes that are using the dead nexthop.

The current logic in fibtableflush() is to only flush error routes (e.g., blackhole) when it is called as part of network namespace dismantle (i.e., with flushall=true). Therefore, error routes are not flushed when their nexthop object is deleted:

# ip link add name dummy1 up type dummy # ip nexthop add id 1 dev dummy1 # ip route add 198.51.100.1/32 nhid 1 # ip route add blackhole 198.51.100.2/32 nhid 1 # ip nexthop del id 1 # ip route show blackhole 198.51.100.2 nhid 1 dev dummy1

As such, they keep holding a reference on the nexthop object which in turn holds a reference on the nexthop device, resulting in a reference count leak:

# ip link del dev dummy1 [ 70.516258] unregisternetdevice: waiting for dummy1 to become free. Usage count = 2

Fix by flushing error routes when their nexthop is marked as dead.

IPv6 does not suffer from this problem.