CVE-2025-68241

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

ipv4: route: Prevent rt_bind_exception() from rebinding stale fnhe

The sit drivers packet transmission path calls: sit_tunnel_xmit() -> update_or_create_fnhe(), which lead to fnhe_remove_oldest() being called to delete entries exceeding FNHE_RECLAIM_DEPTH+random.

The race window is between fnhe_remove_oldest() selecting fnheX for deletion and the subsequent kfree_rcu(). During this time, the concurrent paths __mkroute_output() -> find_exception() can fetch the soon-to-be-deleted fnheX, and rt_bind_exception() then binds it with a new dst using a dst_hold(). When the original fnheX is freed via RCU, the dst reference remains permanently leaked.

CPU 0 CPU 1 __mkroute_output() find_exception() [fnheX] update_or_create_fnhe() fnhe_remove_oldest() [fnheX] rt_bind_exception() [bind dst] RCU callback [fnheX freed, dst leak]

This issue manifests as a device reference count leak and a warning in dmesg when unregistering the net device:

unregister_netdevice: waiting for sitX to become free. Usage count = N

Ido Schimmel provided the simple test validation method [1].

The fix clears oldest->fnhe_daddr before calling fnhe_flush_routes(). Since rt_bind_exception() checks this field, setting it to zero prevents the stale fnhe from being reused and bound to a new dst just before it is freed.

[1] ip netns add ns1 ip -n ns1 link set dev lo up ip -n ns1 address add 192.0.2.1/32 dev lo ip -n ns1 link add name dummy1 up type dummy ip -n ns1 route add 192.0.2.2/32 dev dummy1 ip -n ns1 link add name gretap1 up arp off type gretap local 192.0.2.1 remote 192.0.2.2 ip -n ns1 route add 198.51.0.0/16 dev gretap1 taskset -c 0 ip netns exec ns1 mausezahn gretap1 -A 198.51.100.1 -B 198.51.0.0/16 -t udp -p 1000 -c 0 -q & taskset -c 2 ip netns exec ns1 mausezahn gretap1 -A 198.51.100.1 -B 198.51.0.0/16 -t udp -p 1000 -c 0 -q & sleep 10 ip netns pids ns1 | xargs kill ip netns del ns1

Affected Software

References

• https://git.kernel.org/stable/c/041ab9ca6e80d8f792bb69df28ebf1ef39c06af8
• https://git.kernel.org/stable/c/0fd16ed6dc331636fb2a874c42d2f7d3156f7ff0
• https://git.kernel.org/stable/c/298f1e0694ab4edb6092d66efed93c4554e6ced1
• https://git.kernel.org/stable/c/4b7210da22429765d19460d38c30eeca72656282
• https://git.kernel.org/stable/c/69d35c12168f9c59b159ae566f77dfad9f96d7ca
• https://git.kernel.org/stable/c/ac1499fcd40fe06479e9b933347b837ccabc2a40
• https://git.kernel.org/stable/c/b84f083f50ecc736a95091691339a1b363962f0e
• https://git.kernel.org/stable/c/b8a44407bdaf2f0c5505cc7d9fc7d8da90cf9a94