CVE-2025-68241

In the Linux kernel, the following vulnerability has been resolved:<br /> <br /> ipv4: route: Prevent rt_bind_exception() from rebinding stale fnhe<br /> <br /> The sit driver&amp;#39;s packet transmission path calls: sit_tunnel_xmit() -&gt;<br /> update_or_create_fnhe(), which lead to fnhe_remove_oldest() being called<br /> to delete entries exceeding FNHE_RECLAIM_DEPTH+random.<br /> <br /> The race window is between fnhe_remove_oldest() selecting fnheX for<br /> deletion and the subsequent kfree_rcu(). During this time, the<br /> concurrent path&amp;#39;s __mkroute_output() -&gt; find_exception() can fetch the<br /> soon-to-be-deleted fnheX, and rt_bind_exception() then binds it with a<br /> new dst using a dst_hold(). When the original fnheX is freed via RCU,<br /> the dst reference remains permanently leaked.<br /> <br /> CPU 0 CPU 1<br /> __mkroute_output()<br /> find_exception() [fnheX]<br /> update_or_create_fnhe()<br /> fnhe_remove_oldest() [fnheX]<br /> rt_bind_exception() [bind dst]<br /> RCU callback [fnheX freed, dst leak]<br /> <br /> This issue manifests as a device reference count leak and a warning in<br /> dmesg when unregistering the net device:<br /> <br /> unregister_netdevice: waiting for sitX to become free. Usage count = N<br /> <br /> Ido Schimmel provided the simple test validation method [1].<br /> <br /> The fix clears &amp;#39;oldest-&gt;fnhe_daddr&amp;#39; before calling fnhe_flush_routes().<br /> Since rt_bind_exception() checks this field, setting it to zero prevents<br /> the stale fnhe from being reused and bound to a new dst just before it<br /> is freed.<br /> <br /> [1]<br /> ip netns add ns1<br /> ip -n ns1 link set dev lo up<br /> ip -n ns1 address add 192.0.2.1/32 dev lo<br /> ip -n ns1 link add name dummy1 up type dummy<br /> ip -n ns1 route add 192.0.2.2/32 dev dummy1<br /> ip -n ns1 link add name gretap1 up arp off type gretap \<br /> local 192.0.2.1 remote 192.0.2.2<br /> ip -n ns1 route add 198.51.0.0/16 dev gretap1<br /> taskset -c 0 ip netns exec ns1 mausezahn gretap1 \<br /> -A 198.51.100.1 -B 198.51.0.0/16 -t udp -p 1000 -c 0 -q &amp;<br /> taskset -c 2 ip netns exec ns1 mausezahn gretap1 \<br /> -A 198.51.100.1 -B 198.51.0.0/16 -t udp -p 1000 -c 0 -q &amp;<br /> sleep 10<br /> ip netns pids ns1 | xargs kill<br /> ip netns del ns1