Time-of-check Time-of-use (TOCTOU) Affecting kernel-rt-kvm package, versions *
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Start learning- Snyk IDSNYK-CENTOS8-KERNELRTKVM-14514100
- published18 Dec 2025
- disclosed16 Dec 2025
Introduced: 16 Dec 2025
NewCVE-2025-68241 (opens in a new tab)How to fix?
There is no fixed version for Centos:8 kernel-rt-kvm.
NVD Description
Note: Versions mentioned in the description apply only to the upstream kernel-rt-kvm package and not the kernel-rt-kvm package as distributed by Centos.
See How to fix? for Centos:8 relevant fixed versions and status.
In the Linux kernel, the following vulnerability has been resolved:
ipv4: route: Prevent rt_bind_exception() from rebinding stale fnhe
The sit driver's 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 path's __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
References
- https://access.redhat.com/security/cve/CVE-2025-68241
- 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