HIGH Introduced in 5.8
vxlan FdbNexthop Race
CVE-2026-98023
CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H
KernelScan AI7.0HIGH
01Description
In the Linux kernel, the following vulnerability has been resolved: vxlan: reject dynamic fdb entries that reference a nexthop id The commit cited in the Fixes tag allowed VXLAN FDB entries to point to FDB nexthops so that overlay traffic could be load balanced across multiple VTEPs. Such entries can only be configured from user space, cannot be learned and cannot roam. They only make sense with a user space control plane such as E-VPN where data plane learning is disabled. Despite that, the VXLAN driver does not currently prevent such entries from being configured with the "dynamic" flag. The per-nexthop FDB list is only protected by the per-device hash lock, which is not sufficient when two VXLAN devices point to the same FDB nexthop and therefore share the list. Aging runs in softirq context without RTNL, so an entry deleted by one device can race with an addition or deletion from the other, leading to list corruption: list_del corruption. next->prev should be ffff8881069d9548, but was dead000000000122. (next=ffff8881069d9448) WARNING: CPU: 0 PID: 90 at lib/list_debug.c:65 __list_del_entry_valid_or_report+0x1aa/0x210 ... vxlan_fdb_destroy+0x5b8/0xad0 vxlan_cleanup+0x328/0x450 call_timer_fn+0x2a/0x1c0 run_timer_softirq+0x18c/0x210 BUG: KASAN: slab-use-after-free in vxlan_fdb_destroy Fix this by rejecting the bogus configuration of dynamic FDB entries that point to FDB nexthops, both when created and when an existing entry is updated. As such, the per-nexthop FDB list is only ever mutated under the RTNL lock. Add test cases to make sure that this does not regress in the future.
02KernelScan AI Analysis
Risk summary
A local attacker with CAP_NET_ADMIN (obtainable in a user namespace) can trigger a race condition in the VXLAN driver's FDB nexthop list management, leading to use-after-free and kernel memory corruption. The race occurs when two VXLAN devices share the same FDB nexthop and dynamic entries are aged out in softirq context without the RTNL lock, concurrent with netlink-driven additions or deletions. This can result in kernel crashes or potentially arbitrary code execution.
Vulnerability analysis
A race condition exists in the VXLAN driver when two VXLAN devices share the same forwarding database nexthop and dynamic entries are configured. The shared list is protected by a lock that is specific to each device, so when both devices access the same shared data concurrently, the background cleanup that removes old entries can run at the same time as configuration changes. This allows one device to free an entry while another device is still using it, corrupting the linked list and leaving a dangling reference to freed memory. The fix refuses to create or update dynamic forwarding database entries that point to a nexthop, ensuring the shared list is only ever changed while the configuration is locked. An unprivileged local user who can gain network-admin privileges inside a user namespace can reach this by setting up VXLAN devices and adding forwarding entries through the standard network configuration interface; the race triggers naturally as entries age out without requiring precise attacker timing.
Lifecycle
03Fix Versions
| Branch | Introduced | Fixed in | Patch commit |
|---|---|---|---|
| 5.10 | 5.8 | 5.10.271 | 00c7f4b8144e |
| 5.15 | 5.8 | 5.15.222 | 55a33882808f |
| 6.1 | 5.8 | 6.1.189 | c8f29af91e08 |
| 6.6 | 5.8 | 6.6.158 | b2a8e165ddb6 |
| 6.12 | 5.8 | 6.12.111 | 28f86eec851b |
| 6.18 | 5.8 | 6.18.53 | 53359916ceb6 |
| 7.2 | 5.8 | 7.2.7 | 3fa64d86fc64 |
| mainline | 5.8 | 7.3-rc3 | 98fc57d16744 |
| cip/6.12 | 5.8 | 6.12.111-cip32 | 28f86eec851b |