Description

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.

Executive Summary

Technical Details

Mitigations

References

Additional Metadata

{
  "score": "7.8",
  "pubDate": "2026-09-25T11:17:30.873Z",
  "pubdate": "2026-09-25T11:17:30.873Z",
  "executiveSummary": "A race condition vulnerability exists in the Linux kernel's VXLAN driver involving the improper handling of dynamic Forwarding Database (FDB) entries that reference nexthop IDs.\nThe vulnerability allows the simultaneous creation of dynamic FDB entries linked to nexthops across multiple VXLAN devices, which share the same per-nexthop FDB list.\nThis flaw leads to memory corruption, specifically a use-after-free and list corruption scenario when the kernel's aging mechanism attempts to remove entries.\nThe issue affects systems utilizing VXLAN with user-space control planes like E-VPN where load balancing across VTEPs is enabled via FDB nexthops.\nSuccessful exploitation or triggering of this condition can lead to kernel panic or denial-of-service (DoS) via system instability caused by corrupted internal list structures.\nThe vulnerability occurs because dynamic entries are allowed to be configured for nexthops, bypassing the necessary synchronization protections provided by the RTNL lock.",
  "technicalDetails": "The root cause of this vulnerability is an invalid configuration state where VXLAN FDB entries, which are intended to be managed via user-space control planes, are improperly flagged as 'dynamic'.\nIn the Linux kernel VXLAN implementation, FDB entries linked to nexthop IDs are designed for static, user-controlled overlay configurations. When these entries are erroneously flagged as dynamic, the kernel's aging process (run in softirq context) attempts to clean them up.\nThe vulnerability arises when multiple VXLAN devices are configured to point to the same FDB nexthop. These devices share a common per-nexthop FDB list. This list is protected only by a per-device hash lock, which is insufficient for managing concurrent modifications from multiple devices.\nBecause the aging process executes in a softirq context without the protection of the RTNL (Routing Netlink) lock, it lacks the necessary synchronization to safely traverse and modify the shared list. When one device initiates a deletion of an entry via vxlan_fdb_destroy, it can race against an addition or deletion operation from another device sharing the same nexthop reference.\nThe race condition results in a 'list_del' corruption, where the 'next' and 'prev' pointers of the list nodes become inconsistent, as observed in the reported 'list_del corruption' and 'KASAN: slab-use-after-free' error logs within vxlan_fdb_destroy and vxlan_cleanup.\nThe attack flow requires an attacker or a misconfigured user-space control plane to create dynamic FDB entries linked to a nexthop. Once multiple VXLAN devices reference these entries, the kernel's timer-based cleanup routine triggers the corruption as soon as the aging mechanism attempts to process the affected entries.\nExploitation does not require direct memory manipulation by an external actor, but rather the exploitation of a logic flaw in configuration validation that exposes memory management concurrency issues. The impact is a kernel panic, resulting in a system-wide denial-of-service."
}