CVE-2026-98023

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.