UBUNTU-CVE-2026-31504

In the Linux kernel, the following vulnerability has been resolved: net: fix fanout UAF in packet_release() via NETDEV_UP race packet_release() has a race window where NETDEV_UP can re-register a socket into a fanout group's arr[] array. The re-registration is not cleaned up by fanout_release(), leaving a dangling pointer in the fanout array. packet_release() does NOT zero po->num in its bind_lock section. After releasing bind_lock, po->num is still non-zero and po->ifindex still matches the bound device. A concurrent packet_notifier(NETDEV_UP) that already found the socket in sklist can re-register the hook. For fanout sockets, this re-registration calls __fanout_link(sk, po) which adds the socket back into f->arr[] and increments f->num_members, but does NOT increment f->sk_ref. The fix sets po->num to zero in packet_release while bind_lock is held to prevent NETDEV_UP from linking, preventing the race window. This bug was found following an additional audit with Claude Code based on CVE-2025-38617.