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.
02 / AFFECTED SOFTWARE
Affected packages
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03 / CONNECTIONS
Connected vulnerabilities
04 / EVIDENCE
Source records
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.
05 / REFERENCES
Further evidence
- https://git.kernel.org/linus/42156f93d123436f2a27c468f18c966b7e5db796
- https://ubuntu.com/security/CVE-2026-31504
- https://ubuntu.com/security/notices/USN-8277-1
- https://ubuntu.com/security/notices/USN-8277-2
- https://ubuntu.com/security/notices/USN-8278-1
- https://ubuntu.com/security/notices/USN-8278-2
- https://ubuntu.com/security/notices/USN-8279-1
- https://ubuntu.com/security/notices/USN-8279-2
- https://ubuntu.com/security/notices/USN-8279-3
- https://ubuntu.com/security/notices/USN-8280-1
- https://ubuntu.com/security/notices/USN-8280-2
- https://ubuntu.com/security/notices/USN-8280-3
- https://ubuntu.com/security/notices/USN-8281-1
- https://ubuntu.com/security/notices/USN-8281-2
- https://ubuntu.com/security/notices/USN-8289-1
- https://ubuntu.com/security/notices/USN-8289-2
- https://ubuntu.com/security/notices/USN-8305-1
- https://ubuntu.com/security/notices/USN-8305-2
- https://ubuntu.com/security/notices/USN-8310-1
- https://ubuntu.com/security/notices/USN-8350-1
- https://ubuntu.com/security/notices/USN-8351-1
- https://ubuntu.com/security/notices/USN-8361-1
- https://ubuntu.com/security/notices/USN-8361-2
- https://ubuntu.com/security/notices/USN-8361-3
- https://ubuntu.com/security/notices/USN-8374-1
- https://ubuntu.com/security/notices/USN-8391-1
- https://ubuntu.com/security/notices/USN-8392-1
- https://ubuntu.com/security/notices/USN-8393-1
- https://ubuntu.com/security/notices/USN-8426-1
- https://ubuntu.com/security/notices/USN-8426-2
- https://ubuntu.com/security/notices/USN-8440-1
- https://ubuntu.com/security/notices/USN-8441-1
- https://ubuntu.com/security/notices/USN-8462-1
- https://ubuntu.com/security/notices/USN-8499-1
- https://ubuntu.com/security/notices/USN-8508-1
- https://ubuntu.com/security/notices/USN-8528-1
- https://www.cve.org/CVERecord?id=CVE-2026-31504