CVE-2026-31504net: fix fanout UAF in packet_release() via NETDEV_UP race

Official description

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.

Affected packages & fixed versions

Affected products (as reported)

References

• https://git.kernel.org/stable/c/ee642b1962caa9aa231c01abbd58bc453ae6b66e
• https://git.kernel.org/stable/c/42cfd7898eeed290c9fb73f732af1f7d6b0a703e
• https://git.kernel.org/stable/c/1b4c03f8892d955385c202009af7485364731bb9
• https://git.kernel.org/stable/c/654386baef228c2992dbf604c819e4c7c35fc71b
• https://git.kernel.org/stable/c/75fe6db23705a1d55160081f7b37db9665b1880b
• https://git.kernel.org/stable/c/d0c7cdc15fdf8c4f91aca1928e52295d175b6ec6
• https://git.kernel.org/stable/c/ceccbfc6de720ad633519a226715989cfb065af1
• https://git.kernel.org/stable/c/42156f93d123436f2a27c468f18c966b7e5db796
• https://cert-portal.siemens.com/productcert/html/ssa-082556.html
• https://cert-portal.siemens.com/productcert/html/ssa-019113.html
• https://cert-portal.siemens.com/productcert/html/ssa-457936.html