Linux Kernel Fanout UAF via NETDEV_UP Race
CVE-2026-31504 Published on April 22, 2026
net: fix fanout UAF in packet_release() via NETDEV_UP race
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.
Products Associated with CVE-2026-31504
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Affected Versions
Linux:- Version ce06b03e60fc19c680d1bf873e779bf11c2fc518 and below ee642b1962caa9aa231c01abbd58bc453ae6b66e is affected.
- Version ce06b03e60fc19c680d1bf873e779bf11c2fc518 and below 42cfd7898eeed290c9fb73f732af1f7d6b0a703e is affected.
- Version ce06b03e60fc19c680d1bf873e779bf11c2fc518 and below 1b4c03f8892d955385c202009af7485364731bb9 is affected.
- Version ce06b03e60fc19c680d1bf873e779bf11c2fc518 and below 654386baef228c2992dbf604c819e4c7c35fc71b is affected.
- Version ce06b03e60fc19c680d1bf873e779bf11c2fc518 and below 75fe6db23705a1d55160081f7b37db9665b1880b is affected.
- Version ce06b03e60fc19c680d1bf873e779bf11c2fc518 and below d0c7cdc15fdf8c4f91aca1928e52295d175b6ec6 is affected.
- Version ce06b03e60fc19c680d1bf873e779bf11c2fc518 and below ceccbfc6de720ad633519a226715989cfb065af1 is affected.
- Version ce06b03e60fc19c680d1bf873e779bf11c2fc518 and below 42156f93d123436f2a27c468f18c966b7e5db796 is affected.
- Version 3.1 is affected.
- Before 3.1 is unaffected.
- Version 5.10.253, <= 5.10.* is unaffected.
- Version 5.15.203, <= 5.15.* is unaffected.
- Version 6.1.168, <= 6.1.* is unaffected.
- Version 6.6.131, <= 6.6.* is unaffected.
- Version 6.12.80, <= 6.12.* is unaffected.
- Version 6.18.21, <= 6.18.* is unaffected.
- Version 6.19.11, <= 6.19.* is unaffected.
- Version 7.0, <= * is unaffected.