Bug 2460643 (CVE-2026-31504)

Summary: CVE-2026-31504 kernel: net: fix fanout UAF in packet_release() via NETDEV_UP race
Product: [Other] Security Response Reporter: OSIDB Bzimport <bzimport>
Component: vulnerabilityAssignee: Product Security <prodsec-ir-bot>
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Priority: medium    
Version: unspecifiedCC: rhel-process-autobot, watson-tool-maintainers
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Hardware: All   
OS: Linux   
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A flaw was found in the Linux kernel. A race condition in the `packet_release()` function, specifically during a network device up (NETDEV_UP) event, can lead to a Use-After-Free (UAF) vulnerability. This occurs when a socket is re-registered into a fanout group's array, leaving a dangling pointer. A local attacker could potentially exploit this to cause memory corruption, which may result in a denial of service or privilege escalation.
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Description OSIDB Bzimport 2026-04-22 15:02:41 UTC 
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.
Comment 1 Keith Grant 2026-04-22 19:33:08 UTC 
Upstream advisory:
https://lore.kernel.org/linux-cve-announce/2026042206-CVE-2026-31504-10f4@gregkh/T