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net: fix fanout UAF in packet_release() via NETDEV_UP race

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[ Upstream commit 42156f93d123436f2a27c468f18c966b7e5db796 ]

`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.

Fixes: ce06b03e60 ("packet: Add helpers to register/unregister ->prot_hook")
Link: https://blog.calif.io/p/a-race-within-a-race-exploiting-cve
Signed-off-by: Yochai Eisenrich <echelonh@gmail.com>
Reviewed-by: Willem de Bruijn <willemb@google.com>
Link: https://patch.msgid.link/20260319200610.25101-1-echelonh@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>
(cherry picked from commit ee642b1962caa9aa231c01abbd58bc453ae6b66e)

Orabug: 39250953
CVE: CVE-2026-31504

Change-Id: I002fefc6bd0bbe09ed5e2ad13009c783609ec04a
Signed-off-by: Alok Tiwari <alok.a.tiwari@oracle.com>
Signed-off-by: Tashfin Shakeer Rhythm <tashfinshakeerrhythm@gmail.com>
This commit is contained in:
Yochai Eisenrich 2026-03-19 22:06:10 +02:00 • committed by Tashfin Shakeer Rhythm
commit 214ddb05df
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net/packet/af_packet.c
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@ -3117,6 +3117,7 @@ static int packet_release(struct socket *sock)
spin_lock(&po->bind_lock);
unregister_prot_hook(sk, false);
WRITE_ONCE(po->num, 0);
packet_cached_dev_reset(po);
if (po->prot_hook.dev) {