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

[Yochai Eisenrich <echelonh@gmail.com>]

`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.
Link: https://blog.calif.io/p/a-race-within-a-race-exploiting-cve

Fixes: ce06b03e60fc ("packet: Add helpers to register/unregister ->prot_hook")

Signed-off-by: Yochai Eisenrich <echelonh@gmail.com>
---
 net/packet/af_packet.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/net/packet/af_packet.c b/net/packet/af_packet.c
index 72d0935139f0..bb2d88205e5a 100644
--- a/net/packet/af_packet.c
+++ b/net/packet/af_packet.c
@@ -3135,6 +3135,7 @@ 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) {
--
2.53.0