[PATCH net v3] net/packet: avoid fanout hook re-registration after unregister

[David Lee <david.lee@trailofbits.com>]

packet_set_ring() temporarily detaches a socket from packet delivery while
reconfiguring its ring. It records the previous running state, clears
po->num, unregisters the protocol hook when needed, drops po->bind_lock,
and later restores po->num and re-registers the hook from the saved
was_running value.

That unlocked window can race with NETDEV_UNREGISTER. The notifier can
observe the socket as not running, skip __unregister_prot_hook(), and
invalidate the per-socket binding by setting po->ifindex to -1 and clearing
po->prot_hook.dev. A one-member fanout group can still retain its shared
fanout hook device pointer. When packet_set_ring() resumes, re-registering
solely from the stale was_running state can re-add the fanout hook after
the device has been unregistered.

Treat po->ifindex == -1 as an invalidated binding after reacquiring
po->bind_lock. This is distinct from ifindex 0, the normal
unbound/wildcard state: ifindex -1 marks an existing device binding that
was invalidated when the device was unregistered. Restore po->num as
before, but do not re-register the hook if device unregister already
detached the socket.

Fixes: dc99f600698d ("packet: Add fanout support.")
Link: https://lore.kernel.org/netdev/20260701113947.23180-1-david.lee@trailofbits.com/
Assisted-by: Codex:gpt-5
Signed-off-by: David Lee <david.lee@trailofbits.com>
Reviewed-by: Willem de Bruijn <willemb@google.com>
---
Changes in v3:
- Drop Reported-by due to duplicated patch author.
- Add Link to the v1 discussion.
- Clarify that ifindex -1 is an invalidated-binding state.
- Add Reviewed-by tag.

Changes in v2:
- Add Fixes and Reported-by tags.
- Fix the incorrect Signed-off-by tag from v1.

Trail of Bits has a PoC that achieves local privilege escalation using this
bug on a custom kernel config with CONFIG_LIST_HARDENED disabled, which can
be shared further if needed.

 net/packet/af_packet.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/net/packet/af_packet.c b/net/packet/af_packet.c
index 8e6f3a734ba0..000000000000 100644
--- a/net/packet/af_packet.c
+++ b/net/packet/af_packet.c
@@ -4561,7 +4561,11 @@ static int packet_set_ring(struct sock *sk, union tpacket_req_u *req_u,
 
 	spin_lock(&po->bind_lock);
 	WRITE_ONCE(po->num, num);
-	if (was_running)
+	/*
+	 * NETDEV_UNREGISTER may have invalidated the binding while bind_lock
+	 * was dropped above.  Do not re-add a fanout hook to a dead device.
+	 */
+	if (was_running && READ_ONCE(po->ifindex) != -1)
 		register_prot_hook(sk);
 
 	spin_unlock(&po->bind_lock);
-- 
2.43.0