Bluetooth: ISO: Reject concurrent BIS listener setup

iso_sock_listen() drops the socket lock before calling iso_listen_bis()
to preserve the hdev->lock ordering. Two concurrent listen() calls on
the same broadcast socket can therefore both observe BT_BOUND. Caller
A can attach connA and release the locks, then caller B can acquire
them and attach connB before A publishes BT_LISTEN.

__iso_chan_add() checks whether connB already has a socket, but not
whether the socket already has a connection. The second attachment
overwrites iso_pi(sk)->conn, leaking the connA reference and leaving
connA->sk intact. Socket teardown follows connB, so later events on
connA can cause a stale socket access.

Recheck BT_BOUND and require iso_pi(sk)->conn to be NULL after taking
both the HCI device and socket locks in iso_listen_bis(). The connection
check also covers the interval between attachment and publication of
BT_LISTEN. Reject a competing setup with -EBADFD through the existing
unlock path before allocating another connection.

A recorded KASAN report from an instrumented kernel shows:

  BUG: KASAN: slab-use-after-free in iso_sock_hold+0x119/0x1f0
  Call Trace:
   iso_sock_hold+0x119/0x1f0
   iso_conn_del+0xe4/0x270
   hci_disconn_complete_evt+0x351/0x8c0
   hci_event_packet+0x71b/0xb20
   hci_rx_work+0x293/0x730

  Allocated by task 93:
   sk_prot_alloc+0x113/0x220
   sk_alloc+0x2b/0x6d0
   bt_sock_alloc+0x29/0x370
   iso_sock_alloc.constprop.0+0x19/0x300
   iso_sock_create+0x94/0x100

  Freed by task 93:
   kfree+0x121/0x3c0
   __sk_destruct+0x42b/0x540
   iso_sock_release+0x29d/0x340
   __sock_release+0xa1/0x260
   sock_close+0x10/0x20

Fixes: 168e28305b ("Bluetooth: iso: Fix circular lock in iso_listen_bis")
Cc: [email protected]
Assisted-by: GPT-6 Astra
Signed-off-by: Chengfeng Ye <[email protected]>
Signed-off-by: Luiz Augusto von Dentz <[email protected]>

net/bluetooth/iso.c

@@ -1340,6 +1340,11 @@ static int iso_listen_bis(struct sock *sk)
 	hci_dev_lock(hdev);
 	lock_sock(sk);
 
+	if (sk->sk_state != BT_BOUND || iso_pi(sk)->conn) {
+		err = -EBADFD;
+		goto unlock;
+	}
+
 	/* Fail if user set invalid QoS */
 	if (iso_pi(sk)->qos_user_set && !check_bcast_qos(&iso_pi(sk)->qos)) {
 		iso_pi(sk)->qos = default_qos;