Bluetooth Classic HFP Hands-Free: TX buffer leak on RFCOMM disconnect leads to permanent Bluetooth denial of service

Summary

The Bluetooth Classic Hands-Free Profile HF role in subsys/bluetooth/host/classic/hfp_hf.c does not release the AT command buffers that are still queued on hf->tx_pending when the RFCOMM DLC goes down. hfp_hf_disconnected() only cancelled the work items and cleared hf->acl, and no other code path ever dequeues those buffers, so each one is permanently lost from the fixed hf_pool (CONFIG_BT_MAX_CONN + 1 buffers, i.e. two in the default configuration). The same function also left BT_HFP_HF_FLAG_CONNECTED set, and hfp_hf_create() wiped the object with memset() as soon as hf->acl was NULL, without checking whether hf->work, hf->slc_work or hf->deferred_work was still pending or running.

The remote Audio Gateway peer controls the trigger end to end and needs no application involvement: an unsolicited +BCS: 1 response makes bcs_handle() call bt_hfp_hf_select_codec() and queue an AT+BCS buffer, which cannot be transmitted while BT_HFP_HF_FLAG_RX_ONGOING is set, and an immediately following RFCOMM DISC strands that buffer. Repeating the connect / +BCS / disconnect cycle a couple of times empties hf_pool for good.

With the pool exhausted, HFP command sending fails permanently, and because hfp_hf_send_cmd() allocates with K_FOREVER (bt_rfcomm_create_pdu() → bt_conn_create_pdu()) from the Bluetooth RX context — where that timeout is not downgraded to K_NO_WAIT — one further +BCS from the peer parks the Bluetooth RX thread inside net_buf_alloc() with no possible waker, halting all HCI event processing until the device is rebooted. Separately, reusing the HF slot while a work handler was still queued or running allowed memset() to wipe live k_work items out from under the system workqueue, corrupting shared kernel workqueue state. Exploitation requires an established HFP session (for SSP devices L2CAP enforces authentication before the RFCOMM channel is reachable) and a build with CONFIG_BT_HFP_HF enabled.