Remotely triggerable NULL-pointer dereference in Bluetooth LE Audio BAP unicast client QoS-state handling

The Zephyr Bluetooth LE Audio Basic Audio Profile (BAP) unicast client mishandles peer-supplied ASE state notifications. In unicast_client_ep_qos_state() (subsys/bluetooth/audio/bap_unicast_client.c), the handler writes attacker-controlled QoS fields (interval, framing, phy, sdu, rtn, latency, pd) through the stream->qos pointer with only a stream != NULL guard. stream->qos is NULL for any stream that has been codec-configured via bt_bap_stream_config() but not yet added to a unicast group (it is set only by unicast_group_add_stream()).

A malicious or buggy remote ASCS server, to which the local device is connected as a BAP unicast client, can send a GATT notification announcing the ASE has entered the QoS Configured state while the local endpoint is still in the Codec Configured state — a transition the dispatcher explicitly permits — during that window, causing a write through a NULL pointer and a crash (denial of service). The data written is itself remote-controlled.

The defect shipped in v4.3.0 and v4.4.0 (and earlier). The fix re-points all BAP QoS storage to the always-valid embedded ep->qos struct, eliminating the NULL dereference.