Bluetooth Mesh PB-ADV: invalidated provisioning link kept alive indefinitely, blocking (re)provisioning (DoS)

Summary

In Zephyr's Bluetooth Mesh PB-ADV provisioning bearer (subsys/bluetooth/mesh/pb_adv.c), prov_msg_recv() rescheduled the provisioning protocol watchdog timer unconditionally at the top of the function, before the FCS check and before the ADV_LINK_INVALID check. Once a provisioning attempt fails, prov_failed() sets ADV_LINK_INVALID and the only recovery path is the protocol timer firing (protocol_timeout -> prov_link_close -> close_link -> reset_adv_link and re-enabling of scanning and the unprovisioned device beacon). A remote, unauthenticated attacker on the BLE advertising channel can first induce a provisioning failure (e.g. with a malformed generic-provisioning PDU) and then transmit any FCS-valid PB-ADV transaction PDU on the same link ID more often than once per protocol timeout (60 s, or 120 s for OOB input/output). Because each such packet reset the timer even on an invalidated link, protocol_timeout never fired, the dead link was never torn down, and the device remained pinned in an un-provisionable state with its unprovisioned beacon disabled and new Link Open requests rejected. PB-ADV PDUs are processed without authentication and the FCS is a keyless CRC, so no pairing or prior trust is required and the attacker chooses the link ID itself. The impact is a persistent denial of provisioning/re-provisioning service; there is no memory-safety, confidentiality, or integrity impact. The vulnerable code shipped in releases through v4.4.1. The fix moves the timer reschedule to after the ADV_LINK_INVALID check (and the FCS check before the reset) so an invalidated link can no longer be kept alive by incoming packets.