CVE-2026-90254: Undefined Security Weakness

In the Linux kernel, the following vulnerability has been resolved:

Bluetooth: hci_sync: free the advertising instance on the failure and cancel paths

adv_timeout_expire() hands a kmalloc()ed instance byte to hci_cmd_sync_queue() with a NULL destroy callback, and only adv_timeout_expire_sync() frees it. That leaks on two paths:

- the return value is not checked, and hci_cmd_sync_queue() does not take ownership when it fails (-ENETDOWN, -ENODEV, -ENOMEM);

- a cancelled entry is not released, as _hci_cmd_sync_cancel_entry() does not free entry->data when there is no destroy callback. hci_cmd_sync_clear() cancels every pending entry when the controller is unregistered.

Free the buffer from a destroy callback, and in the caller when the entry could not be queued at all.