dpll: fix NULL deref in dpll_device_ops() during teardown race

In the Linux kernel, the following vulnerability has been resolved: dpll: fix NULL deref in dpll_device_ops() during teardown race When the last owner of a dpll device unregisters while a foreign driver still holds a pin on it via dpll_pin_on_pin_register(), the dpll object stays alive with an empty registration list. A pin notification queued before the unregister (e.g. ice reacting to zl3073x_i2c removal) then walks pin->dpll_refs into dpll_device_ops(), which trips the WARN_ON and dereferences the missing registration. dpll_lock cannot help because the notification work was queued before the unregistering driver took the lock. Treat the empty registration list as a legitimate transient state. Make dpll_priv() and dpll_device_ops() return NULL in that case and make every pin netlink path that resolves a device from a pin skip such dplls. dpll_cmd_pin_get_one() picks a ref with a live registration and returns -ENODEV when there is none, the pin dumpit skips such a pin instead of aborting the dump, dpll_msg_add_pin_dplls() and the frequency, esync, reference sync and phase adjust set paths skip dead refs, and dpll_pin_parent_device_set() validates the parent with dpll_device_get_by_id(). dpll_pin_register() is the last caller that dereferenced the device ops without a check, so move its frequency monitor validation under dpll_lock and tolerate a missing registration there as well. The empty registration list is equivalent to a cleared DPLL_REGISTERED mark, both transitions happen under dpll_lock in dpll_device_register() and dpll_device_unregister(). A pin notification for a pin whose dplls are all gone is now dropped with -ENODEV instead of crashing, all callers in the core ignore that return value. WARNING: drivers/dpll/dpll_core.c:1092 at dpll_device_ops+0x24/0x40, CPU#83: kworker/u576:3/23471 Modules linked in: ... ice ... zl3073x_i2c(-) ... zl3073x ... Workqueue: ice_dpll_wq ice_dpll_pin_notify_work [ice] RIP: 0010:dpll_device_ops+0x24/0x40 Call Trace: <TASK> dpll_cmd_pin_get_one+0x336/0x520 dpll_pin_event_send+0x82/0x140 dpll_pin_on_pin_unregister+0xbb/0x160 ice_dpll_pin_notify_work+0x1bc/0x1f0 [ice] process_one_work+0x19e/0x370 worker_thread+0x1a6/0x310 kthread+0xe4/0x120 ret_from_fork+0x1a1/0x270 ret_from_fork_asm+0x1a/0x30 </TASK> ---[ end trace 0000000000000000 ]--- BUG: kernel NULL pointer dereference, address: 0000000000000010 #PF: supervisor read access in kernel mode #PF: error_code(0x0000) - not-present page