CVE-2026-97925: tick/broadcast: Plug clockevents replacement race

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

tick/broadcast: Plug clockevents replacement race

朱恺乾 reported and decoded the following race condition when a broadcast device is replaced:

CPUA CPUB tickbroadcastoneshotcontrol() bc = tickbroadcastdevice.evtdev; tickinstallbroadcastdevice(dev) clockeventsexchangedevice(cur, dev) shutdown(cur); detach(cur); cur->handler = noop; tickbroadcastdevice.evtdev = dev;

tickbroadcastsetevent(bc, nextevent); <- FAIL: arms a detached device.

If the original broadcast device has a restricted interrupt affinity mask and the last CPU in that mask goes offline then the BUG() in tickcleanupdeadcpu() triggers because the clockevent device is not in detached state.

The reason for this is that tickinstallbroadcastdevice() is not serialized vs. tick broadcast operations.

The obvious cure is to serialize tickinstallbroadcastdevice() with tickbroadcastlock against a concurrent tick broadcast operation.

That requires to split clockeventsexchangedevice() into two parts, one which does the exchange, shutdown and detach operation and the other which drops the module reference count. This is required because the module reference cannot be dropped while holding tickbroadcastlock.

Let clockeventsexchangedevice() do both operations as before, but let the broadcast device code take the two step approach and do the device exchange under tickbroadcastlock and drop the module reference count after releasing it.