CVE-2025-40327

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

perf/core: Fix system hang caused by cpu-clock usage

cpu-clock usage by the async-profiler tool can trigger a system hang, which got bisected back to the following commit by Octavia Togami:

18dbcbfabfff ("perf: Fix the POLL_HUP delivery breakage") causes this issue

The root cause of the hang is that cpu-clock is a special type of SW event which relies on hrtimers. The __perfeventoverflow() callback is invoked from the hrtimer handler for cpu-clock events, and __perfeventoverflow() tries to call cpuclockeventstop() to stop the event, which calls htimercancel() to cancel the hrtimer.

But that's a recursion into the hrtimer code from a hrtimer handler, which (unsurprisingly) deadlocks.

To fix this bug, use hrtimertrytocancel() instead, and set the PERFHESSTOPPED flag, which causes perfsweventhrtimer() to stop the event once it sees the PERFHES_STOPPED flag.

[ mingo: Fixed the comments and improved the changelog. ]