CVE-2024-46845: tracing/timerlat: Only clear timer if a kthread exists

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

tracing/timerlat: Only clear timer if a kthread exists

The timerlat tracer can use user space threads to check for osnoise and timer latency. If the program using this is killed via a SIGTERM, the threads are shutdown one at a time and another tracing instance can start up resetting the threads before they are fully closed. That causes the hrtimer assigned to the kthread to be shutdown and freed twice when the dying thread finally closes the file descriptors, causing a use-after-free bug.

Only cancel the hrtimer if the associated thread is still around. Also add the interfacelock around the resetting of the tlatvar->kthread.

Note, this is just a quick fix that can be backported to stable. A real fix is to have a better synchronization between the shutdown of old threads and the starting of new ones.