tracing/timerlat: Drop interface_lock in stop_kthread()

Vulnerability Description

In the Linux kernel, the following vulnerability has been resolved: tracing/timerlat: Drop interface_lock in stop_kthread() stop_kthread() is the offline callback for "trace/osnoise:online", since commit 5bfbcd1ee57b ("tracing/timerlat: Add interface_lock around clearing of kthread in stop_kthread()"), the following ABBA deadlock scenario is introduced: T1 | T2 [BP] | T3 [AP] osnoise_hotplug_workfn() | work_for_cpu_fn() | cpuhp_thread_fun() | _cpu_down() | osnoise_cpu_die() mutex_lock(&interface_lock) | | stop_kthread() | cpus_write_lock() | mutex_lock(&interface_lock) cpus_read_lock() | cpuhp_kick_ap() | As the interface_lock here in just for protecting the "kthread" field of the osn_var, use xchg() instead to fix this issue. Also use for_each_online_cpu() back in stop_per_cpu_kthreads() as it can take cpu_read_lock() again.

References

• https://git.kernel.org/stable/c/a4a05ceffe8fad68b45de38fe2311bda619e76e2
• https://git.kernel.org/stable/c/09cb44cc3d3df7ade2cebc939d6257a2fa8afc7a
• https://git.kernel.org/stable/c/db8571a9a098086608c11a15856ff585789e67e8
• https://git.kernel.org/stable/c/b484a02c9cedf8703eff8f0756f94618004bd165