clocksource: Use migrate_disable() to avoid calling get_random_u32() in atomic context

Vulnerability Description

In the Linux kernel, the following vulnerability has been resolved: clocksource: Use migrate_disable() to avoid calling get_random_u32() in atomic context The following bug report happened with a PREEMPT_RT kernel: BUG: sleeping function called from invalid context at kernel/locking/spinlock_rt.c:48 in_atomic(): 1, irqs_disabled(): 0, non_block: 0, pid: 2012, name: kwatchdog preempt_count: 1, expected: 0 RCU nest depth: 0, expected: 0 get_random_u32+0x4f/0x110 clocksource_verify_choose_cpus+0xab/0x1a0 clocksource_verify_percpu.part.0+0x6b/0x330 clocksource_watchdog_kthread+0x193/0x1a0 It is due to the fact that clocksource_verify_choose_cpus() is invoked with preemption disabled. This function invokes get_random_u32() to obtain random numbers for choosing CPUs. The batched_entropy_32 local lock and/or the base_crng.lock spinlock in driver/char/random.c will be acquired during the call. In PREEMPT_RT kernel, they are both sleeping locks and so cannot be acquired in atomic context. Fix this problem by using migrate_disable() to allow smp_processor_id() to be reliably used without introducing atomic context. preempt_disable() is then called after clocksource_verify_choose_cpus() but before the clocksource measurement is being run to avoid introducing unexpected latency.

References

• https://git.kernel.org/stable/c/d9c217fadfcff7a8df58567517d1e4253f3fd243
• https://git.kernel.org/stable/c/60f54f0d4ea530950549a8263e6fdd70a40490a4
• https://git.kernel.org/stable/c/852805b6cbdb69c298a8fc9fbe79994c95106e04
• https://git.kernel.org/stable/c/8783ceeee797d9aa9cfe150690fb9d0bac8cc459
• https://git.kernel.org/stable/c/cc3d79e7c806cb57d71c28a4a35e7d7fb3265faa
• https://git.kernel.org/stable/c/0fb534187d2355f6c8f995321e76d1ccd1262ac1
• https://git.kernel.org/stable/c/6bb05a33337b2c842373857b63de5c9bf1ae2a09