CVE-2025-21767

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.