CVE-2025-21767

/// Technical 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

/// Remediation

Check the references section for vendor advisories, patches, and mitigation guidance. If immediate patching is not possible, review the CVSS vector to understand the attack surface and apply compensating controls such as network segmentation or access restrictions.

/// References