CVE-2025-21767: clocksource: Use migrate_disable() to avoid calling get_random_u32() in atomic context

Severity: medium (CVSS 5.5 v3.1)
Exploited in the wild: not listed in CISA KEV
EPSS: 0.2% probability of exploitation in the next 30 days
Public exploit or detection template: none known
Published: 2025-02-27 · updated 2026-10-15
Weakness: CWE-667

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.

Affected products

Linux kernel: from 5.10.50 before 5.10.235; from 5.12.17 before 5.13; from 5.13.2 before 5.15.179; from 5.16 before 6.1.129; from 6.2 before 6.6.79; from 6.7 before 6.12.16 — fixed in 5.10.235, 5.13, 5.15.179, 6.1.129, 6.12.16, 6.13.4

How to fix

Upgrade to a fixed version: Linux kernel 5.10.235, 5.13, 5.15.179, 6.1.129. Check the vendor advisory for mitigations.

References

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