CVE-2025-21767 in Linux
Summary
by MITRE • 02/27/2025
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.
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Analysis
by VulDB Data Team • 05/25/2026
The vulnerability CVE-2025-21767 represents a critical issue within the Linux kernel's clocksource subsystem that manifests specifically under PREEMPT_RT kernel configurations. This flaw occurs when the system attempts to invoke random number generation functions within atomic contexts where such operations are not permitted. The vulnerability stems from the improper handling of lock acquisition during CPU selection processes in the clocksource verification mechanism, creating a scenario where sleeping locks are accessed while preemption is disabled.
The technical implementation flaw resides in the clocksource_verify_choose_cpus() function which is executed with preemption disabled in real-time kernel environments. When this function attempts to call get_random_u32() to select CPUs for clocksource verification, it triggers a cascade of lock acquisitions through the batched_entropy_32 local lock and base_crng.lock spinlocks. These locks are implemented as sleeping locks in PREEMPT_RT configurations, making their acquisition impossible within atomic contexts where preemption is disabled and interrupts are not allowed to sleep.
The operational impact of this vulnerability is severe as it can cause system instability and potential kernel panics when the watchdog thread kwatchdog attempts to perform clocksource verification under real-time scheduling constraints. The error message indicates that sleeping functions are being called from invalid contexts, specifically showing in_atomic() returning 1, which demonstrates that the kernel is attempting to execute blocking operations while in an atomic execution context. This situation violates fundamental kernel design principles and can lead to system hangs or crashes during normal operation.
The fix implemented addresses this by utilizing migrate_disable() to properly manage the context switching requirements for smp_processor_id() calls without introducing atomic context constraints. This approach allows the function to reliably determine the current CPU without triggering the atomic context restrictions. The solution employs preempt_disable() after the clocksource_verify_choose_cpus() function completes but before actual clocksource measurements begin, thereby maintaining the necessary execution context while avoiding the problematic lock acquisition scenarios. This remediation aligns with CWE-362 which addresses concurrent execution using improper locking mechanisms and follows ATT&CK techniques related to privilege escalation through kernel exploitation.
The vulnerability demonstrates the complexity of real-time kernel implementations where standard locking mechanisms must be carefully adapted to maintain both performance and safety requirements. The fix ensures that random number generation operations occur in appropriate contexts while maintaining the integrity of the clocksource verification process. This resolution prevents potential denial of service conditions and maintains system stability under real-time scheduling constraints where deterministic behavior is essential for proper system operation. The solution maintains backward compatibility while addressing the specific requirements of PREEMPT_RT configurations where the distinction between atomic and non-atomic contexts becomes critical for system reliability.